Inheritance Flashcards
What type of cells are gametes, and how are they produced in parents?
Gametes are sex cells and are haploid, meaning they have only one copy of each chromosome. They are produced through meiosis.
What process occurs during fertilization, and what type of cell is formed?
During fertilization, gametes fuse. This involves the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell). The zygote is diploid, containing two copies of each chromosome.
What is the significance of gametes having only one copy of each chromosome in terms of alleles?
As there is only one chromosome from each homologous pair in gametes, there is only one allele of each gene present. This allele may be dominant, recessive, or co-dominant.
What type of process is sexual reproduction in terms of genetic variation in offspring?
Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other.
Explain how the fusion of gametes contributes to genetic variation.
Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring.
After male and female gametes fuse, what is the chromosome number of the resulting zygote?
46 because when a male and female gamete fuse their chromosomes are combined, meaning the resulting zygote is diploid.
What does it mean for a zygote to be diploid in terms of its genes and alleles?
The zygote contains two chromosomes of each copy. It will therefore have two alleles of each gene.
Describe the difference between a homozygous and a heterozygous genotype.
If the two alleles for a particular gene are the same then the genotype is described as homozygous. If the two alleles for a particular gene are different then the genotype is described as heterozygous.
To what extent is the pattern of inheritance involving haploid gamete production and fusion to form a diploid zygote common among organisms?
This pattern of inheritance is common to all eukaryotes with a sexual life cycle.
How many copies of each autosomal gene does a diploid cell possess?
A diploid cell has two copies of each autosomal gene.
What does the “P generation” refer to in genetic crosses?
The “P generation” refers to the parental generation; the initial individuals used in a genetic cross. These are typically pure-breeding (homozygous) for the traits of interest.
What do “F1 generation” and “F2 generation” stand for in the context of genetic crosses?
The “F1 generation” is the first filial generation, which is the offspring resulting from the cross of the P generation. The “F2 generation” is the second filial generation, resulting from the cross (or self-fertilization) of the F1 generation.
In flowering plants, what is contained in pollen, and where are female gametes located?
Pollen contains the male gamete and is located on the anther of the flower. The female gametes are located in the ovary.
Why is pollination necessary in flowering plants?
Plants reproduce sexually and require pollination for fertilisation. Pollination is needed to transfer the male gametes from the pollen to the female gametes in the ovary.
What is self-pollination and self-fertilization, and what type of plant commonly exhibits this?
Plants with male and female reproductive parts on the same plant can be capable of self-pollination and self-fertilisation. Plants such as peas produce both male and female gametes on the same plant, allowing self-pollination and therefore self-fertilization.
What is the purpose of a Punnett grid?
The purpose of a Punnett grid is to predict the probability of a certain offspring displaying a certain genotype or phenotype. In the case where multiple offspring are produced, Punnett grids can predict the numbers of offspring that will display a certain genotype or phenotype after a cross
How are genetic crosses utilized in agriculture and horticulture?
Genetic crosses are widely used to breed new varieties of crop or ornamental plants. Growers can also cross-pollinate by artificial pollination between different plants with favoured traits, with the goal to create new generation of plants will possess the desirable traits from both parent plants.
How can farmers and ornamental plant growers control the way their plants reproduce?
Farmers and ornamental plant growers can control the way their plants reproduce by artificially pollinating them.
If a grower thinks a trait is useful or profitable what may they do?
If a grower thinks a trait is useful or profitable they may choose to self-pollinate the favoured plants to keep the desirable traits in the next generation.
What is a gene, and where is it found?
A gene is a short length of DNA found on a chromosome that codes for a particular characteristic (by coding for the production of a specific protein).
What are alleles?
Alleles are variations of the same gene.
Why do individuals typically have two alleles for each gene?
As we have two copies of each chromosome, we have two copies of each gene and therefore two alleles for each gene. One of the alleles is inherited from the mother and the other from the father.
What is the difference between homozygous and heterozygous genotypes?
When the two alleles at a locus are the same/identical they are described as homozygous. When the two alleles at a locus are different they are described as heterozygous.
What is the difference between a gene and an allele?
A gene is a segment of DNA that codes for a particular trait, while an allele is a specific variation of that gene. The gene determines the characteristic, and the allele determines the specific form of that characteristic.
What is the phenotype of an organism?
The observable characteristics of an organism (seen just by looking - like eye colour, or found – like blood type) is called the phenotype.
What factors determine the phenotype of an organism?
The phenotype of all characteristics is determined by the following factors: The genotype only, the environment only and the interaction between both the genotype and the environment.
Give an example of a human trait primarily determined by genotype only.
Blood group is determined this way (genotype only).
Give an example of a human trait primarily determined by the environment only.
Surroundings such as chemical or radiation exposure, diet or exercise can affect physical characteristics, for example scars and accent are determined this way (environment only).
Give an example of a human trait determined by the interaction between genotype and environment.
Height and skin colour are determined this way (interaction between both the genotype and the environment).
How does the genotype contribute to the phenotype?
A genotype is the allele combination of an organism.
A phenotype is the observable characteristics resulting from the genotype and environmental influences.
How genotype influences phenotype:
- Different versions of genes that can produce varied traits.
- Genotype determines which proteins are produced, affecting physical traits.
- Some alleles are dominant over others, influencing trait expression.
- The degree to which genotype predicts phenotype, which can vary based on environmental factors.
How does the environment contribute to the phenotype?
Surroundings such as chemical or radiation exposure, diet or exercise can affect physical characteristics; for example, scars and accent are determined this way. The environment can modify the expression of genes.
What is a dominant allele?
A dominant allele only needs to be inherited from one parent in order for the characteristic to be expressed in the phenotype.
What is a recessive allele?
A recessive allele needs to be inherited from both parents in order for the characteristic to be expressed in the phenotype. If there is only one recessive allele, it will remain hidden, and the dominant characteristic will show.
If an individual is homozygous dominant, how will the trait coded by that allele be expressed?
If an individual is homozygous dominant (having two copies of the dominant allele), the dominant trait will be expressed in the phenotype.
If an individual is heterozygous for a trait with a dominant and recessive allele, how will the trait coded by those alleles be expressed?
If an individual is heterozygous (having two different alleles), the dominant allele will mask the effect of the recessive allele, and the dominant trait will be expressed in the phenotype.
Why do both a homozygous-dominant genotype and a heterozygous genotype often result in the same phenotype?
Because the presence of just one dominant allele is sufficient to mask the expression of the recessive allele. Therefore, whether an individual has two copies of the dominant allele (homozygous dominant) or one copy of the dominant allele and one copy of the recessive allele (heterozygous), the dominant trait will be expressed.
What is phenotypic plasticity?
Phenotypic plasticity is the capacity to develop traits suited to the environment experienced by an organism, by varying patterns of gene expression. Although genotype remains fixed throughout an organism’s lifetime, the way that the phenotype is expressed can vary during this time.
Is phenotypic plasticity due to changes in an organism’s genotype?
Phenotypic plasticity is not due to changes in genotype.
Can changes in traits due to phenotypic plasticity be reversed during the lifetime of an individual?
The changes in traits due to phenotypic plasticity may be reversible during the lifetime of an individual.
Give an example of phenotypic plasticity and explain how it relates to the environment.
An organism’s interaction with their environment determines how their phenotype is expressed. For example, aquatic plants can be the same species but appear morphologically different based on different water conditions.
How does phenotypic plasticity influence the traits that develop?
The organism can develop traits that allow for adaptation and survival in a certain environment.
In humans, how many chromosomes are contained in the nucleus of sperm and egg cells?
In humans, the sperm and egg cells contain 23 single chromosomes in their nucleus (as opposed to diploid cells which contain 46 chromosomes, or 23 pairs).
Who is known as the father of genetics and how did he contribute to understanding genetics?
Gregor Mendel is known as the Father of Genetics and contributed through breeding experiments on pea plants.
How did Mendel carry out his breeding experiments?
Mendel carefully transferred pollen from one pea plant to the reproductive parts of another, collecting pea seeds and growing them to find out their characteristics.
What characteristics of pea plants did Mendel investigate?
Mendel investigated the height of pea plants, the colours of their flowers and the smoothness of their seed coat.
What was the offspring ratio for the second generation, and how were traits inherited?
The offspring plants in the second generation had characteristics of both parent plants in a 3:1 ratio.
What does it mean to have multiple alleles for a gene?
Multiple alleles can exist for a gene that determines a specific trait, with each allele resulting in a different variation of that trait, like blood types A, B, AB, and O.
How many copies of each gene do cells possess, and why?
Cells have two copies of every gene because the chromosomes of eukaryotic cells occur in homologous pairs.
What is an individual described as if the two alleles of a gene are the same or different?
If the two alleles of a gene are the same, we describe the individual as being homozygous. If the two alleles of a gene are different, we describe the individual as being heterozygous
What is phenylketonuria (PKU), and what type of allele causes it?
Phenylketonuria (PKU) is a recessive genetic condition caused by by mutation in an autosomal gene that codes for the enzyme needed to convert phenylalanine to tyrosine.
What does the mutated gene in PKU code for, and what is the consequence of this mutation?
Phenylalanine is an amino acid, not an enzyme. It’s obtained through diet and is a building block of proteins.
Phenylalanine hydroxylase is the enzyme responsible for breaking down phenylalanine. This enzyme converts phenylalanine to another amino acid called tyrosine.
The PAH gene provides instructions for making the phenylalanine hydroxylase enzyme.
Phenylketonuria (PKU) is indeed caused by mutations in the PAH gene, which result in a deficient or non-functional phenylalanine hydroxylase enzyme.
When the enzyme is deficient or non-functional, phenylalanine builds up in the body because it cannot be properly broken down.
How does a child receive PKU?
In order for a child to have PKU they must first inherit two recessive alleles from each of their parents because it is caused by a recessive allele it means that two non-PKU sufferers could have a child with PKU if both parents are heterozygous carriers of the mutated PAH gene.
What does the genetic cross shown on the Punnett square demonstrate about the offspring of PKU carrier parents?
The genetic cross shown on the Punnett square above demonstrates that the offspring of the PKU carrier parents have a 75% chance of not having PKU and a 25% chance of inheriting 2 PKU alleles and therefore having the condition.
Is this pattern of inheritance unique to PKU?
This pattern of inheritance is the same with any autosomal recessive condition, for example cystic fibrosis.
What genetic testing is performed on newborns in the UK and many other countries?
Every baby born in the UK and in many other countries around the world are tested for several genetic conditions including PKU.
How are babies tested for PKU?
The babies have a small prick of blood taken from the sole of their foot a few days after being born in order to be screened for the condition.
How many alleles of a gene can exist in the gene pool, and how many does an individual inherit?
Any number of alleles of a gene can exist in the gene pool but an individual only inherits two.
What is the number of alleles that many genes have?
Many genes have more than two alleles.
Though a gene may have many alleles how many will a diploid individual inherit?
However, a diploid individual will still only inherit two of the possible alleles.
How much do alleles differ from each other?
Alleles differ from each other by one or only a few bases.
How does a very small change in base sequence affect gene function and phenotype?
Even a very small change in base sequence can bring about a large effect in gene function, with a large knock-on effect on the phenotype.
Do different alleles of a gene still occupy the same locus on the chromosome, even if they have slightly different base sequences?
Even though different alleles of a gene have slightly different base sequences, they still occupy the same locus on the chromosome.
What are the exact positions where bases differ between alleles called?
The exact positions where bases differ between alleles are called SNPs or snips (Single Nucleotide Polymorphisms).
What is the potential number of SNPs in a specific allele?
An allele can have several SNPs but still only differ by a few bases from its other allele.
What type of alleles can co-dominant alleles have on the phenotype?
Co-dominant alleles have a combined effect on the phenotype.
What is the extent of allele expression in codominance?
The alleles are both expressed to an equal extent in the phenotype.
Give an example of codominance.
An example of codominance is in speckled chickens.
In chickens, how do we denote the gene for color and its alleles in chickens?
We can denote the gene for colour using the capital letter C. The two alleles for this gene are white for white feather colour, and black for black feather colour.
What is the difference between codominance and incomplete dominance?
Codominance:
- Both alleles are fully expressed in the phenotype
- Parent traits appear together in offspring
- Example: Red and white flowers produce offspring with red and white patches
Incomplete dominance:
- Alleles blend to create an intermediate phenotype
- New phenotype is a mix of parent traits
- Example: Red and white flowers produce pink offspring
Give an example of incomplete dominance.
An example is incomplete dominance can be found in the four o’clock flower or marvel of Peru (Mirabilis jalapa).
In Marvel of Peru plants, how do we denote the gene for color and its alleles?
We can denote the gene for colour using the capital letter C. The two alleles for this gene are white for white flower colour, and red for red flower colour
What is the name of the interaction that results in acclimatization and survival in a certain environment?
The organism can develop traits that allow for acclimatisation and survival in a certain environment.
What is the role of each blood group allele?
IA allele produces A antigen. IB allele produces B antigen and ii alleles do not produce antigen
If the combination of parental alleles is IAIB what blood group will the offspring have?
The offspring will have AB blood type
What blood group will result from IAi?
The result is A. As a result, the offspring of the first cross have a 1:2:1 ratio of A:AB:B blood type
What type of diagrams can we use to determine blood groups?
We can use genetic diagrams to predict the outcome of crosses that involve the codominant alleles controlling blood groups.
What is the blood type ratio of the children if both parents are heterozygous, type A?
Using the Punnett square of blood group with two heterozygous parents, type A and type B, gives the possibility of the children having all 4 blood types with the following ratio Type A : Type B: Type AB : Type O = 1 : 1: 1: 1.
What is the inheritance of blood group an example of?
Inheritance of blood group is an example of co-dominance with multiple alleles.
Why is understanding blood group inheritance important?
This is of critical importance when deciding to give blood transfusions following injury or illness.
What can happen if the wrong blood group is used in a transfusion?
Use of the wrong blood group can cause an immune response that coagulates (solidifies) blood, leading to clots and serious illness/death.
How many alleles are there for the gene controlling a person’s blood group?
There are three alleles of the gene controlling a person’s blood group instead of the usual two.
What symbol represents the gene for blood group?
I represents the gene.
How are the codominant alleles for blood group represented?
Superscripts A and B represent the codominant alleles, I^A and I^B for example.
How is the recessive allele for blood group represented?
Lowercase i with no superscript represents the recessive allele.
What does the I^A allele result in?
I^A results in the production of antigen A on the surface of red blood cells.
What does the I^B allele result in?
I^B results in the production of antigen B on the surface of red blood cells.
What does the i allele result in?
i results in no antigens being produced on the surface of red blood cells.
What can we use to predict the outcome of crosses involving codominant alleles controlling blood groups?
We can use genetic diagrams to predict the outcome of crosses that involve the codominant alleles controlling blood groups.
What is the result in heterozygotes with codominance?
In codominance, heterozygotes have a dual phenotype.
What is the difference between codominance and incomplete dominance at the phenotypic level?
Codominance is when both traits are fully and distinctly expressed. Incomplete Dominance is when traits blend, creating an intermediate appearance.
Give an example of codominance.
Include the AB blood type (I^A I^B) as an example of codominance.
What is the result in heterozygotes with incomplete dominance?
In incomplete dominance, heterozygotes have an intermediate phenotype.
Give an example of incomplete dominance.
Include four o’clock flower or marvel of Peru (Mirabilis jalapa) as an example of incomplete dominance.
What names are acceptable when students are referring to organisms in an examination?
When students are referring to organisms in an examination, either the common name or the scientific name is acceptable.
What is an example of codominance in chickens?
A chicken with the genotype CWCW has white feathers as their phenotype. A chicken with the genotype CBCB has black feathers as their phenotype. A chicken with the genotype CWCB has a combination of both feather colours, they are called speckled colour chickens
What is an example of incomplete dominance in Marvel of Peru plants?
A plant with the genotype CWCW has white flowers as their phenotype. A plant with the genotype CRCR has red flowers as their phenotype. A plant with the genotype CWCR has a blend of both colours, which is expressed in the phenotype as a pink flower colour
What determines sex in humans?
Sex is determined by an entire chromosome pair (as opposed to most other characteristics that are just determined by one or a number of genes).
What are the sex chromosomes in human females?
Females have the sex chromosomes (pair 23 in humans) XX.
What are the sex chromosomes in human males?
Males have the sex chromosomes (pair 23 in humans) XY.
Does the XX/XY rule for sex determination apply to all species?
Note that the rule XX for females and XY for males applies to mammals, but not to all species.
What are autosomes?
All other chromosomes (pairs 1 - 22 in humans) are autosomes and have no influence on determining the sex of offspring.
Which parent determines the sex of the child, and why?
Because only a father can pass on a Y chromosome, he is responsible for determining the sex of the child.
What chromosomes are contained in male sperm cells due to meiosis?
Due to meiosis, half of his sperm cells will carry his X chromosome, half his Y chromosome.
What determines the sex of the child?
The chromosome carried by the sperm that fertilises the egg will determine the sex of the child.
What chromosomes do a father pass on to his daughters and sons?
His daughters receive a copy of his X chromosome. His sons receive a copy of his Y chromosome.
How can the inheritance of sex be shown?
The inheritance of sex can be shown using a genetic diagram (known as a Punnett square), with the X and Y chromosomes taking the place of the alleles usually written in the boxes.
How does the size and centromere placement differ between the X and Y chromosomes?
The X chromosome is larger than the Y, and has its centromere more central than on the Y chromosome.
Which chromosome carries more genes, and what is the approximate difference in the number of genes coded for?
The X carries around 16 × more genes than the Y chromosome.
What types of traits are coded for on the X chromosome but not on the Y?
Non-sex phenotypic traits, including certain blood clotting factors, are coded for on the X chromosome but not on the Y.
What do the genes on the Y chromosome code for?
The Y chromosome carries genes that code for male characteristics.
What is the SRY gene, and what is its role?
One of these genes is the SRY gene which is involved in the development of testes in male embryos and Production of testosterone.
Which sex hormones are expressed by the male sex genes, and which organ do they affect?
- SRY (Sex-determining Region Y) gene on Y chromosome.
- Triggers testes development.
- Testes produce testosterone, leading to male characteristics.
- Key role in male sex determination.
What is the result of females not receiving the male sex genes?
Females don’t receive these genes, so instead, ovaries develop and female sex hormones are expressed.
What sex hormones are expressed by the female sex genes, and which organ do they affect?
Females that develop in the womb without the SRY gene have ovaries that produces oestrogen and progesterone.
What is one of the most common sex linked diseases and what causes it?
Haemophilia is one of the most common and well known genetic disorders and it affects the ability of the blood to clot. It is caused by a faulty recessive allele located on the X chromosome.
What is a carrier?
A carrier is an individual that has one copy of the faulty allele and one normal allele, but does not suffer from the disease.
Are some genetic diseases in humans sex-linked?
Some genetic diseases in humans are sex-linked.
How does the inheritance of sex-linked diseases differ between males and females?
Inheritance of these diseases is different in males and females.
What are sex-linked genes?
Sex-linked genes are only present on one sex chromosome and not the other.
How does an individuals sex affect what alleles they pass on to offspring?
This means the sex of an individual affects what alleles they pass on to their offspring through their gametes.
Which sex chromosomes are affected by this disease?
If the gene is on the X chromosome, males (XY) will only have one copy of the gene, whereas females (XX) will have two.
What are the possible phenotypes for females in the presence of a sex-linked disease?
There are three phenotypes for females: normal, carrier, has the disease.
What are the possible phenotypes for males in the presence of a sex-linked disease?
Males have only two phenotypes: normal, has the disease.
Give an example of a well-known sex-linked disease.
Haemophilia is a well known sex-linked disease.
What does the gene found on the X chromosome code for in terms of blood clotting?
There is a gene found on the X chromosome that codes for a protein called factor VIII. Factor VIII is needed to make blood clot.
What are the two alleles for factor VIII?
There are two alleles for factor VIII: The dominant F allele which codes for normal factor VIII and The recessive f allele which results in a lack of factor VIII, meaning a person has haemophilia.
What is the result when a person possesses only the recessive allele f?
When a person possesses only the recessive allele f, they don’t produce factor VIII and their blood can’t clot normally.
What is the result of males having an abnormal allele f?
If males have an abnormal allele, f, they will have the condition as they have only one copy of the gene.
What is the result of females being heterozygous for the faulty gene?
Females can be heterozygous for the faulty gene and not suffer from the condition but act as a carrier.
What are the alleles of the mother and father?
Parental phenotypes: carrier female x normal male
Parental genotypes: X^FX^f x X^FY.
Which gametes can be formed during meiosis
Parental gametes: X^F or X^f X^F or Y.
What ratio of phenotypes can we predict from this.
Predicted ratio of phenotypes in offspring
1 female with normal blood clotting : 1 carrier female : 1 male with haemophilia : 1 male with normal blood clotting.
What ratio of phenotypes can we predict from this.
Predicted ratio of genotypes in offspring: 1 X^FX^F : 1 X^FX^f : 1 X^fY: 1 X^FY.
What does the exam tip say about formatting?
The expected notation when writing about sex linked alleles is to use upper case ‘X’ and ‘Y’ for the chromosome, next to superscript letters to represent the allele. For example X^FX^F Homoz ygous female who has haemophilia or X^FX^f Heteroz ygous female who is a carrier X^fY Male who has haemophilia.
What are family pedigree diagrams used for?
Family pedigree diagrams are usually used to trace the pattern of inheritance of a specific characteristic (usually a disease) through generations of a family.
How are males and females represented in pedigree charts?
Males are indicated by the square shape and females are represented by circles.
How are affected and unaffected individuals shown in pedigree charts?
In this diagram, affected individuals are red and unaffected are blue. Shading or cross-hatching may also be used to show affected individuals.
What do horizontal lines between males and females indicate in pedigree charts?
Horizontal lines between males and females show that they have produced children (which are linked underneath each couple).
How are generations indicated in pedigree charts?
Roman numerals may be used to indicate generations.
Why is marriage between close relatives prohibited in many countries?
Reproducing with close relatives increases the chance that both individuals possess harmful recessive alleles that can be passed onto the offspring. This causes the offspring to have a much higher chance of inheriting genetic diseases.
What is inductive reasoning in the context of pedigree charts?
Inductive reasoning is the idea of making generalised conclusions based on specific evidence taken from a small sample. For example, we could observe a sample of evidence from a pedigree chart and if that observation deviated from what we would expect we could surmise that the condition could be sex-linked.
What is deductive reasoning in the context of pedigree charts?
Deductive reasoning is making specific deductions about something unknown based on known evidence. For example, if two non-affected parents have a child that is affected by a genetic condition we can deduce that the condition is caused by a recessive allele, and that the parents are both carriers of the allele.
What exam tip is given for answering questions about pedigree charts for genetic diseases?
When answering questions about pedigree charts for genetic diseases, it is always useful to remember which phenotype is caused by the recessive allele. You can write these genotypes onto your chart and it will give you a good starting point for working out the possible genotypes of the rest of the individuals in the chart.
How can scientists draw general conclusions from pedigree charts?
Scientists draw general conclusions by inductive reasoning when they base a theory on observations of some but not all cases. A pattern of inheritance may be deduced from parts of a pedigree chart and this theory may then allow genotypes of specific individuals in the pedigree to be deduced.
Definition of continuous variation.
Features can be measured across a complete range (from one extreme to another). Data collected are quantitive data.
Continuous data is data that can take any value. Height, weight, temperature and length are all examples of continuous data.
Definition of discontinuous variation.
Features from distinct classes or categories. Data collected are qualitative data (i.e. discrete or categorical data). iscontinuous data refers to information that is collected at specific intervals or points in time. For example eye color.
What is the gene locus of continuous variation?
Many loci (that may be on different chromosomes).
What is the gene locus of discontinuous variation?
Usually only one but may be a very small number.
What is the number of alleles of continuous variation?
Many pairs of alleles as many genes contribute to the inheritance (polygenic).
What is the number of alleles of discontinuous variation?
Usually only one pair of alleles (monogenic) but may be a very small number.
What is the effect on phenotype for continuous variation?
Many intermediate phenotypes between the extremes (e.g. between shortest and tallest).
What is the effect on phenotype for discontinuous variation?
Feature either present or absent (the differences are discrete categories).
What is the environmental influence on continuous variation?
Environment has a significant influence.
What are examples of continuous variation?
Height in humans and milk yield in cattle.
What is the environmental influence on continuous variation?
Environment has little to no influence.
What are examples of discontinuous variation?
Ability to roll tongue and human blood groups.
What are the two types of variation?
Variation can be discrete or continuous.
What is discrete variation?
Discrete variation is variation that falls into two or more clear-cut categories with no overlap or in-between categories.
Give an example of discrete variation.
Blood group is an example of discrete variation. All human blood is either group O, A, B or AB, each with a Rhesus factor (+ or -). This gives just 8 distinct blood groups.
What is continuous variation?
Continuous variation occurs when two or more genes affect the final characteristic.
Give an example of continuous variation.
For example, height in humans is determined by many genetic factors: Bone length, Skeletal muscle structure, Ability to absorb food substances effectively, Hormone production. As well as environmental factors like diet, exercise, prenatal nutrition, lifestyle etc.
What term describes characteristics determined by more than one gene?
Most characteristics are determined by more than one gene - a polygenic characteristic.
Give an example of a misleading continuous trait.
Even grouped data like shoe size appears to be discrete but in fact, peoples’ feet vary continuously in size. Shoe size is merely a practicality for shoe manufacturers, who cannot make exactly the right-sized shoes for everybody.
What term describes characteristics determined by more than one gene?
Most characteristics are determined by more than one gene - a polygenic characteristic.
What distribution do we expect for continuous variation?
Continuous variation in birth mass results in the population displaying a normal distribution (bell-shaped curve).
What is the relationship between phenotype, genotype and environment?
Phenotype = genotype + environment
How do different alleles affect the phenotype?
At the genetic level different alleles at a single locus have a small effect on the phenotype.
How do different genes have the same effect on the phenotype?
Different genes can have the same effect on the phenotype and these add together to have an additive effect.
What are polygenes?
If a large number of genes have a combined effect on the phenotype they are known as polygenes.
Give an example of a continuous polygenic trait.
An example of a continuous polygenic trait is skin colour.
How does UV exposure influence the skins melanin production?
Skin color is also influenced by environmental factors such as UV exposure, which can cause the skin color to become darker.
What are box plots also known as, and when should they be used?
Box plots are also known as box-and-whisker diagrams. They are used when we are interested in splitting data up into quartiles.
What does using quartiles and drawing a box plot allow us to see?
Using quartiles and drawing a box plot allows us to see what is happening at the low, middle and high points and consider any possible extreme values.
What five values do you need to know to draw a box plot?
You need to know five values to draw a box plot: Lowest data value, First quartile, Median, Third quartile, Highest data value.
How are box plots usually drawn?
Usually on graph paper, box plots are drawn accurately with the five points marked by short vertical lines. The middle three values then form a box with the median line inside.
Where are box-and-whisker plots usually drawn?
Usually on graph paper, box plots are drawn accurately with the five points marked by short vertical lines.
Where is the median located?
The middle three values then form a box with the median line inside.
What is a quartile?
The four values that divide the population into four equal segments
Does the median always lay in the middle?
The median will not necessarily be in the middle of the box.
What does the box represent?
The box represents the interquartile range (middle 50% of the data).
What joins the lowest and highest value to the box?
The lowest data value and highest data value are joined to the box by horizontal lines.
What are outliers?
Outliers are data points that exist at the extremes above and below the rest of the data.
When will a data point be considered an outlier?
In order for a data point to be categorised as an outlier it needs to be more than 1.5 × the interquartile range above the third quartile or below the first quartile.
How are outliers plotted?
As shown in the worked example above, the outliers are plotted separately to the whiskers of the plot.
What is the relationship between unlinked genes, segregation, and meiosis?
Unlinked genes segregate independently as a result of meiosis.
What are unlinked genes?
Unlinked genes are genes that an organism carries on separate chromosomes, not on homologous copies of the same chromosome.
What is an example of a pair of unlinked genes in fruit flies (Drosophila melanogaster)?
An example of a pair of unlinked genes in fruit flies (Drosophila melanogaster) is The gene for curly wings on chromosome 2, and The gene for mahogany eyes on chromosome 3.
What is an example of a pair of unlinked genes in humans?
An example of a pair of unlinked genes in humans is The gene for trypsin (a stomach enzyme) on chromosome 7, and The gene for human growth hormone on chromosome 17.
What does assortment of chromosomes refer to?
Assortment of chromosomes refers to their alignment in metaphase I of meiosis.
How do bivalents assort themselves?
Each bivalent assorts (aligns) itself independently of all the others.
How is the segregation of chromosomes governed?
Segregation of chromosomes (i.e. how they get separated) is governed by their pattern of assortment.
What does segregation refer to?
Segregation just refers to which pole of the cell the whole chromosomes are pulled to in anaphase I.
What does segregation determine?
Segregation determines which combinations of alleles end up in which gamete cells by the end of meiosis II.
What does Mendel’s Law of Independent Assortment state?
This is called Mendel’s Law of Independent Assortment which states that alleles of different genes are inherited independently of one another; in other words inheriting a particular allele for one gene doesn’t affect the ability to inherit any other allele for another gene.
How are linked and unlinked genes inherited?
By contrast, linked genes (on the same chromosome) tend to be inherited together.
What should students understand about dihybrid crosses involving pairs of unlinked autosomal genes?
Students should understand how the 9:3:3:1 and 1:1:1:1 ratios are derived. 9:3:3:1 and 1:1:1:1 ratios for dihybrid crosses are based on what has been called Mendel’s second law. This law only applies if genes are on different chromosomes or are far apart enough on one chromosome for recombination rates to reach 50%. Students should recognize that there are exceptions to all biological “laws” under certain conditions.
What are the phenotypes and genotypes of the parents if BbEe x bbEe
Parental phenotypes are black coat, brown eyes x chestnut coat, brown eyes. Parental genotypes are BbEe x bbEe.
What are the gametes of each parent with the genotypes are parental genotypes BbEe x bbEe?
Parental gametes: BE or Be or bE or be x bE or be.
What are the resulting offspring phenotypes if the parental genotypes are parental genotypes BbEe x bbEe?
Predicted ratio of phenotypes in offspring
3 black coat, brown eyes : 3 chestnut coat, brown eyes : 1 black coat, blue eyes : 1 chestnut coat, blue eyes.
What are the resulting offspring genotypes if the parental genotypes are parental genotypes BbEe x bbEe?
Predicted ratio of genotypes in offspring = 3 BbEE : 3 bbEE : 1 Bbee : 1 bbee
What are the resulting offspring phenotypes if the parental genotypes are parental genotypes BbEe x BbEe?
Predicted ratio of phenotypes in offspring
9 black coat, brown eyes : 3 chestnut coat, brown eyes : 3 black coat, blue eyes : 1 chestnut coat, blue eyes.
What are the phenotypes and genotypes of the parents if BbEe x bbee?
Parental phenotypes: black coat, brown eyes x chestnut coat, blue eyes parental genotypes: BbEe x bbee.
What are the gametes of each parent with with the genotypes are parental genotypes BbEe x bbee?
Parental gametes BE or Be or bE or be x be.
What are the resulting offspring phenotypes if the parental genotypes are parental genotypes BbEe x bbee?
Predicted ratio of phenotypes in offspring = 1 black coat, brown eyes : 1 chestnut coat, brown eyes : 1 black coat, blue eyes : 1 chestnut coat, blue eyes
What phenotypic ratio are you expected to remember for the double-heterozygous cross for unlinked genes?
For the double-heterozygous cross for unlinked genes above, you’re expected to remember the phenotypic ratio 9:3:3:1.
What phenotypic ratio are you expected to remember for the a heterozygous and homozygous recessive cross?
For one heterozygous parent and one homozygous recessive parent, you’re expected to remember the phenotypic ratio 1:1:1:1.
What Mendel’s second law, also known as the law of independent assortment, states?
Mendel’s second law, also known as the law of independent assortment, states that a pair of traits segregates independently of another pair during meiosis when the sperm and egg cells are created during meiosis.
Why does Menderls second law not work on linked genes?
Since DNA and the chemical basis of inheritance was unknown, it has since been discovered that the law is not universally true, but requires certain conditions in order to occur. For example, linked genes on the same chromosome do not follow the same observed ratios of phenotypes than would be expected if Mendel’s second law was always correct.
What can online databases be used for in relation to human genes and their polypeptide products, following the sequencing of the human genome?
Following the sequencing of the whole human genome, we now know the exact locus (position) of every gene across the 23 pairs of chromosomes.
Online databases have been built that are able to locate any known gene or allele.
Provide an example of an online database that can be used to locate human genes and alleles.
One example is the European Molecular Biology Laboratory database (EMBL).
Give two examples of genes that can be located using these databases, along with their associated conditions and chromosome locations.
- The CFTR protein, critical to cystic fibrosis, on chromosome 7.
- HBB, a faulty allele of which is the cause of sickle-cell anaemia, on chromosome 11.
How can knowing the locus of a particular gene aid in medicine?
If we know the locus of a particular gene, medicine can establish the location of a faulty allele, which is often recessive.
How can the knowledge of a faulty allele’s location be used in genetic engineering and potentially lead to genetic therapy?
A faulty allele can be cut out of the chromosome by genetic engineering using recombinant DNA technology. Replacing a faulty allele could lead to genetic therapy.
What role do location databases play in cancer genetics?
Location databases of cancer-related genes are often vital information to researchers, doctors, and patients involved in cancer genetics.
What kind of information can be found and compared in these databases?
Databases can be used to find and compare information about DNA, mRNA, and protein sequences.
In crosses involving linkage, how should alleles be represented?
The symbols used to denote alleles should be shown alongside vertical lines representing homologous chromosomes.
Why do alleles of linked genes sometimes fail to assort independently?
Because they are located close together on the same chromosome and tend to be inherited together.
What does “7q31.2” tell you about the CFTR gene (associated with cystic fibrosis)?
- 7: It is found on chromosome 7.
- q: It is found on the long arm (q arm) of the chromosome.
- 31.2: It indicates the gene’s distance from the centromere on chromosome 7. Smaller numbers are closer to the centromere.
What does “13q34” tell you about the coagulation factor X gene (mutation can lead to hemophilia)?
- 13: It is found on chromosome 13.
- q: It is found on the long arm (q arm) of the chromosome.
- 34: It indicates the gene’s distance from the centromere on chromosome 13.
Based on their locations (7q31.2 and 13q34), are the CFTR gene and the coagulation factor X gene likely to be linked? Why or why not?
No, they are NOT likely to be linked. They are found on different chromosomes (chromosome 7 and chromosome 13), so their inheritance is not linked.
What increases the likelihood of genes being inherited together?
When genes are located close together on the same chromosome, they are likely to be inherited together due to autosomal linkage.
Why is a gene located at 3p24.1 unlikely to be linked to a gene at 6q13.2?
Because they are found on different chromosomes (chromosome 3 and chromosome 6).
Which gene is more likely to be linked to a gene located at 3p24.1: a gene found at 3q27.8, or a gene found at 3p18.9? Why?
A gene found at 3p18.9 is most likely to be linked to a gene found at 3p24.1 because they are not only on the same chromosome, but on the same arm of the chromosome and located close together (a similar distance from the centromere).
What might you be asked to do in an exam regarding this type of gene location data?
You may be asked to compare this data and comment on the likelihood of linkage between genes.
What are gene loci considered to be linked?
Gene loci are said to be linked if they are on the same chromosome.
What is a locus?
Locus (plural of loci) refers to the specific linear position on the chromosome that genes occupy.
What characteristics do sex-linked genes have?
Sex-linked genes have characteristics that generally only affect one gender of a species.
What are genes called if they are located on the sex chromosome?
If genes are on the sex chromosome, they are said to be sex-linked.
Which chromosome are sex-linked genes usually on?
These genes are usually on the X chromosome because the Y chromosome contains very few genes.
Give examples of genetic conditions in humans that are sex-linked and primarily affect males.
In humans, color-blindness and hemophilia are notable examples of genetic conditions that only affect males.
What are linked genes located on chromosomes 1-22 called?
Linked genes located on chromosomes 1-22, or any chromosome that is not a sex chromosome (called autosomes) are said to be examples of autosomal linkage.
How is the likelihood of genes being inherited together measured?
The likelihood of genes being inherited together, or the extent to which they are linked, is measured in units called centimorgans, in honor of Thomas Hunt Morgan’s work.
How can linked genotypes be written to make them easier to understand?
When writing linked genotypes it can be easier to keep the linked alleles within a bracket.
Give an example of writing a genotype with linked genes using brackets.
Give an example of writing a genotype with linked genes using brackets.
Describe another commonly used way of denoting linked alleles.
Another commonly used way of denoting linked alleles is to link them with a line.
Give an example of denoting linked alleles using a line.
So, for example, linkage between genes F and G might be shown as:
F G
_
_
f g
What should you remember to distinguish between?
Remember to distinguish between sex linkage and autosomal linkage.
What does the explanation of non-Mendelian ratios fall into for IB?
The explanation of non-Mendelian ratios falls into the domain of autosomal linkage for IB.
What assumption do dihybrid crosses and their predictions rely on?
Dihybrid crosses and their predictions rely on the assumption that the genes being investigated behave independently of one another during meiosis.
Do all genes assort independently during meiosis?
However, not all genes assort independently during meiosis.
What do some genes located on the same chromosome display?
Some genes which are located on the same chromosome display autosomal linkage and stay together in the original parental combination.
How does linkage between genes affect the alleles passed onto offspring?
Linkage between genes affects how parental alleles are passed onto offspring through the gametes.
What is a dihybrid cross used for in the theoretical example involving newts?
In the following theoretical example, a dihybrid cross is used to predict the inheritance of two different characteristics in a species of newt.
What characteristics are being investigated in the newt example?
The genes are for tail length and scale color.
What are the two alleles for tail length in the newt example?
The gene for tail length has two alleles: Dominant allele T produces a normal length tail, and Recessive allele t produces a shorter length tail.
What are the two alleles for scale color in the newt example?
The gene for scale color has two alleles: Dominant allele G produces green scales, and Recessive allele g produces white scales.
If the genes are unlinked, what is the predicted phenotypic ratio of offspring in the dihybrid cross?
Predicted ratio of phenotypes in offspring = 1 normal tail, green scales : 1 normal tail, white scales : 1 short tail, green scales : 1 short tail, white scales
If the genes are unlinked, what is the predicted genotypic ratio of offspring in the dihybrid cross?
Predicted ratio of genotypes in offspring = 1 TtGg : 1 Ttgg : 1 ttGg : 1 ttgg
What phenotypic ratio is observed if the same dihybrid cross is carried out with linked genes?
There would be a 1 : 1 phenotypic ratio (1 normal tail, green scales : 1 short tail, white scales).
Why does the phenotypic ratio change when the genes are linked?
This change in the phenotypic ratio occurs because the genes are located on the same chromosome.
What does the unexpected phenotypic ratio indicate?
The unexpected phenotypic ratio, therefore, shows us that the genes are linked.
Describe the parental phenotypes in the worked example of autosomal linkage in newts.
Parental phenotypes: normal tail, green scales x short tail, white scales.
Describe the parental genotypes in the worked example of autosomal linkage in newts.
Parental genotypes: (TG)(tg) x (tg)(tg).
Describe the parental gametes in the worked example of autosomal linkage in newts.
Parental gametes: (TG) or (tg) x (tg).
What is the predicted genotypic ratio of offspring in the worked example?
Predicted ratio of genotypes in offspring = 1 (TG)(tg) : 1 (tg)(tg).
What is the predicted phenotypic ratio of offspring in the worked example?
Predicted ratio of phenotypes in offspring = 1 normal tail, green scales : 1 short tail, white scales.
What type of offspring do test crosses involving autosomal linkage predict?
When you are working through different genetics questions you may notice that test crosses involving autosomal linkage predict solely parental type offspring (offspring that have the same combination of characteristics as their parents).
Why are recombinant offspring often produced in reality?
However in reality recombinant offspring (offspring that have a different combination of characteristics to their parents) are often produced. This is due to the crossing over that occurs during meiosis.
How does crossing over affect gene linkage?
The crossing over and exchanging of genetic material breaks the linkage between the genes and recombines the characteristics of the parents. So if a question comes along that asks you why recombinant offspring are present you now know why.
What are recombinant offspring and how are they produced in the context of autosomal linkage?
Recombinant offspring have a different combination of characteristics compared to their parents. They are produced due to crossing over during meiosis, which breaks the linkage between genes and recombines parental characteristics.
In a population, how does the frequency of recombinant offspring compare to non-recombinant offspring?
The frequency of recombinants is nearly always less than that of non-recombinants.
Explain the relationship between the distance between two linked genes on a chromosome and the recombination frequency.
Recombination frequency between two linked genes is greater when the genes are further apart on the same chromosome. This is because there are more possible locations for a chiasma to form between the genes.
Describe the process of crossing over and its impact on phenotypes.
Crossing over involves the exchange of genetic material during meiosis. This process results in recombinant phenotypes that can differ from the parental phenotype. Crossing over is random and chiasmata form at different locations with each meiotic division.
How to determine the outcomes of crosses between an individual heterozygous for both genes and an individual homozygous recessive for both genes, in order to identify recombinants in gametes, in genotypes of offspring and in phenotypes of offspring.
Set up a punnet square with the possible gametes from each parent. The heterozygous parent (AaBb) can produce four types of gametes: AB, Ab, aB, and ab. The homozygous recessive parent (aabb) can only produce one type of gamete: ab.
After completing the punnet square, the offspring genotypes will include both parental and recombinant combinations. Parental genotypes resemble the original parents (AaBb and aabb). Recombinant genotypes have new combinations of alleles (Aabb and aaBb).
The phenotypes of the offspring reflect their genotypes. Parental phenotypes will be those associated with the original parents. Recombinant phenotypes will display traits that are different from either parent, due to the new allele combinations.
What is a test cross and how is it used to identify recombinants?
A test cross involves crossing an individual with a homozygous recessive individual (for both traits). It is used to determine unknown genotypes and identify recombinant phenotypes in offspring. If any offspring possess a non-parental phenotype, they are labeled as recombinants. These individuals have new allele combinations due to crossing over during meiosis, leading to the exchange of genetic material between chromosomes.
How do you draw a Punnett square for dihybrid inheritance of linked genes?
- Cross double-homozygous dominant plants (PL)(PL) with double-homozygous recessive plants (pl)(pl) to produce 100% heterozygous F1 generation (PL)(pl).
- Interbreed F1 generation to produce F2 generation.
- Create a Punnett square showing possible genotypes for F2.
- Note that the frequency of recombinant gametes will be lower than parental gametes, affecting offspring phenotypes.
What are the alleles in the sweet pea plant example?
P = purple flowers, dominant to p = red flowers
L = long seeds, dominant to l = round seeds
How do the expected vs. predicted phenotypes differ in linked genes?
Expected (Mendelian ratios): 9:3:3:1 ratio
Predicted (Linked genes):
More F2 offspring show parental phenotypes than expected
Fewer plants with recombinant phenotypes are produced than the 9:3:3:1 ratio would suggest
What is a recombinant in genetic crosses?
A recombinant is an offspring that possesses a non-parental phenotype due to new allele combinations. These combinations result from crossing over during meiosis, which leads to the exchange of genetic material between chromosomes.
How do students determine the outcomes of crosses between an individual heterozygous for both genes and an individual homozygous recessive for both genes?
Students should:
- Identify possible gametes for each parent
- Create a Punnett square to show possible offspring genotypes
- Determine phenotypes from genotypes
- Identify recombinants in gametes, offspring genotypes, and offspring phenotypes
How do you determine the outcomes of crosses between an individual heterozygous for two genes and an individual homozygous recessive for both genes? How do you identify recombinants?
- Cross:Perform a test cross (heterozygous individual crossed with a homozygous recessive individual).
- Gametes: Identify gametes with new combinations of alleles compared to the parental gametes. These are the recombinant gametes.
- Genotypes: In the offspring, identify genotypes that are different from the parental genotypes due to recombination.
- Phenotypes: Identify offspring phenotypes that do not match the parental phenotypes. These arise from the recombinant genotypes.
What are non-Mendelian ratios, and why were the actual ratios found in some genetic crosses referred to as such?
- Non-Mendelian Ratios are the actual ratios found were referred to as ‘non-Mendelian’ as they didn’t follow Mendel’s pattern.
- Recombinants were still being produced
What theories were developed to explain non-Mendelian ratios?
Genes could be inherited together, not by the law of independent assortment as put forward by Mendel.
The idea of linkage of genes was developed to explain the non-Mendelian ratios.
What factors influence the frequency of recombinant phenotypes and gametes?
The frequency of recombinant phenotypes is lower because crossing over is a random process, and the chiasmata do not always form in the same place for each meiotic division. Also, the frequency of recombinant gametes also depends on the closeness of linkage between the two genes. Genes located close together on a chromosome are less likely to be separated by crossing over, so recombinants of those two genes will be less frequent.
What does a chi-squared test determine in the context of dihybrid crosses?
Whether there is a significant difference between the observed and expected results in an experiment. This helps determine if deviations are due to chance or another factor like gene linkage. Remember that statistical testing often involves using a sample to represent a population. In this case the sample is the F2 generation. In many experiments the sample is the replicated or repeated measurements.
What type of data is required for a chi-squared test?
Categorical data (data that can be grouped).
Outline the steps for calculating a chi-squared value.
- Obtain the expected and observed results.
- Calculate the difference between each set of results.
- Square each difference.
- Divide each squared difference by the expected value, then sum these answers to obtain the chi-squared value.
How do you interpret a chi-squared value?
- Compare the chi-squared value to a critical value from a chi-squared distribution table, using the appropriate degrees of freedom.
- If the chi-squared value represents a larger probability than the critical probability (typically 0.05), the differences are likely due to chance.
- If it represents a smaller probability than the critical probability, the differences are significant and something else may be causing the differences.
What is a null hypothesis (H₀), and what is the standard null hypothesis in a chi-squared test?
A null hypothesis (H₀) is what we believe to already be true. The standard null hypothesis is that there is no significant difference between the expected and observed frequencies, and any difference that does occur is due to chance.
What is an alternative hypothesis (H₁), and what is the standard alternative hypothesis in a chi-squared test?
The alternative hypothesis (H₁) sets out how we think the population parameter could have changed. The standard alternative hypothesis is that there is a significant difference between the expected and observed frequencies.
How are degrees of freedom (df) calculated in a chi-squared test for dihybrid crosses?
Degrees of freedom are calculated by subtracting one from the number of classes (phenotype categories). For example, with four phenotypes, df = 4 - 1 = 3.
In a chi-squared test, if the calculated chi-squared value is greater than the critical value at p=0.05, what does this indicate?
This indicates that there is a significant difference between the expected and observed results. We reject the null hypothesis. The differences are likely due to a factor other than chance (e.g., gene linkage).
