Patterns of inheritance Flashcards
Define the term variation
The differences in characteristics between organisms are called variation
Define the term interspecific variation
The differences between organisms of different species
Define the term intraspecific variation
The differences between organisms of the same species
Name and describe the two causes of variation.
- An organism’s genetic material- differences in the genetic material an organism inherits from its parents leads to genetic variation
- The environment in which the organism lives- this causes environmental variation.
Describe 5 causes of genetic variation between individuals within a population.
- Alleles- with a gene for a particular characteristic, different alleles produce different affects. Individuals in a species population may inherit different alleles of a gene.
- Mutations- Changes to the DNA sequence and therefore to genes can lead to changes in the proteins that are coded for.
- Meiosis- gametes are produced by meiosis. Each gamete receives half the genetic content of a parent cell. The genetic material is mixed up by independent assortment and crossing over.
- Sexual reproduction- offspring produced from tow individuals inherits genes form each of the parents
- Chance- different gametes are produced and in reproduction it is chance as to which two combine- individuals differ from siblings
Define phenotype
Observable characteristics of an organism
Define genotype
Genetic makeup of an organism
Describe how chlorosis is an example of a phenotype influenced by both genetic and environmental factors.
- Chlorosis, is when the leaves look pale or yellow, this occurs because the cells are not producing the normal amount of chlorophyll
- Most plants which show chlorosis have normal genes coding for chlorophyll production. The change in their phenotype is the result of environmental factors:
- 1) lack of light- in absence of light plants will turn of chlorophyll production to conserve resources
- 2) Mineral deficiencies- lack of iron or magnesium. Iron is needed as a cofactor by some of the enzymes that make chlorophyll and magnesium is found at the heart of the chlorophyll molecule
- 3) Virus infections- interfere with the metabolism of cells
Describe how body mass is an example of a phenotype influenced by both genetic and environmental factors.
- In most cases dramatic variations in size such as obesity and being severely underweight are a result of environmental factors
- e.g. amount of foods eaten and quantity of exercise or presence of disease
- Occasionally obesity can be a result of the genetic make-up of an organism
- Mutations can cause a pattern of fat deposition in the body to be altered.
- Scientific studies have shown that this gene acts in conjunction with other genes that regulate the energy balance in the body.
Define allele
Different versions of the same gene
Define dominant allele
Version of the gene that will always be expressed if present
Define recessive allele
Version of a gene that will only be expressed if two copies of this allele are present in an organism
Define homozygous
Two identical alleles for a characteristic
Define heterozygous
Two different alleles for a characteristic
Define carrier
A person who has one copy of a recessive allele coding for a genetically inherited condition
Define monogenic inheritance
A characteristic inherited on a single gene
Define dihybrid inheritance
A characteristic inherited on two genes
Define autosomal linkage
Genes present on the same non-sex chromosome
Define sex linked genes
Genes carried on the sex chromosomes
Define codominance
When different alleles of a gene are equally dominant and both are expressed in the phenotype
Define epistasis
The effect of one gene on the expression of another gene
Describe the 6 steps for drawing a genetic cross diagram.
- State the phenotype of both the parents
- State the genotype of both parents. Assign a letter code to represent the alleles of the gene being studied. A capital letter should be used to represent the dominant allele and its lowercase form to represent the recessive allele.
- State the gametes of each parent and circle the letters
- Use a Punnet Square to show the results of the random fusion of gametes during fertilisation. Remember to label the gametes on the edges of the square.
- State the proportion of each genotype which are produced among the offspring.
- State the corresponding phenotype for each of the possible genotypes. It must be clear you know which phenotype results from each genotype
Describe what codominance is
- Occurs when two different alleles occur for a gene- both of which are equally dominant.
- As a result both alleles of the gene are expressed in the phenotype of the organism
- e.g colours of flowers red and white make pink
- When studying codominance upper and lower case letters are not used to represent the alleles
- Instead a letter is chosen to represent the gene e.g C
The different alleles are then represented using a second letter which is shown as a superscript
Describe what happens when there are multiple alleles for a gene
- Some genes have more than two versions- multiple alleles
- However, as an organism carries only two versions of the gene (one on each homologous chromosome) only two alleles can be present in an individual
- Blood group is determined by a gene with multiple alleles
- This results in many different crosses
Describe how sex is determined in humans
- Humans have 23 pairs of chromosomes of varying shapes and sizes
- In 22 pairs both members of the pair are the same but the 23rd pair known as the sex chromosomes are different
- Human females have two x chromosomes, whereas a male has an X and Y
- The X chromosome is large and contains many genes not involved in sexual development
- The Y chromosome is very small containing almost no genetic information, but does carry the gene that causes the embryo to develop as a male
Describe what sex linkage is
- Some characteristics are determined by genes carried on the sex chromosomes- sex linked
- As the Y chromosome is much smaller than the X chromosome, there are a number of genes in the X chromosome that males have only one copy of.
- This means that any characteristic caused by a recessive allele on the section of the X chromosome, which is missing in the Y chromosome occurs more frequently in males.
- Because females will also have a dominant allele present in their cells
Give an example of a sex-linked genetic disorder
- Haemophillia
- Patients with haemophilia have blood which clots extremely slowly due to the absence of a protein blood-clotting factor. This can lead to prolonged bleeding with is potentially fatal
- It is a recessive allele on the X chromosomes so more frequent in males
Describe what dihybrid inheritance and how a dihybrid cross would look
- A dihybrid cross is used to show the inheritance of two different characteristics, caused by two genes, which may be located on different pairs of homologous chromosomes. Each of these genes can have two or more alleles.
- A dihybrid cross is set out in a very similar format to the one used when studying a monohybrid cross but 4 alleles are shown at each stage instead of two
- e.g peas can be yellow or green and round or wrinkled
State the expected phenotypic ratio for dihybrid crosses involving: i) two double heterozygotes, ii) one double heterozygote and one double homozygous recessive
- If a true breeding homozygous pea plant with yellow round seeds is crossed with a true breeding homozygous pea plant with green wrinkled seeds. All the F1 generation will have a heterozygous genotype YyRr
- When the F1 generation are crossed there are 16 possible combinations of alleles and the expected ratio is 9:3:3:1
- 9 is the characteristics where both are dominant
- 3 is characteristics where one is dominant and one recessive
- 1 is phenotype which shows both the recessive characteristics
Explain why expected phenotypic ratios may not occur if there is linkage between two genes.
- The fertilisation of gametes is a random process so in a small sample a few chance events can lead to a skewed ratio
- The genes being studied are both on the same chromosome. These are known as linked genes. If no crossing over occurs the alleles for the two characteristics will always be inherited together.
Explain why crossing over disrupts autosomal linkage, but only occasionally.
- When genes that are linked are found on one of the oher pairs of chromosomes it is called autosomal linkage.
- Linked genes are inherited as one unit- no independent assortment during meiosis unless alleles are separated by chiasmata- tend to be inherited together
- Linked genes cannot undergo the normal random shuffling of alleles and the expected rations will not be produced in the offspring.
- Occasionally they will be separated- recombinant offspring
- The closer the genes are on a chromosome the less likely they are to be separated during crossing over and the fewer recombinant offspring produced.
Define recombinant offspring
New combination of alleles- different allele combination than either parent.
Define recombinant frequency
Proportion of recombinant offspring resulting from a cross
Describe how epistasis can occur
- We talk about a the expression of a gene to give a particular characteristic.
- Another gene may interact with this gene to change its expression
- Gene regulation is an example.
What are the different forms of epistasis
- One gene can interact with a second gene
- One gene may prevent the expression of a second gene- antagonistic epistasis
- It may allow a second gene to be expressed- complementary epistasis
Define hypostatic gene
A gene that is affected by another gene
Define epistatic gene
A gene that affects the expression of another gene
Define dominant epistasis
- Dominant epistasis occurs if a dominant allele results in a gene having an effect on another gene.
- This would happen if an epistatic gene coded for an enzyme that modified one of the precursor molecules in the pathway.
- The next enzyme in the pathway would then lack a suitable substrate molecule and so the pigment would again not be produced.
- All the genes in the sequence would be effectively masked.
Define recessive epistasis
- If the presence of two recessive alleles at a gene locus led to the lack of an enzyme it would be called recessive epistasis
Describe when a chi-squared (χ2) test would be used to analyse data.
- When you are trying to see if differences between the observed and expected value are significant or down to chance.
- Used to test the null hypothesis
- The values calculated are used to find the probability of the difference being due to chance alone.
Describe the meaning of each of the symbols in the equation for calculating the χ2-value (the test statistic) from a chi-squared (χ2) test
- O= observed frequencies
- E= expected frequencies
- χ^2 = the test statistic
- df= degrees of freedom and is n-1
Describe how to conduct a chi-squared test to determine whether data from a genetic cross supports the hypothesis about how the characteristic(s) is/are inherited.
- Calculate the observed and expected frequencies
- Calculate the test statistic
- Find the critical value from a table
- If the test statistic is smaller than the critical value the null hypothesis is accepted and the difference is not significant
Define continuous variation
A characteristic that can take any value any value within a range e.g height
Define discontinuous variation
A characteristic that can only result in discrete values e.g. blood type
Describe the causes of variation that result in discontinuous variation.
Variation determined purely by genetic factors falls into the discontinuous category. Most characteristics are controlled by a single gene.
Define polygenic
A characteristic that is controlled by a group of genes
Describe the term multifactorial
Characteristics which are dependent on a number of factors.
Describe the causes of variation that result in continuous variation.
Characteristics which show continuous variation are controlled by a number of genes and are also often influenced by environmental factors.
Define evolution
The change in allele frequency within a gene pool over time
Define natural selection
The process by which organisms best suited to their environment survive and reproduce, passing on their characteristics to their offspring through their genes
Define the term allele frequency
The relative frequency of a particular allele in a population in a given time
Define the term gene pool
Sum total of all the genes in a population at a given time
Define the term selection pressure
Factors that affect an organisms chance of survival or reproductive success
Define the term selectively neutral allele
A variety of a gene that doesn’t provide a selective advantage or disadvantage to the organism
Define advantageous allele
A variety of a gene that provides a selective advantage to the organism
Define advantageous characteristic
One form of a part of an organism’s phenotype that provides it with a selective advantage
Describe the steps in the process of adaptations evolving by natural selection.
- Organisms within a species show variation in their characteristics that are caused by genetic variation.
- Organisms whose characteristics are best adapted to a selection pressure- predation, competition- have an increased chance of surviving and successfully reproducing
- Successful organisms pass the allele encoding the advantageous characteristic onto their offspring.
- This process is repeated for every generation. Over time, the proportion of individuals with the advantageous adaptation increases. There fore the frequency of the allele that codes for this increases in the population’s gene pool.
- Over a very long periods of time this process can lead to the evolution of new species
Name 5 factors that can affect the evolution of a specie
- Mutations
- Sexual selection
- Gene flow
- Genetic drift
- Natural selection
Define the term “genetic drift” and describe the consequence of it on the genetic diversity of a population.
- Random change of allele frequency
- This is change in allele frequency due to random nature of mutation.
- The appearance of a new allele will have a greater impact (is more likely to increase in number) in a smaller population than in a much larger population where there is a greater number of alleles present in the gene pool
Define the term “genetic bottleneck” and describe the consequence of it on the genetic diversity of a population.
- Large reductions in population size which last for at least one generation are called population bottlenecks
- The gene pool along with genetic diversity, is greatly reduced and the effects will be seen in future generations.
Define the term “founder effect” and describe the consequence of it on the genetic diversity of a population
- Small populations can arise due to the establishment of new colonies by a few isolated individuals- founder effect- an extreme example of genetic drift
- These populations have much smaller gene pools than the original populations and display less genetic variation
- If carried to the new population, the frequency of any alleles that were rare in the original population will be much higher in the new, smaller population and so they will have a much bigger impact during natural selection.
Define the term “stabilising selection” and describe the consequences of it for a population.
- The norm or average is selected for (positive selection) and the extremes are selected against (negative selection)
- It results in a reduction of frequency of alleles at the extremes and an increase in the frequency of average alleles.
e. g. medium baby weight
Define the term “directional selection” and describe the consequences of it for a population.
- Occurs when there is change in the environment and the normal phenotype is no longer the most advantageous.
- Organisms which are less common and have more extreme phenotypes are positively selected.
- The allele frequency then shifts towards the extreme phenotypes and evolution occurs.
e. g peppered moths colours
Define the term “disruptive selection” and describe the consequences of it for a population.
- The extremes are selected for and the norm selected against. Opposite of stabilising selection
- E.g Darwin finches
State what the Hardy-Weinberg principle says
In a stable population with no disturbing factors, the allele frequencies will remain constant from one generation to the next and there will be no evolution
Describe the assumptions made in the Hardy-Weinberg principle.
- No selection occurs- all alleles are equally advantageous
- No mutations are occuring
- Mating is random
- Population is not large
- No migration into or out of population
Describe what the symbols represent in the Hardy-Weinberg equations
- p + q =1
p= frequency of dominant allele
q= frequency of recessive allele - p^2 +2pq +q^2 = 1
p^2= frequency of homozygous dominant genotype
2pq= frequency of heterozygous genotype
q^2= frequency of homozygous recessive genotype
Define speciation
The formation of new species through evolution
Define allopatric speciation
Speciation that occurs as a result of a physical barrier between populations
Define sympatric speciation
Speciation that occurs when there is no physical barrier between populations
Outline the stages in the process of speciation.
- Members of a population become isolated an no longer interbreed with the rest of the population resulting in no gene flow between the two groups
- Alleles within the groups continue to undergo random mutations. The environment of each group may be different or change (resulting in different selection pressures) so different characteristics will be selected for and against.
- The accumulation of mutations and changes in allele frequencies over many generations eventually lead to large changes in phenotype.
- The members of different populations become so different they are no longer able to interbreed (to produce fertile offspring). They are now reproductively isolated and are different species.
Describe how geographical isolation can lead to the evolution of new species.
- Allopatric speciation is the more common form of speciation and happens when some members of a population are separated from the rest of the group by a physical barrier e.g. river of sea.
- The environments of the different groups will often be different and so will the selection pressures resulting in difference physical adaptations.
- Separation of a small group will often result in the founder effect leading to genetic drift further enhancing the differences between the populations
e. g. Galapagos Islands and finches
Define prezygotic reproductive barrier
Prevents fertilisation and formation of a zygote
Define postzygotic reproductive barrier
Zygotes are formed but do not produce mature fertile offspring
Describe 5 types of prezygotic reproductive barrier and 2 types of postzygotic reproductive barrier.
Prezygotic
1. Geographical isolation- populations never meet because they inhabit different areas
2. Ecological isolation- Inhabit same area but they live in different habitats within that area
3. Temporal isolation- inhabit the same habitat but are active at different times of the day and reproduce at different times of year
4. Behavioral isolation- Populations meet, but don’t recognise each others courtship displays, so fail to mate
5. Mechanical isolation- Can’t interbreed because reproductive parts don’t fit each other
Postzygotic
1. Hybrid inviability- Interbreed but the hybrid offspring produced fail to live to maturity
2. Hybrid sterility- The populations interbreed but the hybrid offspring are sterile.
Describe the role of hybridisation and polyploidy in the process of speciation in plants
- When members of two different species interbreed and form fertile offspring- often in plants
- The hybrid formed which is a new species will have a different number of chromosomes to either parent and may no longer be able to interbreed with members of either parent population.
- This stops gene flow and reproductively isolated the hybrid organisms.
Define artificial selection and describe the process
- Selection of individuals for breeding with desirable characteristics.
- Artificial selection or selective breeding is fundamentally the same as natural selection except for the nature of the selection pressure applied.
- Instead of environment leading to survival of the fittest, it is selection for breeding of plants or animals with desirable characteristics by farmers or breeders
- Individuals with the desired characteristics are selected and interbred
- Offspring from this cross showing the best examples of the desired traits are selected to breed.
- This breeding of closely related individuals is called interbreeding.
- The process is repeated over many generations resulting in changes to the frequency of alleles within the population and eventually speciation
Describe the problems caused by interbreeding
- Limiting the gene pool and so decreasing genetic diversity reduces the chances of a population of inbred organisms evolving and adapting to changes in their environment
- Many genetic disorders are caused by recessive alleles, cystic fibrosis.
- Recessive alleles are not uncommon in mos populations but most individuals will be heterozygous.
- Organisms that are closely related are genetically similar and likely to have the same recessive alleles- results in offspring which have a greater chance of being homozygous and affected by the genetic disorder
- OVer time this reduces the ability of these organisms to survive and reproduce.
- Results in the organisms being less biologically fit- less likely to survive and produce two surviving offspring to replace themselves.
Give 1 example of artificial selection in crop plants.
Brassica oleracea is a wild mustard which has been selectively bred for many centuries producing a number of common vegetables.
Describe the ethical considerations surrounding the use of artificial selection in dogs (and other species).
- With the limited gene pool and lack of outbreeding it is inevitable that unwanted traits are selected for also.
- Big dogs have hip or heart problems.
- The skull of King charles spaniel is too small to accomodate the brain comfortably leading to pain and discomfort
- Bull dogs have breathing problems.
- Diseased dogs have effectively been deliberately interbred leading to a lot of problems for the animals.
Define wild type
The allele that codes for the most common phenotype in a natural population
Define the term outbreeding
Breeding of distantly related organisms
Define seed bank
A store of genetic material from plants in the form of seeds. Store both wild type and domesticated varieties
Define gene bank
Store of genetic material- usually frozen
Describe what populations characteristics are normally
- Populations are usually polymorphic (display more than one distinct phenotype).
- The allele coding for the most common characteristic is called the wild type allele.
- Other forms of that allele, resulting from mutations are called mutants.
Explain the value of seed and gene banks for artificial selection in plants and animals.
- Owing to problems caused by interbreeding, alleles from gene banks are used to increase genetic diversity in a process called outbreeding
- Breeding unrelated or distantly related varieties is also a form of out breeding
- This reduces occurrence of homozygous recessives and increases the potential to adapt to environmental change
