Chapter 17- Inherited Change Flashcards
3.7.1
What is the significance of Mendel in inheritence?
Gregor Mendel- work rediscovered to show the laws characteristics are inherited by.
What is the genotype?
- Genetic constitution of an organism- all the alleles an organism has.
- Determines the limits within which the characteristics of an individual may vary, however, the actual characteristics of the individual are affected by other elements, such as diet.
- E.g. lack of calcium leading to maximum height not being reached.
What is the phenotype?
- Observable/ biochemical characteristics of an organism.
- The expression due to the genetic constitution and its interaction with the environment.
- Expression of the genotype and the environmental effects can alter the phenotype.
What is a gene?
- Length of DNA- sequence of nucleotide bases on a DNA molecule that code for a particular polypeptide (or RNA).
- Polypeptides may then determine characteristics e.g., enzymes, pigments. All individuals of the same species have the same genes.
Describe the features of alleles.
- Individuals of a species share the same genes but (usually) different combinations of alleles of these genes.
- An individual inherits alleles from their parent or parents.
- Allele- one of the different forms of a gene. Most plants and animals carry two alleles of each gene- one from each parent- may be more.
- There may be many alleles of a single gene.
- Order of bases in each allele slightly different- each allele codes for different versions of the same characteristic.
- Represented using letters.
What is a locus?
- Fixed position of a gene on a DNA molecule.
- Only one allele of a gene can occur at the locus of any one chromosome.
- Diploid organisms- two copies of each chromosome- alleles of each gene are found at the same locus on each chromosome in a pair.
What are homologous chromosomes?
Chromosomes occur in pairs in diploid organisms- two loci that carry one allele of a gene- two alleles in total- one from mother and one from father.
Name and describe the two types of allele combinations.
- Homozygous- allele on each chromosome of the diploid organism is the same- carries two copies of the same allele at the specific locus.
- Heterozygous- allele on each chromosome of the diploid organism is different- carries two different alleles for a gene at a specific locus. Usually only one gene shows itself in the phenotype.
Name and describe the two idfferent types of alleles.
- Dominant- the allele of the heterozygote that is always expressed in the phenotype even when there’s only one copy- shown by capital letters.
- Recessive- the allele in a heterozygote that is not expressed- only appears in the phenotype if two copies present- shown by lower case letters.
Name and describe the possible ararngements of alleles.
- Homozygous dominant- homozygous organism with two dominant alleles.
- Homozygous recessive- homozygous organism with two recessive alleles. The only time the recessive allele is expressed in the diploid organism.
- Codominant-** two alleles both expressed in the phenotype** as neither one is recessive. When both alleles occur the alleles either blend together (e.g. roan coat from red and white hairs) or both features are represented (e.g. both A and B antigens in blood group AB).
Describe the meaning of multiple alleles.
- Gene with more than two allele forms.
- As there are always only two chromosomes in a homologous pair, only two out of the three or more alleles can be present in one organism. E.g. ABO blood group.
Draw a diagram showing how the terms for inheritance interlink.
Answer on revision card.
Describe how fertilisation works with inheritance.
- Two alleles for each gene.
Gametes only contain one allele for each gene- haploid. When haploid gametes fuse together- alleles they contain form the genotype of diploid offspring produced.
What are genetic diagrams?
Diagrams that predict genotypes and phenotypes of offspring when two parents are crossed.
What is monohybrid inheritence?
Inheritance of characteristic controlled by a single gene e.g. colour of pea pods.
What are monohybrid crosses?
Show the likelihood of different alleles of a gene (and so different versions of the characteristic) being inherited by offspring of certain parents in monohybrid inheritance. Crosses may be used instead of punnet squares so know how to use both.
How should you perform a monhybrid cross?
Hint: 7 steps
- The question usually gives the symbols- use a single letter to represent each characteristic. If doesn’t give you symbol use the first letter of each one of the different features, but try to choose one where the upper and lower case are distinguishable.
- Upper case- represents dominant feature. Lower case- represents recessive feature. Never use two different letters where one character is dominant. The upper case always goes first.
- Make it clear what the letters mean first. Use appropriate letters to make dominant/ recessive alleles clear- avoid s, w, u, o, p, z, x, c, v.
- Represent parents’ genotypes and phenotypes with appropriate letters- label them as parents.
- State the gametes- label them as gametes and circle them.
- Use a Punnett square to show the results of the random crossing. Label male and female gametes, keeping them encircled. May also need to use genetic cross- write out all the possible combinations in the bottom boxes.
- State the phenotype of each genotype and indicate the number of each type.
What does pure breeding mean?
If organisms with the same characteristic are bred repeatedly with another so that they consistently produce the same feature. They are homozygous for the allele that produces the characteristic.
Describe what F1 means and how it results in the monohybrid cross ratio?
- If a pure breeding dominant is bred with a pure breeding recessive the offspring are known as the first filial or F1 generation. All the offspring will express the dominant allele as they will all be heterozygous.
- When the heterozygous organisms from the F1 generation are crossed the offspring known as the second filial- F2- generation are always in the ratio 3:1 for monohybrid inheritence where the 3 is dominant and the 1 is recessive.
- The larger the number of offspring the more likely the ratio will reach 3:1.
What is the basic law of genetics?
In diploid organisms characteristics are determined by alleles that occur in pairs. Only one of each pair of alleles can be present in a single gamete.
What are phenotype ratios?
Ratio of different phenotypes in the offspring. Genetic diagrams allow you to predict the phenotypic ratios in F1 and F2 generations
What is the phenotypic ratio for a genetic cross.
Two heterozygous parents in monohybrid cross- F2 generation- 3:1 ratio of dominant : recessive characteristics.
When may the expected phenotypic ratio not occur for monohybrid crosses?
In codominant alleles and sex linkage- alter phenotypic ratios in monohybrid crosses.
Perform a monohybrid cross for a homozygous recessive and homozygous dominant pea plant. Green is dominant and yellow is recessive.
Answer on revision card.
How are ratios often written.
n:1 format- divide the left-hand value by the right hand value to find n.
Why are expected ratios in genetic crosses rarely achieved?
- **Fusion of gametes/ fertilisation of gametes- random- **due to chance- statistical error and likely to not reach the ratio.
- The larger the sample, the more likely the results match the ratio- small sample size= not likely. Large samples- representative results.
- Ratios may also not occur due to linked genes, epistasis, selection advantage or lethal genotypes/alleles.
When explaining the type of allele/genetic cross, what should you remember?
**Use examples of the phenotype/ genotype and state what alleles they have/ should have and why. **
What must you be able to understand with regards to genetic diagrams?
- You need to be able to use fully labelled genetic diagrams to interpret, or predict, the results of a cross- this could be a punnet square or a genetic cross (see below for genetic cross).
- You need to then be able to calculate the phenotypic ratios in monohybrid and dihybrid crosses or probabilities.
- You need to understand probabilities associated with inheritance and the phenotypic ratios of certain F2 (second filial- parents both heterozygous) generations.
Draw a genetic cross for a homozygous dominant and homozygous recessive pair of bats with normal wings (dominant) and vestigal wings (recessive)
Answer on revision card.
What is dihybrid inheritence?
- Inheritance of two characteristics determined by two different genes located on different chromosomes.
- Each of the two genes has different alleles.
What are dihybrid crosses?
Diagrams show the likelihood of offspring inheriting combinations of two characteristics from particular parents in dihybrid inheritance.
What are the features of pure-breeding in dihybrid inheritence?
Both dominant alleles and both recessive alleles.
What is the F1 generation in dihybrid inheritence?
All show dominant features (round, green coloured seeds)
What is the phenotypic ratio of dihybrid crosses.
9:3:3:1
Describe how the phenotypic ratio for dihybrid inheritance is derived/
- F1 generation- 4 types of gamete- RG, Rg, rG, rg. Genes are on separate chromosomes and as the chromosomes randomly arrange themselves on the equator during meiosis, any one of the two alleles of the gene for seed colour combine with any one other gene for seed shape.
- Law of independent assortment- each member of a pair of alleles may combine randomly with either of another pair.
- Fertilisation is random- so any of the four types of gamete can combine with any four types of gamete from the other plant.
- F2 generation- both heterozygous parents- has a theoretical phenotypical ratio of 9:3:3:1 (dominant both: dominant first, recessive second: recessive first, dominant second: recessive both) however, statistical error should be allowed for.
Why may there be a change in the phenotypic ratio of dihybrid alleles.
- Dihybrid crosses can occur with codominant alleles which work in the same way but will produce different phenotypic rations- more than 4 possible phenotypes.
- Changes to the phenotypic ratio could also be due to linkage or epistasis.
Perform a dihybrid cross where the two parents are heterozygous for both features. Round, green= dominant. Wrinkled, yellow= recessive.
Answer on revision card.
What is codominance?
- Both alleles are expressed in the phenotype.
- Occurs where both alleles are equally dominant and neither is recessive instead of one allele being dominant and one being recessive.
- This means both alleles are expressed in the phenotype.
- May be an explaination for a change in a phenotypic ratio.
- E.g.- shorthorn cattle:
- o One allele codes for an enzyme that catalyses the formation of red pigment.
- o The other allele codes for another enzyme- lacks catalytic activity- doesn’t produce pigment- hairs are white.
- o The alleles are not dominant or recessive so there are not just red and white coats- they are codominant so three coat colours are found.
- o Homozygous for red pigment enzyme- both alleles code for enzyme and therefore the red pigment is produced- red coat.
- o Homozygous for enzyme that won’t catalyse- no pigment produced- white hairs.
- o Heterozygous- red pigment enzyme and enzyme that won’t catalyse produced so both white and red hairs produced- roan.
What phenotypic ratio does a monohybrid cross with two heterozygous parents with codominant alleles produce?
1:2:1
How do you denote codominance?
- Codominance- can’t use upper and lower case letters so use different letters but put them as superscripts.
- E.g. coat colour- CR= red pigment, CW= no pigment.
Perform a cross of two parents who are homozygous white and homozygous red, where the alleles are codominant and a combination of white and red produces a roan coat.
Answer on revision card.
What is meant by multiple alleles?
A gene that has more than two alleles- only two may be present at the loci of an individual’s homologous chromosomes as there are only two homologous chromosomes and therefore two gene loci.
Describe how human blood groups work with multiple alleles.
- Human blood groups- ABO- three alleles associated with the gene I (immunoglobin gene)- different antigens produced on the cell surface membrane of red blood cells.
- IA- produces antigen A.
- IB- produces antigen B.
- IO- doesn’t produce either antigen.
- IA and IB are codominant. IO recessive to both.
Describe sex-linked chromosomes.
- Humans- 23 pairs of chromosomes. 22 pairs have identical homologous chromosomes. One pair- sex chromosomes.
- Sex chromosomes- carry genetic information for biological sex.
- Females- two X chromosomes- XX- same chromosome. All the gametes contain X chromosomes so share same genes (but can be different alleles.
- Males- single X chromosome but also a Y chromosome which is smaller and shaped differently- XY. Two different types of gametes- half have X chromosome and half have Y chromosome.
- There is a 50% chance of an organism being female (XX) or male (XY)
What are sex linked alleles and describe how this works.
- X chromosome- longer than Y chromosome- carries more genes. For most of the length of the X chromosome there is no equivalent homologous part of the Y chromosome- the X chromosome carries most of the genes in the sex chromosomes.
- Males have only one X chromosome so they often have only one allele for sex-linked genes.
- This means characteristics controlled by recessive alleles on the non-homologous portion of the X chromosome appear more frequently in males, as there is no homologous portion on the Y chromosome that might have the dominant allele. Males need only one recessive allele is needed in the X chromosome for it to be expressed.
- Females need two copies of the recessive alleles to have the feature whereas males need only one copy. This means features are much rarer in woman than men.
How are sex linked features denoted?
Sex linked features are written as superscript above the X chromosome as they are linked to the X chromosome. There is no equivalent allele on the Y chromosome so no superscript is written above it. E.g. XHXH or XHY.
Where do males get their X chromosomes from.
As males can only obtain their Y chromosome from the father, X chromosomes come from the mother.
Describe X-linked genetic disorders and where they come from.
- X-linked genetic disorder- disorder caused by faulty alleles on the X chromosome. Usually caused by mutation in the eggs. In males, always inherited from the mother as it is linked to the X chromosome. (Can also have Y-linked disorders but less common).
- If the mother does not have the disease, they may be heterozygous for the character e.g. XHXh and therefore may be carriers- they carry one copy of the allele- don’t show signs of the disease- not expressed in the phenotype but can be passed onto the offspring.
What should be noted with regards to sex-linkage and phenotypic ratios.
- The ratio of male to female with the characteristic may be different to the ratio of offspring with the characteristic or the ratio of a specific sex (male or female) with the characteristic- make sure to narrow down the categories.
- The ratio of the offspring a female carrier XNXn to a male with the disease XnY is 1:1- same for each sex- this is the ratio of a monohybrid F2 cross with a sex-linked characteristic.
Describe haemophillia and sex-linkage.
- Blood clots very slowly so leads to internal bleeding.
- Mainly occurs in males as haemophiliac females die due to menstruation at puberty.
- Haemophilia is caused by a recessive allele which codes for a faulty protein which doesn’t function to clot blood. (Haemophilia can now be treated by giving sufferer the functional protein).
- The allele for the protein is linked to the X chromosome and there is no equivalent allele on the Y chromosome to code for the clotting proteins.
- The alleles from the mother are therefore XH- dominant where the clotting protein is coded for or Xh- recessive where the clotting protein is faulty.
- As the allele is linked to the X chromosome, males can only inherit the disease from their mother as they have to inherit a Y chromosome from their father and the Y chromosome does not carry the allele for the clotting protein.
- If the mother doesn’t carry the disease she may be a carrier- XHXh. The mother doesn’t show any signs of the disease because the carriers possess one dominant H allele- this leads to the production of enough functional clotting protein.
- Males cannot pass haemophilia to their sons due to the Y chromosome but they can pass it to their daughters via the X chromosome- they may then become a carrier.
Describe what pedigree charts are and how they work.
- Show the inheritance of sex-linked characteristics.
- Male usually represented by a square.
- Female represented by a circle .
- Shading indicates presence of a character e.g. haemophilia in the phenotype.
Describe how linkage works.
- Each chromosome possesses many different genes.
- Any two genes occurring on the same chromosome are linked (not on the same homologous pair).
- All the genes on a single chromosome form a linkage group.
- If genes occur on separate chromosomes they are not linked.
What are autosomes.
Chromosomes that are not sex chromosomes- 22 in humans.
What are autosomal genes?
Genes located on the autosome.
What is autosomal linkage?
- Where two or more genes are carried on the same autosome.
- If two genes are autosomal linked and there is no crossing over there are only two possible combinations of gametes- usually AB and ab (dominant and recessive paired together), opposed to a dihybrid cross where there could be AB, Ab, aB and ab.
How does autosomal linkage work with crossing over.
- Assuming there is no crossing over, the linked genes remain together during meiosis I and so pass to the gametes and offspring together. They do not segregate in line with Mendel’s Law of Independent Assortment.
- The closer together two genes are on the autosome the more closely they are said to be linked- crossing over is less likely to split them up.
What is the effect of autosomal linkage on the genotype of offspring.
- ,akes it more likely the offspring will have their parent’s genotype and phenotype (e.g. heterozygous if two heterozygous parents are crossed).
- Look out for this being a cause of certain genotypes being more common in offspring than others.
- The others will be due to crossing over (make sure to name which genes are linked/crossed).
How does autosomal linkage alter phenotypic ratios?
The autosomal linkage will not follow the 9:3:3:1 phenotypic ratio and will be closer to the monohybrid ratio of 3:1 because they are inherited together.
Draw an example cross for two parents with autosomally GgNn genotypes.
Answer on revision card.
What is epistasis?
- Epistasis- when the allele of one gene affects or masks the expression of another in the phenotype.
- Many genes control the same characteristic- interact to form the phenotype.
When does epistasis occur?
- Epistasis may mask genes for enzymes resulting in e.g. one pigment forming but not another, or may mask the effects of a pigment/ shape by changing the patterning e.g. baldness over a certain shape of hair.
- Forms of epistasis may occur where genes act in a sequence by determining the enzymes in a biochemical pathway- one gene produces a non-functional enzyme- may affect the other gene as if gene produces the functional enzyme it still will not be expressed as the substrate was not made by the enzyme before it e.g. in the production of a pigment in plants.
What is helpful to do when performing an epistatic genetic cross.
List the genotypes and the phenotypes alleles produce before starting a genetic cross to avoid confusion.
Describe how melanin distribution in mice is determined by epistasis and draw an example cross.
- Gene A- controls distribution of melanin- black pigment in hairs. Dominant allele A leads to hairs that have black bands. Recessive allele a- produces uniform black hairs.
- Gene B- controls coat colour by determining the expression of gene A. Dominant allele B- produces melanin. Recessive allele b- no pigment produced- hair white.
- Agouti mice- have hairs with black bands.
- Black mice- uniform black hairs.
- Albino mice- white- lack melanin.
- If gene B is in the form bb then no melanin is produced and the coat is albino, as in the absence of melanin gene A cannot be expressed so regardless of which alleles are present in A it will always be albino.
- If dominant allele B is present melanin is produced.
- If B is present with dominant allele A banding occurs creating an agouti coat.
- If B is present with two recessive A alleles- aa- the hairs and coat are uniform black.
- Example cross on revision guide.
What is the F1 generation for epistatic crosses.
Pure breeding dominant- AABB, crossed with pure breeding recessive- aabb- F1 generation produced- all AaBb.
When can the Chi squared test be used?
- The sample is relatively large- over 20.
- Data falls into discrete categories- used for categorical data.
- Raw counts and not percentages/ rates/ other values are used.
Describe the ratios of recessive epistatic alleles.
- Crossing doesn’t result in the expected phenotypic ratios- wouldn’t get 9:3:3:1 like in a dihybrid cross. Phenotypic ratio expected from dihybrid cross depends on whether epistatic allele is recessive or dominant.
- Recessive epistatic alleles- two copies mask the expression of the other gene. Crossing homozygous recessive parent (aabb) with a homozygous dominant parent (AABB) produces 9:3:4 phenotypic ratio (dominant both: dominant epistatic, recessive other: recessive epistatic) in the F2 generation.
- Dominant epistatic allele- having at least one copy will mask the expression of the other gene. Crossing a homozygous recessive parent with a homozygous dominant parent will produced a 12:3:1 phenotypic ratio (dominant epistatic: recessive epistatic, dominant other: recessive both) in the F2 generation.
What is the Chi squared test?
- Tests if difference in frequencies is due to chance or whether there is a bias.
- Used to test the null hypothesis- assumption there is no statistically significant different between observations with any difference being due to chance.
- Chi squared- tests (X2)- tests whether deviation between observed and expected numbers in an investigation is significant or not.
How does the Chi squared test work?
- Statistical test is used to see if the results of an experiment supports a theory- for example testing expected Mendelian ratios.
- The theory is used to predict a result- expected result. Experiment carried out and actual result recorded- observed result.
- To see if results support theory- make a hypothesis- the null hypothesis- always there is no significant difference between the observed and expected values.
- Experiment results- different to expected but need to test if the difference is due to chance or because the theory is wrong.
What is the Chi squared test used for?
- Chi squared test- carried out to compare the goodness of fit of the observed and expected results- how well the observed match the expected- outcome either supports or rejects the null hypothesis.
- You may need to use the chi-squared ( Χ2 ) test to compare the goodness of fit of observed phenotypic ratios with expected ratios.
- You may need to use the Χ2 test to investigate the significance of differences between expected and observed phenotypic ratios.
How do you perform the Chi squared test?
- Test used to test the theory of the alleles within the species- e.g. if the allele is monohybrid, if one parent is homozygous dominant etc.
- Null hypothesis- no significant difference between the observed and expected results.
- Expected results- phenotypic ratio/ ratio of genetic cross.
- Observed results- experiment carried out and numbers with different phenotypes counted.
- Chi squared test- chi squared value is calculated and compared to the critical value. If the test shows the observed and expected results are not significantly different- unable to reject the null hypothesis- data supports theory.
- Use the formula (make sure to know how to plug in values) to calculate the chi squared value.
- The value is then read off the chi squared distribution table to determine whether the deviation from the expected results is significant or not.
- The significance is calculated by finding the degrees of freedom- the number of classes minus one e.g. coin toss- heads or tails- 2 classes so the degrees of freedom is 1.
- Then you look along the row for the degrees of freedom to find the calculated value (will probably lie between two values). This will then show the probability value. (p-value) or the range the p-value is in.
How can you use the degrees of freedom and critical value to find the value of chi squared.
- Find the degrees of freedom and the critical value at the 0.05 level by matching the row and column. Then you could compare the chi squared value to this:
- X2 greater than or equal to the critical value- significant difference between the observed and expected results- something other than chance is causing the difference- reject the null hypothesis. There is a less than 5% probability the results are due to chance.
- X2 smaller than the critical value- no significant difference between the observed and expected results- the null hypothesis cant be rejected. There is a greater than 5% probability the results are due to chance.
Give examples of experiments tested with Chi Squared.
Experiments to be tested could involve genetic ratios using crosses of Drosophila or Fast Plant.
