20.4

Question 1

Describe and explain autosomal linkage

Answer 1

Autosome is the name for any chromosome that isn’t a sex chromosome.
Autosomal genes are the genes located on the autosome

Genes on the same autosome are said to be linked - that’s because they’ll stay together during the independent assortment of chromosome in meiosis 1 and their alleles will be passed on the offspring together

The closer together two genes are on the autosome, the more closely they are said to be linked, because crossing over is less likely to split them up

Question 2

What is the phenotypic ratio for autosomal linkage not as we expect?

Answer 2

In a genetic cross between two heterozygous parents, you’d expect a 9:3:3:1 ratio in the offspring. Instead the phenotypic ratio is 3:1 because the two autosomally linked alleles are inherited together, this means a higher proportion of the offspring will have their parents’ (heterozygous) genotype and phenotype (less likely to be recombinant offspring- different combinations of allele than parent)

Question 3

Recombinant frequency

Answer 3

A measure of the amount of crossing over that has happened in meiosis

Recombinant frequency = number of recombinant offspring / total number of offspring

Recombinant frequency of 50% indicated that there is no linkage and the genes are on separate chromosomes
Less than 50% indicated that there is gene linkage and the random process of independent assortment has been hindered

Question 4

Define and explain epistasis?

Answer 4

Many different genes can control the same characteristic — they interact to form the phenotype.
Epistasis is when the allele of one gene masks (blocks) the expression of other genes.

E.g. in humans a widows peak is controlled by one gene and baldness by others. If you have both genes, the baldness genes are epistatic to the widows peak gene, as the baldness gene masks the widows peak gene

Question 5

Recessive epistatic genes

Answer 5

If the epistatic gene is recessive then two copies of it will mask (block) the expression of the other gene

If you cross a homozygous recessive parent with a homozygous dominant parent you will produce a 9:3:4 phenotypic ratio of dominant both: dominant epistatic , recessive other: recessive epistatic in the F2 generation

Question 6

Dominant epistatic alleles

Answer 6

If the epistatic allele is dominant, then having at least one copy of it will mask the expression of the gene.

Crossing over a homozygous recessive parent with a homozygous dominant parent will produce a 12:3:1 phenotypic ratio of dominant epistatic : recessive epistatic, dominant other: recessive both in the F2 generation

Question 7

Chi squared test

Answer 7

A statistical test that measures the size of difference between the results you actually get (observe) and those you expected to get. Helped you determine whether the differences in the expected and observed results are significant or not

• you can accept or reject the null hypothesis (if the difference between the observed and expected results are significant you can reject the null hypothesis)

Chi squared = sum of ((O—E)^2 /E)

To find out whether there is a significant difference between your observed and expected results you need to compare your chi squared value to a critical value (5%/ 0.05 level of probability that the difference is due to chance)

Degrees of freedom: n - 1

If chi squared value is more than or equal to the critical value = reject null hypothesis (there is a significant difference, differences are due to another factor)
If chi squared value is less than critical value = accept null hypothesis (these is no significant difference, difference is due to chance)