7A - Linkage and epistasis

Question 1

When is a characteristic said to be sex-linked?

Answer 1

When the allele that codes for it is located on a sex-chromosome.

Question 2

What are examples of genetic disorders caused by faulty alleles on sex chromosomes?

Answer 2

Colour blindness and haemophilia.

Question 3

Why are males more likely than females to show recessive phenotypes for genes that are sex-linked?

Answer 3

Because males only have one X chromosome so they often only have one allele for sex-linked genes. Therefore, because they only have one copy, they express the characteristic of this allele even if it’s recessive.

Question 4

What is it called when the faulty alleles are carried on the X chromosome?

Answer 4

X-linked disorders.

Question 5

What is an autosome?

Answer 5

Any chromosomes that isn’t a sex chromosome.

Question 6

What are autosomal genes?

Answer 6

Genes located on the autosomes.

Question 7

Why are genes on the same autosome said to be linked?

Answer 7

Because they’re on the same autosome they’ll stay together during the independent segregation of chromosomes in meiosis I, and their alleles will be passed on to the offspring together. The only reason this won’t happen is if crossing over splits them up first.

Question 8

The closer together two genes are on the autosome…

Answer 8

…the more closely they are said to be linked.

Question 9

Why is it that the closer together two genes are on the autosome, the more closely they are said to be linked?

Answer 9

Because crossing over is less likely to split them up.

Question 10

What would you expect the phenotypic ratio to be like if two genes are autosomally linked?

Answer 10

You won’t get the ratio you expect in the offspring of a cross.

Question 11

How are the X and Y chromosomes different (structurally)?

Answer 11

Y chromosomes are smaller and carry fewer genes.

Question 12

What is an X-linked gene?

Answer 12

Most genes on the sex chromosomes are only carried on the X chromosome and this means they are X-linked genes.

Question 13

What are genetic disorders caused by?

Answer 13

Faulty alleles on sex chromosomes.

Question 14

Are males or females more likely to show recessive phenotypes for genes that are sex-linked?

Answer 14

Males

Question 15

Explain how colour blindness (a sex-linked disorder) becomes expressed in the phenotype.

Answer 15

1) Colour blindness is a sex-linked disorder caused by a faulty allele carried on the X-chromosome.
2) As it is sex-linked both the chromosome and the allele are represented in the genetic diagram, e.g. Xn, where X represents the X chromosome and n the faulty allele for colour vision.
3) The Y chromosomes doesn’t have an allele for colour vision so is just represented by Y.
4) Females would need two copies of the recessive allele to be colour blind, while males only need one copy. This means colour blindness is much rarer in women than men.

Question 16

What would the ratio be of not colour-blind to colour-blind if you crossed a carrier female and an unaffected male?

Answer 16

3:1 (normal to colour-blind).

Question 17

What would to ratio be of female offspring without colour-blindness:male offspring without colour-blindness:male offspring with colour-blindness if you crossed a carrier female and an unaffected male?

Answer 17

2:1:1 (female offspring without colour-blindness:male offspring without colour-blindness:male offspring with colour-blindness).

Question 18

What would the ratio be of offspring with colour-blindness:offspring without colour-blindness if you did a monohybrid cross between a carrier female and a colorblind male?

Answer 18

1:1 (with:without).

Question 19

What are the genes located on the autosomes called?

Answer 19

Autosomal genes.

Question 20

Why is it that if two genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring of a cross?

Answer 20

Because the two autosomally-linked alleles are inherited together meaning that a higher proportion of the offspring will have their parents’ genotype and phenotype.

Question 21

Why is it that if two genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring of a cross?

Use an example this time…

Answer 21

For example, in a dihybrid cross between two heterozygous parents you’d expect a 9:3:3:1 ratio in the offspring. Instead the phenotypic ratio is more likely to be that expected for a monohybrid cross between two heterozygous parents (3:1) (because the two autosomally-linked alleles are inherited together meaning that a higher proportion of the offspring will have their parents’ genotype and phenotype).

Question 22

What does the fact that if two genes are autosomally linked, you won’t get the phenotypic ratio you expect in the offspring of a cross because the two autosomally-linked alleles are inherited together meaning that a higher proportion of the offspring will have their parents’ genotype and phenotype allow us to do?

Answer 22

Allows us to use the predicted phenotypic ratio to identify autosomal linkage.

Question 23

What is a back cross?

Answer 23

Crossing the offspring with one of the parents.

Question 24

What is it called when the offspring is crossed with one of the parents?

Answer 24

A back cross.

Question 25

What does an epistatic gene do?

Answer 25

Masks the expression of another gene.

Question 26

What is epistasis?

Answer 26

When the allele of one gene masks (blocks) the expression of the alleles of the genes.

Question 27

What do crosses involving epistatic genes not result in?

Answer 27

The expected phenotypic ratios.

Question 28

Predict the ratio of a dihybrid cross involving a recessive epistatic allele

Answer 28

9:3:4 (dominant both:dominant epistatic recessive other:recessive epistatic).

Question 29

Explain why the ratio of a dihybrid cross involving a recessive epistatic allele is 9:3:4

Answer 29

Having two copies of the recessive epistatic allele masks (blocks) the expression of the other gene. If you cross a homozygous recessive parent with a homozygous dominant parent you will get a 9:3:4 phenotypic ratio of dominant both:dominant epistatic recessive other:recessive epistatic in the F2 generation.

Question 30

Predict the ratio of a dihybrid cross involving a dominant epistatic allele

Answer 30

12:3:1 (dominant epistatic:recessive epistatic dominant other:recessive both).

Question 31

Explain why the ratio of a dihybrid cross involving a dominant epistatic allele is 12:3:1

Answer 31

Having at least one copy of the dominant epistatic allele masks (blocks) the expression of the other gene. Crossing 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.