G&E: Linkage and Epistasis

Question 1

Sex-linked characteristics

Answer 1

The allele that codes for it is located on a sex chromosome.

Question 2

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

Answer 2

• Most genes on sex chromosomes are only carried on the X chromosome.
• Males only have one X chromosome and so often only have one allele for sex-linked genes.
• As they only have one copy, they express the characteristic even if it is recessive.

Question 3

Why is colour blindness much rarer in women than men?

Answer 3

Sex-linked disorder.

Males only need one copy of the recessive allele to be colourblind, while females need two.

Question 4

Autosome

Answer 4

Chromosome that isn’t a sex chromosome.

Question 5

Autosomal linkage

Answer 5

When two genes are on the same chromosome so stay together during the independent segragation of chromosomes in meiosis I.

Question 6

Epistasis

Answer 6

When the allele of one gene mask the expression of the alleles of other genes.

Question 7

What is the phenotypic ratio of a dihiybrid cross involving a recessive epistatic allele?

2 homozygous parents

Answer 7

9:3:4

dominant both : dominant epistatic recessive other : recessive epistatic

Question 8

What is the phenotypic ratio of a dihydrib cross involving a dominant epistatic allele?

2 homozygous parents

Answer 8

12 : 3 : 1

dominant epistatic : recessive epistatic dominant other : recessive both