5. Epistasis

Question 1

What do linkage / epistasis affect ?

Answer 1

The phenotypic ratios of dihybrid crosses

Question 2

Epistasis

Answer 2

When many different genes control the same characteristic they interact to form phenotype.

This can be because the allele of one gene masks expression of alleles of other gene.

Question 3

Example

Widows peak

Answer 3

Human widows peak is controlled by one gene and baldness by others.

If you have allele that codes for baldness, it doesn’t matter whether you have the allele for windows peak or not as you have no hair.

Baldness gene is epistasis to widows peak gene as baldness gene masks the expression of widows peak gene

Question 4

Example

Flower colour

Answer 4

Flower pigment controlled by 2 genes.

Gene1 codes for yellow Y
Gene2 codes for an enzyme that turns yellow pigment orange R

If you don’t have Y allele it won’t matter if you have the R allele or not as flower will be colourness.

Gene1 is epistasis to gene 2 as it make the expression of gene2

Question 5

Where are epistatic genes located?

Answer 5

On different loci - different positions on chromosomes

Question 6

What is the expected rate you would get from a dihybrid cross involving epistatic allele?

Answer 6

Depends if allele is recessive or epistatic.

Question 7

Recessive epistatic alleles

Answer 7

If epistatic allele is recessive then 2 copies of it will mask expression of other gene.

If you cross homozygous recessive (yy) patents will homozygous dominant (YY) you will produce 9:3:4 phenotypic ratio

Question 8

Dominant epistatic allele

Answer 8

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

Cross a homozygous recessive (tt) with homozygous dominant (TT) it will produce a 12 : 3 : 1 phenotypic ratio