Topic 7—A: Genetics- 5. Epistasis

Question 1

What does epistasis effect?

Answer 1

The phenotypic ratios of dihybrid crosses

Question 2

Definition of epistasis

Answer 2

• when an allele of one gene masks (blocks) the expression of the alleles of other genes

Question 3

Why can many different genes control the same characteristic- they interact to form the phenotype?

Answer 3

They are epistasis

Question 4

Example (epistasis) - widows peak

Answer 4

• in humans a widows peak is controlled by one gene and baldness by others
• if you have the alleles that code for baldness it doesn’t matter whether you have the allele for a widows peak or not as you have no hair
• the baldness genes are epistatic to the widows peak gene as the baldness genes mask the expression of the widows peak gene

Question 5

Example (epistasis)- flower colour

Answer 5

• flower pigment in a plant is controlled by two genes
• gene 1 codes for a yellow pigment (Y is the dominant yellow allele)
• Gene 2 codes for an enzyme that turns the yellow pigment orange (R is the dominant orange allele)
• If you don’t have the Y allele it wont matter if you have the R allele or not as the flower will he colourless
• Gene 1 is epistatic to gene 2 as it than mask the expression of gene 2

Question 6

Does crossing over epistatic genes result in expected phenotypic ratios?

Answer 6

No
- the phenotypic ratio you would expect to get from a dihybrid cross involving an epistasis allele depends on whether the epistasis allele is recessive or dominant

Question 7

Recessive epistatic alleles

Answer 7

• If the epistatic allele 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 8

Dominant epistatic alleles

Answer 8

• if the epistatic allele is dominant then having at least one copy of it will mask (block) 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