7.1: Epistasis

Question 1

What is epistasis?

Answer 1

Epistasis is when the phenotypic expression of one gene is affected by a different gene.

Aka multiple genes affect one character.

Question 2

What is the result of epistasis?

Answer 2

Epistatic interactions produce a modified mendelian ratio of phenotypes in offspring.

Question 3

What are the key types of epistasis and the ratios they produce (under a double heterozygous cross)?

Answer 3

Complementary genes
–> 9:7

Dominant epistasis
–> 12:3:1

Recessive epistasis
–> 9:3:4

BOTH dominant AND recessive epistasis
–> 13:3

Duplicate genes
–> 15:1

(NO INTERACTION: 9:3:3:1)

Question 4

What is pleiotropy?

Answer 4

Pleiotropy is when one gene affects multiple characters.

Question 5

Dominance vs epistasis?

Answer 5

Dominance describes relationship of alleles

Epistasis describes relationship of genes

Question 6

Types of dominance?

Answer 6

Complete: recessive allele doesn’t effect expression of dominant: heterozygous and homozygous dominant expressed the same.

Incomplete: heterozygous a blending intermediate of homozy dom and homozy recessive types: e.g. pink flower

Co-dominance: het. a patchwork of both phenotypes separately visible at once: e.g. splotchy white and red flower. e.g. AB blood.

Question 7

How does epistasis work for complementary genes?

Answer 7

When genes are complementary, two genes work together for a phenotypic effect.

• -> Must have at least one dominant allele for BOTH genes to be purple.
• -> 9:7 ratio.

Question 8

How does dominant epistasis work?

Answer 8

A dominant allele at one locus prevents expression of an allelic difference at another locus.

Question 9

How does recessive epistasis work?

Answer 9

in recessive epistasis a homozygous recessive gene at one locus prevents expression of an allelic difference at 2nd locus.

Question 9

How does recessive epistasis work?

Answer 9

in recessive epistasis a homozygous recessive gene at one locus prevents expression of an allelic difference at 2nd locus.

Question 10

Recessive vs dominant epistasis?

Answer 10

Both recessive and dominant epistasis have the same effect but different genotypes responsible.

Dominant: 12:3:1 ratio
Recessive: 9:3:4 ratio

Question 11

How does having both dominant and recessive epistasis work?

Answer 11

In dominant and recessive epistasis you might have a dominant blocking an expression but a homozy recessive allowing it so you need right combo of both for expression.
–> 13:3 ratio

Question 12

How does epistasis work for duplicate genes?

Answer 12

For duplicate genes you only need one dominant allele of either gene to express the trait, but must be double homozygous to display recessive trait.