Lecture 6: two gene inheritance patterns

Question 1

Law of Independent Assortment

Answer 1

Alleles of different genes segregate independently during sexual reproduction
The inheritance of an allele for one gene is unaffected by the inheritance of an allele at another gene locus

Question 2

How to do a test cross for two genes

Answer 2

The result of a test cross helps determine the unknown genotype of a phenotypically dominant individual
A_B_ x aabb
A test cross ratio of 1:1:1:1 phenotypic classes indicates the dominant individual is a double heterozygote (AaBb)
If test cross yields all dominant phenotype offspring the dominant parent is AABB
If test cross yields a 1:1 ratio the dominant parent is AABb or AaBB

Question 3

Epistasis is

Answer 3

Epistasis is when alleles of one gene mask the expression of alleles of another gene
Gene interaction that modifies the 9:3:3:1 ratio
Observed between the alleles of two genes that determine the coat colour in Labrador retriever dogs

Question 4

Epistasis and albinism

Answer 4

Albinism regularly occurs in many mammals and some other species
The white coat colour is generally the result of a recessive epistatic allele
When a dominant allele present at one locus masks expression of alleles of another gene
Dominant epistasis
12:3:1 phenotype ratio in F2 generation

Question 5

Grey horses

Answer 5

G_ _ _ - grey horse at maturity
gg_ _ - horse that is not grey at maturity
ggE_ - black horse
ggee – chestnut horse
F2 results (cross GgEe by GgEe):
12 grey: 3 black: 1 chestnut

Question 6

Labs with epistasis

Answer 6

Dominant allele B
black pigment → black
Recessive allele b
brown pigment → chocolate
Dominant allele E
needed to deposit pigment in hair shaft
Recessive allele e
pigment cannot be deposited → yellow
When recessive alleles present at one locus mask a different gene locus
Recessive epistasis
9:3:4 phenotype ratio in F2 generation
The ‘4’ encompasses the 3:1 classes of the 9:3:3:1 segregation ratio