Epistasis. In Labrador retrievers coat color is controlled by two loci each with

Question

Epistasis. In Labrador retrievers coat color is controlled by two loci each with two alleles: B, b and E, e. When pure breeding black labs with genotype BBEE are crossed with pure breeding yellow labs of genotype bbee the resulting F1 offspring all have black coat. When a test cross is done with the F1 using a double homozygous recessive individual the following phenotypic ratios are typically seen. 1 Black: 1 Chocolate: 2 Yellow a. List the alleles, genotypes and phenotypes. Which gene is epistatic? b. What phenotypic ratios would you then expect from a heterozygous cross (BbEe times BbEe)? X linkage a. Red/Green colorblindness is a sex-linked recessive trait. A son and father are colorblind. The mother has normal vision. Did the son inherit colorblindness from his father? b. Hemophilia is an X-linked recessive disorder. A normal woman has a father afflicted with hemophilia. She and a normal man have four children. Of these children two are male and two are female. Draw a pedigree assuming ideal proportions in the progeny. Indicate genotypes.

Explanation / Answer

Answer a.

Alleles: B (black), b (brown), E (epistatic), e (resessive- hypostatic)

Results of test cross between BbEe and double homogygous (bbee)

Alleles

Be

Be

be

be

bE

Genotypes

Phenotypes

BbEe

(Black)

BbEe

(Black)

bbEe

(Chocolate)

bbEe

(Chocolate)

bE

Genotypes

Phenotypes

BbEe

(Black)

BbEe

(Black)

bbEe

(Chocolate)

bbEe

(Chocolate)

Be

Genotypes

Phenotypes

BBee

(Yellow)

BBee

(Yellow)

Bbee

(Yellow)

Bbee

(Yellow)

be

Genotypes

Phenotypes

Bbee

(Yellow)

Bbee

(Yellow)

Bbee

(Yellow)

Bbee

(Yellow)

The gene ‘E’ is epistatic, as it will only allow the formation of coat color in homogygous dominant or heterozygous condition. In homozygous recessive (ee) condition, it will completely suppress the coat color and lead to the appeanace of yellow or golden color Labrador.

Answer b. Results of heterozygous cross

Alleles

BE

Be

bE

be

BE

Genotypes

Phenotypes

BBEE

(Black)

BBEe

(Black)

BbEE

(Black)

BbEe

(Black)

Be

Genotypes

Phenotypes

BBEe

(Black)

BBee

(Yellow)

BbEe

(Black)

Bbee

(Yellow)

bE

Genotypes

Phenotypes

BbEE

(Black)

BbEe

(Black)

bbEE

(Chocolate)

bbEe

(Chocolate)

be

Genotypes

Phenotypes

BbEe

(Black)

Bbee

(Yellow)

bbEe

(Chocolate)

bbee

(Yellow)

  Phenotypic ratio- 9 Black: 3 Chocolate: 4 yellow

Alleles

Be

Be

be

be

bE

Genotypes

Phenotypes

BbEe

(Black)

BbEe

(Black)

bbEe

(Chocolate)

bbEe

(Chocolate)

bE

Genotypes

Phenotypes

BbEe

(Black)

BbEe

(Black)

bbEe

(Chocolate)

bbEe

(Chocolate)

Be

Genotypes

Phenotypes

BBee

(Yellow)

BBee

(Yellow)

Bbee

(Yellow)

Bbee

(Yellow)

be

Genotypes

Phenotypes

Bbee

(Yellow)

Bbee

(Yellow)

Bbee

(Yellow)

Bbee

(Yellow)

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