There is a rat species whose fur color can be brown, black, or white. Enzyme A (
ID: 94683 • Letter: T
Question
There is a rat species whose fur color can be brown, black, or white. Enzyme A (encoded by gene "a") is required to convert a brown pigment into a black pigment. The deposition of pigment onto the fur is controlled by a pathway that is normally inhibited by Protein B (encoded by gene "b"). When no pigment is deposited the fur looks white. The dominant alleles A and B specify functional protein, whereas the recessive mutant alleles a and b specify no protein.
a. What genotypes give rise to brown, black, and white fur?
b. You cross pure-breeding white rates to pure-breeding brown rats. What ratio of phenotypes do you see in the F1s?
c. In a dihybrid cross (Aa Bb x Aa Bb), what ratio of phenotypes do you expect?
Explanation / Answer
Ans A) Enzyme A encoded by “gene a” convert the Brown fur of the rats to the Black fur. Protein B encoded by gene B inhibit the activity of enzyme A and the fur colour becomes “White”. The genotypes of fur colours are explained below:-
The genotype for Black fur is “AAbb”. This is because enzyme A is encode by gene A which is dominant ( as given in the question.)
The genotype for the Brown fur is “AaBb”.
The genotype for white fur is “aaBB”. This is because the gene B encodes the protein B which inhibit the activity of Enzyme A. It is given that the dominant alleles A and B specify functional proteins.
Ans B) The genotype of the pure breeding white rats are “aaBB” and genotypes of the pure breeding Brown rats “AaBb” and when they are crossed for F1 generation then the phenotypes are given here:-
Parental Generation
Pure White rats (aaBB)
Pure Brown rats (AaBb)
Gametes aB × AB, Ab, aB, ab
Now F1 generation
AB
Ab
aB
ab
aB
AaBB (Brown)
AaBb (Brown)
aaBB (white)
aaBb (white)
The phenotype for the F1 generation are 2 ( Brown) : 2 (White)
Or 1:1 ratio of brown and white
Ans C) The dihybrid cross (Aa Bb x Aa Bb), the ratio of phenotypes are given below
The gametes for “Aa Bb” are “AB, Ab, aB, ab”
Now the dihybrid cross (Aa Bb x Aa Bb), is given below
AB
Ab
aB
ab
AB
AABB (Brown)
AABb (Brown)
AaBB (Brown)
AaBb (Brown)
Ab
AABb (Brown)
AAbb (Black)
AaBb (Brown)
Aabb (Black)
aB
AaBB (Brown)
AaBb (Brown)
aaBB (white)
aaBb (white)
ab
AaBb(Brown)
Aabb (Black)
aaBb (white)
aabb (Black)
Now the phenotypes of the F2 generations are:-
9 (Brown) : 3 (Black) : 4 (Black)
Parental Generation
Pure White rats (aaBB)
Pure Brown rats (AaBb)
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.