Genetics: Uncovering the Genotype of Black Mouse with Tan Parent

[Dooga Blackrazor]

In mice, the gene C causes pigment to be produced, while the recessive gene c makes it impossible to produce pigment (albino). Another gene, B, on a different chromosome, causes a chemical reaction with the pigment and produces a black coat colour. The recessive gene, b, causes an incomplete breakdown of the pigment, and a tan colour results. The genes that produce black or tan coat colour rely on the gene C, which produces pigment, but are independent of it.

The mating of a tan mouse and a black mouse produces many different offspring. One of the offspring is albino.

The tan mouse is Ccbb
How do you know the genotype of the black mouse?

I know it's CcB? but I don't know how the ? allele is found.

CcBB will produce CB, CB, cB, CB gametes and and can cross to give albino offspring that are ccBc and albino.

CcBb can give offspring that are albino as ccbb.

My textbook says the black mouse is CcBb since an allele must be taken from both parents. That isn't any justification for why that's the answer is it?

 

[kalladin]

If the tan mouse is Ccbb, then the cross must be Ccbb x CcB-.
The genotype of the tan mouse is a given, and we know that the genotype of a black mouse must be C-B-. Since the tan mouse is Ccbb, the only possible genotype for the black mouse is CcB- in order to have albino (cc--) progeny. I don't know what the genotype for the albino mouse is supposed to be, but from the information you give it's cc--, so the problem really ends at Ccbb x CcB-. Assuming that the albino genotype is ccbb, then the cross must be Ccbb x CcBb.

 

[Dooga Blackrazor]

That's what I thought, but the albino genotype can also be ccBb or ccBB. The answer might be to use a test cross but that isn't what the book is saying.

 

[Dooga Blackrazor]

For those wondering: I discerned that the mystery allele must be "b" because it said many different offspring resulted. Since there are 3 possible offsprings and 2 isn't usually considered many, I assumed it wanted a genotype combination which would result in all 3 offspring being possible.

 

[Moonbear]

kalladin has properly deduced how you determine that the black mouse has the genotype Cc for the allele associated with albinism.

I think you're right that the answer depends on the word "many" since a black mouse with the genotype CcBB crossed with a tan mouse of genotype Ccbb would only produce black and albino offspring. Without the use of the word "many," you would have no way to determine the genotype of the Black mouse for the B gene, other than that one allele must be B and the other could be B or b.