Question
Pigment in mouse fur is only produced when the C allele is present and cc mice are white. If pigment is produced, the mice can be either agouti or black. Agouti mice are AA or Aa and black mice are aa.
If we start with a cross between AACC and aacc mice, what are the expected genotypic and phenotypic ratios in the F1 progeny.
If we cross F1 mice with each other, what are the expected genotypic and phenotypic ratios in the F2.
In three crosses between different agouti females and aacc males, the following phenotypic ratios were obtained:
Cross 1: ½ agouti, ½ white
Cross 2: ½ agouti, ½ black
Cross 3: ¼ agouti, ¼ black, ½ white.
What are the genotypes of the agouti females parents in the three crosses?
Explanation / Answer
OK, for the first part, if we cross AACC and aacc (each individual is homozygous for both traits), all of the F1 generation will be exactly the same. The only alleles the first individual can contribute are AC and the only alleles the other individual can contribute are ac. Using that, you can figure out what the genotype and phenotype will be. If we take those hybrids and cross them to each other (a dihybrid cross), the ratio can best be figured out using a Punnett square. You have to figure out what the allele combinations are for each gamete. The FOIL (first, outer, inner, last) method works well for this. So, our F1 generation has the AaCc genotype. Using the FOIL, the possible gametes are AC (first ones of each pair), Ac (outer of each), aC (inner of each pair) and ac (last of each pair). It's the same for both parents. Then you just have to fill it in. Finally, I suggest going down each column and, on a separate sheet, write down each possible outcome and keep a tally. You then use the tally to come up with the genotype ratio. Once you have the genotype ratio, you can look at each. You know that if there if it is A-C- you will have agouti. aaC- gives you black and --cc gives you white. You can use that to figure out what your phenotypic ratios are for your F2 generation. In the last crosses, a little logic goes a long way. If the father is aacc, that means he can only contribute an ac gamete. If the mother is agouti she has to have one dominant C allele and one dominant A allele. So, she can only be one of these: AACC, AaCC, AACc or AaCc. For cross 1: The results have 1/2 agouti and 1/2 white. The only way for white to happen is if the offspring have --cc in their genotype. The only way you can get two cc is for one to come from the father and one from the mother. That narrows it down really fast to the right genotype for the female parent (without also having black offspring) Cross 2 has 1/2 agouti and 1/2 black. Black only happens with aaC-. Same as the first one, the only way to get two recessive a alleles is to get one from each parent. Easy to pick which female parent has a recessive a allele that won't also give you white offspring. Cross 3 has 1/4 agouti, 1/4 black and 1/2 white. Since you know black is aaC- and white is --cc, you know the mother must have one recessive allele of each. You also know she is agouti, so she must also have one dominant A allele and one dominant C allele. Easiest one of the bunch. Good luck!!