3. Two black rabbits thought to be homozygous for coat colour were mated and produced a litter that contained all black babies. The F2 , however, resulted in some white babies, which meant that one of the grandparents was heterozygous for coat colour. How would you find out which grandparent was heterozygous.

Cross each to a rabbit who you know is homozygous - i.e. a white rabbit. If there are any white offspring you know this is the heterozygote.

Explanation:

Heterozygous individuals are those with 2 different alleles of a gene. Homozygous individuals have have 2 alleles that are the same.

Imagine the allele for coat color is B black, or b white. If you cross 2 individuals who are homozygous for the black gene (BB), they would only produce black rabbits in the F1 (BB). If you cross the F1, they could only produce BB rabbits in the F2.

However, if you cross a heterozygous rabbit with a homozygous black rabbit (Bb x BB), you would get either BB or Bb rabbits in the F1. However, intercrossing them could produce BB, Bb, or bb rabbits. Therefore, white rabbits can be produced.

You know that white rabbits are bb. So if you are unsure about the genotype of the black rabbit, you can cross it with a white rabbit (either BB x bb or Bb x bb). If any white rabbits appear in the F1, you know there must be a b allele in the black rabbit genotype, so that rabbit must be heterozygous.