If a plant has a genotype $AaBb$ and undergoes self-pollination, what is the probability that the resulting endosperm will have the genotype $AAaBBb$?

Solution

1. The endosperm genotype $AAaBBb$ arises from the fusion of two polar nuclei ($AaBb$) and one male gamete ($AB$).
2. The probability of a male gamete being $AB$ is $1/4$.
3. The probability of two polar nuclei being $AaBb$ is $(1/2)\times (1/2)=1/4$.
4. The total probability is $(1/4)\times (1/4)\times (1/4)=1/64$.
5. Hence the answer is (B).