A heterozygous gene is known as having two different alleles of a particular gene or genes. The correct answer is option A because both letters are different, versus the other two options where both are capital or lowercase.
There are two types of genotypes: heterozygous and homozygous. Homozygous consists of the SAME letters: BB and bb. Heterozygous consists both a dominant AND recessive gene therefore it would look like Bb. With this question one could infer answer A, Aa is the answer.
