In horses, the allele for black coat color (E) results in production of a hair pigment called eumelanin and is dominant to the allele for chestnut (e). A horse breeder mates a black mare to a chestnut stallion and a chestnut offspring results. What are the possible genotypes of the mare

Ee

Explanation:

The genotype of the mare can be EE or Ee. This is because she is black and as the colour black comes from a dominant allele, only one of those alleles is needed for the black phenotype. On the other hand, the stallion is chestnut, that is to say he has no dominant allele. So its genotype can only be ee.

So the next step is to find the gametes produced by the horses. During fertilization, the egg and sperm fuse. Each has one allele (one copy of the gene) and together they form a diploid zygote, with two alleles (one from the father and one from the mother)

While the organisms are diploid (two copies of each gene), the gametes are haploid (one copy of each gene). If the mare is Ee, she can produce gametes with an E or an e allele. And if it is EE, it can only produce gametes with an E allele.

Since it is already known that the offspring is chestnut (genotype ee), this tells us that it has inherited an e allele from the mother, not an E allele (since it can only inherit the other e allele from the father because that is the only one allele in his genotype). So, the mother can only be Ee, and the offspring received the allele e from her, and together with the allele e from the father, it formed an ee genotype.