Why do you think the puppy litter was all dark-haired?

Because dark hair is represented by a pair of dominant alleles.

Explanation:

Allele genes are segments of DNA (deoxyribonucleic acid) made up of pairs. One is from the mother (egg) and the other from the father (sperm), which are found at the same locus on homologous chromosomes. They are classified into:

The dominant genes are those that determine an inherited trait even when in a simple dose in the genotype. That is, they determine their character even in the absence of another dominant allele. In short, when the offspring receives at least one dominant allele, they will express that trait. In the case of the question above, black hair is a characteristic represented by the dominant allele, if all the offspring expressed this characteristic it is because everyone has at least one copy of that allele.

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