The round pea seed trait is dominant over the wrinkled pea seed trait. If the phenotype of a pea seed is wrinkled, which combination of genes must it have?

The wrinkled pea will have two recessive alleles on the locus.

Explanation:

Two forms of same gene are called alleles of that gene.

The allele that expresses itself under heterozygous condition is called the Dominant allele.

The recessive allele can express itself only if they are under homozygous combination i. e. both the alleles on homologous chromosome are recessive.

Here wrinkled pea is a recessive trait and if it is expressed in the phenotype then the genotype of the plant must be homozygous for the recessive allele.

Two wrinkled seeds combination in cross test

Explanation:

Whenever one or more of the parents' genes are dominant, the resulting daughter gene will be dominant. Thus, in order to produce a recessive gene, BOTH parent genes must be recessive as well. Because wrinkled genes are recessive, you're going to need both wrinkled genes. For example, when Mendel cross-prepared plants with wrinkled seeds to those with smooth seeds, he didn't get descendants with semi-wrinkly seeds. Rather, the descendants from this cross had just smooth seeds. It must have two wrinkled genes. Hence, the combination of genes are two wrinkled seeds is the right answer.