Solutions For An Introduction to Genetic Analysis Chapter 2 Problem 39P

When a plant with three cotyledons was crossed with wild-type, the F₁ progeny obtained was half of the progeny and had three cotyledons (mutant). Half of them had two cotyledons (wild-type).

This shows that the mutant is dominant trait and the original mutant was heterozygous.

If we consider the mutant allele as C and the wild-type as c, then the cross will be as follows:

The progeny or the F₁ generation would be Cc, Cc, cc, cc showing half of the progeny with three cotyledons and half of the progeny with two cotyledons.