In tomato plants the allele T for tall stems is dominant to the allele t for short stems. A plant carrying one T allele and one t allele is crossed with a plant carrying two t alleles. What phenotype ratio is expected?

Explanation

The mixed parent passes on T half the time and t half the time, while the other parent can only pass on t, so half the offspring receive a T allele and grow tall and half receive two t alleles and stay short. A 3 to 1 split needs both parents able to hand on the dominant allele, and here only one of them can.