In garden peas, the allele for tall plants is dominant over the allele for short plants. Imagine that a true-breeding tall plant is crossed with a short plant. Then one of their offspring is test crossed. Out of 20 offspring resulting from the test cross, about how many should be tall?
A) 0
B) 10
C) 15
D) 20
E) 5

Question

Quizplus · Accepted Answer

A true-breeding tall plant has the genotype TT (where T is the dominant allele for tallness), and a short plant has the genotype tt (since shortness is recessive and can only be expressed in the homozygous condition). When a TT plant is crossed with a tt plant, all offspring (F1 generation) will be Tt (heterozygous but tall, due to the dominance of the T allele). A test cross involves crossing an individual with a homozygous recessive individual (tt) to determine the genotype of the first individual. In this case, crossing a Tt plant with a tt plant would result in approximately half of the offspring being tall (Tt) and half being short (tt). Therefore, out of 20 offspring, about 10 should be tall.

In Garden Peas, the Allele for Tall Plants Is Dominant