In garden peas, a single gene controls stem length. The recessive allele (t) produces short stems when homozygous. The dominant allele (T) produces long stems. A short-stemmed plant is crossed with a heterozygous long-stemmed plant.

Which of the following represents the expected phenotypes of the offspring and the ratio in which they will occur?

Question

In garden peas, a single gene controls stem length. The recessive allele (t) produces short stems when homozygous. The dominant allele (T) produces long stems. A short-stemmed plant is crossed with a heterozygous long-stemmed plant.

Which of the following represents the expected phenotypes of the offspring and the ratio in which they will occur?

BlackTom AI · Accepted Answer

The cross involves a short-stemmed plant (tt) fertilizing with a heterozygous long-stemmed plant (Tt).
First, determine the gametes: tt can only give t; Tt can give T or t. When we set up a Punnett square, the possible offspring genotypes are Tt and tt, each arising in a 1:1 ratio (50% each).
Phenotypes follow the dominant-recessive relationship: T is dominant for long stems, t is recessive for short stems. Therefore, Tt plants will have long stems, and tt plants will have short stems.
Option 1 describes a 1 long-stemmed : 1 short-stemmed outcome, which aligns with the 1:1 genotype and the 1:1 phenotype proportions.
Option 2 asserts short-stemmed plants only, which would require all offspring to be tt; this ignores the presence of the T allele from the heterozygous parent and is incorrect.
Option 3 proposes a 3:1 ratio of long to short stems. That ratio would arise from a monohybrid cross between two heterozygotes (Tt x Tt) or a different cross, not from tt x Tt.
Option 4 claims long-stemmed plants only; again, since tt is a possible offspring genotype, this is incorrect.
Option 5 states a 1 long-stemmed : 3 short-stemmed ratio, which would be a reverse 3:1 and does not fit the actual cross outcomes here.
In summary, the correct expectation for this cross is a 1:1 phenotypic ratio of long-stemmed to short-stemmed plants, reflecting half Tt (long) and half tt (short).

APBIO141-031 with labs S14 - CollegeBoard* AP Exam Prep: Multiple Choice

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