4.
A geneticist crossed two pure-bred (homozygous) tall and short plants. In the F1
generation she observed that all the offspring were of intermediate height. If she crosses
one of the offspring with the short parent what kind of progeny would you expect?
Assume that plant height is dictated by a single locus.
a. All short
b. 50 % tall and 50 % short
c. 50 % intermediate and 50 % short
d. 50 % intermediate, 25 % tall and 25 % short
The correct answer is option c. 50% intermediate and 50% short. This scenario demonstrates incomplete dominance, where heterozygous plants show intermediate height, and a backcross of F1 (intermediate) with the short parent yields a 1:1 phenotypic ratio.
Genotypes Assigned
Pure-bred tall plants are TT (tall phenotype), short plants are tt (short phenotype), and all F1 offspring from TT × tt are Tt (intermediate height) due to neither allele fully dominating. Crossing one F1 (Tt) with the short parent (tt) is a test cross equivalent.
Punnett Square Analysis
The gametes from Tt are T and t (each 50%), while tt produces only t gametes.
| t | t | |
|---|---|---|
| T | Tt | Tt |
| t | tt | tt |
Progeny: 50% Tt (intermediate), 50% tt (short).
Option Explanations
-
a. All short: Incorrect; this would occur only if F1 were tt, but F1 is heterozygous Tt.
-
b. 50% tall and 50% short: Incorrect; applies to complete dominance test cross (Tt × tt), but no tall (TT) progeny here, and Tt is intermediate.
-
c. 50% intermediate and 50% short: Correct; matches the 1:1 ratio from Tt × tt in incomplete dominance.
-
d. 50% intermediate, 25% tall and 25% short: Incorrect; resembles F2 (Tt × Tt) 1:2:1 ratio, not backcross.
This CSIR NET life sciences question on incomplete dominance plant height backcross illustrates non-Mendelian inheritance at a single locus, where pure-bred tall (TT) and short (tt) plants produce all intermediate (Tt) F1 progeny. Backcrossing F1 (Tt) to short parent (tt) reveals the classic 1:1 ratio, vital for genetics exam preparation.
Core Concept
Incomplete dominance occurs when heterozygotes (Tt) express a blended phenotype, unlike complete dominance. Plant height examples, like snapdragons or hypothetical peas, show F1 uniformity at intermediate levels. The backcross tests genotype by pairing with recessive homozygote.
Progeny Ratio Breakdown
-
50% Intermediate (Tt): Retains partial tall influence.
-
50% Short (tt): Fully recessive phenotype.
This differs from F2 (1 tall : 2 intermediate : 1 short).
CSIR NET Relevance
Such problems test allelic interactions, Punnett squares, and ratio predictions—key for Unit 8 (Inheritance Biology). Practice distinguishes backcross from self-cross ratios.
