Two pea plants, each carrying one dominant allele for tall stems (T) and one recessive allele for short stems (t), are crossed with each other. Using a Punnett square for this Tt x Tt cross, what proportion of their offspring would be expected to have short stems, and why?
- None of the offspring, because the dominant tall allele always completely replaces the recessive short allele in every offspring's genetic makeup
- Half of the offspring, because each parent contributes one allele, and exactly half of all combinations must contain the recessive allele
- All of the offspring, because both parents carry the recessive allele, so it is guaranteed to be passed on and expressed in every offspring
- One quarter of the offspring, because the only combination that produces a short phenotype is the one where an offspring inherits the recessive allele from both parents (tt), which happens in one of the four equally likely combinations in the Punnett square
Why D? And why not the others?
Correct answer: D. One quarter of the offspring, because the only combination that produces a short phenotype is the one where an offspring inherits the recessive allele from both parents (tt), which happens in one of the four equally likely combinations in the Punnett square
Crossing two Tt parents produces four equally likely combinations in the Punnett square: TT, Tt, Tt and tt; only the tt combination, where an offspring inherits the recessive allele from both parents, produces the short-stem phenotype, and that combination makes up one of the four boxes, giving a quarter of the expected offspring. The option claiming none of the offspring would be short is wrong because it ignores that two carriers of the recessive allele can still produce offspring with two recessive alleles between them. The option claiming half the offspring would be short is wrong because it correctly identifies that half of the offspring carry at least one recessive allele through the Tt combinations, but a single recessive allele paired with a dominant one still produces the tall phenotype, since the dominant allele is expressed. The option claiming all offspring would be short is wrong because carrying the recessive allele is not the same as having two copies of it; the dominant allele from either parent is still enough to produce a tall plant unless both parents happen to contribute the recessive allele to the same offspring.
Source: BBC Bitesize GCSE Biology: Monohybrid crosses and Punnett squares