Question

in the f₂ generation of mendels crosses,
all plants had the dominant trait.
half of the plants had the dominant allele.
all plants had both the dominant and recessive alleles.
one in four plants had two alleles for the recessive trait.

Explanation:

In Mendel's experiments, when he crossed \(F_1\) generation plants (heterozygous, e.g., \(Aa\times Aa\)), the \(F_2\) generation had a phenotypic ratio of \(3:1\) (dominant : recessive) and a genotypic ratio of \(1:2:1\) (\(AA:Aa:aa\)). The statement "one in four plants had two alleles for the recessive trait" (\(aa\)) is correct as per the \(1:2:1\) genotypic ratio. The first option is wrong as there are recessive - trait plants in \(F_2\). The second option is incorrect because more than half (\(\frac{3}{4}\) in terms of phenotype and \(\frac{3}{4}\) in terms of having at least one dominant allele \(A\) in genotype) have the dominant allele. The third option is wrong as only the heterozygous (\(Aa\)) have both dominant and recessive alleles, not all.

Answer:

one in four plants had two alleles for the recessive trait.