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:

Mendel's experiments showed that in the \(F_{2}\) generation, when considering a single - trait cross (e.g., pea plant height with \(T\) (tall, dominant) and \(t\) (short, recessive)), the phenotypic ratio is \(3:1\) (dominant : recessive) and the genotypic ratio is \(1:2:1\) (\(TT:Tt:tt\)).

• Option 1: In \(F_{2}\), there are both dominant and recessive - trait plants (e.g., tall and short in pea plant height example), so “all plants had the dominant trait” is wrong.
• Option 2: The genotypic ratio \(TT:Tt:tt = 1:2:1\). The proportion of plants with the dominant allele (\(TT\) and \(Tt\)) is \(\frac{3}{4}\), not \(\frac{1}{2}\), so “half of the plants had the dominant allele” is wrong.
• Option 3: Plants can be \(TT\) (only dominant alleles), \(Tt\) (dominant and recessive alleles), or \(tt\) (only recessive alleles). So “all plants had both the dominant and recessive alleles” is wrong.
• Option 4: From the genotypic ratio \(TT:Tt:tt=1:2:1\), the proportion of \(tt\) (two alleles for the recessive trait) is \(\frac{1}{4}\), which means one in four plants had two alleles for the recessive trait.

Answer:

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