Mendel accounted for the observation that traits that had disappeared in the F1 generation reappeared in the F2 generation by proposing that Mendel accounted for the observation that traits that had disappeared in the F1 generation reappeared in the F2 generation by proposing that traits can be dominant or recessive, and the recessive traits were "hidden" by the dominant ones in the F1.

a. the mechanism controlling the appearance of traits was different between the F1 and the F2 plants.

b. the traits were lost in the F1 due to the dominance of the parental traits.

c. new mutations were frequently generated in the F2 progeny, "reinventing" traits that had been lost in the F1.

d. members of the F1 generation had only one allele for each trait, but members of the F2 had two alleles for each trait.

The question is incorrect. The correct question with options is:

Mendel accounted for the observation that traits that had disappeared in the F1 generation reappeared in the F2 generation by proposing that:

A. traits can be dominant or recessive, and the recessive traits were "hidden" by the dominant ones in the F1.

B. the mechanism controlling the appearance of traits was different between the F1 and the F2 plants.

C. the traits were lost in the F1 due to the dominance of the parental traits.

D. new mutations were frequently generated in the F2 progeny, "reinventing" traits that had been lost in the F1.

E. members of the F1 generation had only one allele for each trait, but members of the F2 had two alleles for each trait.

Answer:

A. traits can be dominant or recessive, and the recessive traits were "hidden" by the dominant ones in the F1.

Explanation:

Mendel crossed two pure breeding plants that differ from each other with respect to one genetic trait. He obtained all the F1 progeny resembling the one of the parent plants. However, the self-pollination of F1 plants obtained the F2 progeny in a 3: 1 ratio. He observed that one of the parental traits that was not observed in F1 progeny reappeared in the F2 generation.

On the basis of this observation, he proposed the law of dominance. He suggested that a genetic trait may be dominant or recessive. A recessive trait is expressed only in the homozygous state. Its expression is masked in the heterozygous state due to the presence of the dominant allele. For instance, he obtained all tall (Tt) F1 plants by a cross between pure breeding tall (TT) and dwarf (tt) parent plants. However, the self-pollination of F1 obtained the tall and dwarf F2 progeny in a 3: 1 ratio. Here, the hybrid F1 plants (Tt) expressed the dominant trait (tallness) and the expression of the recessive trait (dwarfism, t) was obscured.

The dominant and recessive alleles present together in the F1 hybrids are segregated during gamete formation resulting in a reappearance of the recessive trait in the F2 generation.