Question

In one of his experiments with pea plants, Mendel observed that when a pure tall pea plant is crossed with a pure dwarf pea plant, in the first generation $F_1$ only tall plants appear.

(i)When the $F_1$ generation plants were self-fertilized, he observed that in the plants of second-generation $F_2$ both tall plants and dwarf plants were present. Why did it happen? Explain briefly.
(ii)What happens to the traits of the dwarf plants in this case?

Answer 1

The $F_1$ generation has the genotype Tt which is why all the plants are tall in height since all of them contain the allele for tall height (dominant allele).

In the presence of a dominant allele, the recessive allele is not expressed when all the plants have tall height. But when the plants of this generation are self-crossed we get the following genotypes in the new generation: TT :Tt: tt in the ratio 1:2:1. Hence 25% of the plants have both the alleles as the recessive allele. When the dominant allele is absent the recessive allele is expressed in the plant is short in height. The recessive allele was suppressed in the $F_1$ generation but expressed in the $F_2$ generation.

This can be understood by looking at the punnet square in the image.