Two independent monohybrid crosses were carried out involving a tall pea plant with a dwarf pea plant. In the first cross, the offspring population had equal number of tall and dwarf plants, whereas in the second cross it was different. Work out the crosses, and explain giving reasons for the difference in the offspring populations.

A monohybrid cross in which two forms of single trait are hybridised.

First cross:

The first cross is between tall pea plant and a dwarf pea plant. In this the tall pea plant is heterozygous dominant i.e., Tt and the dwarf plant if homozygous recessive i.e., tt, so the genotype of parents would be Tt and tt. Hence the offspring population produced would be equal number of tall and dwarf plants.

The progeny would be:

The ratio of progeny is 1:1, i.e., 50% tall and 50% dwarf.

Second Cross:

The second cross is between tall pea plant and a dwarf pea plant. In this the tall pea plant is homozygous dominant i.e., TT and the dwarf plant if homozygous recessive i.e., tt, so the genotype of parents would be TT and tt. Hence the offspring population produced would be all tall plants.

The progeny would be:

The ratio of the progeny is 100% tall.

The difference is due to the differences in the genotypes of the parents.

These types of cross shows the law of dominanace, i.e., all traits are controlled by genes. In dissimilar genes, one member of the pair is dominant and the other is recessive.