Question

When a heterozygous dominant is crossed with homozygous recessive then the ratio of progeny will be

Answer 1

Gregor Mendel has first experimented with the test cross method. He crossed the pea plants and obtained the genotype and phenotype of the progenies. From the observation, he had given the law of inheritance.

Complete answer:
Mendel took a homozygous tall (TT) pea plant and crossed it with a homozygous dwarf (tt) pea plant. The phenotype of progenies obtained in the first generation was in a 3:1 ratio. While the genotype ratio was 1:2:1. Here, one of both homozygous tall (TT) and homozygous dwarf (tt) plants are produced. While the other two were heterozygous tall plants (Tt). A heterozygous tall pea plant is taken with the genotype formula Tt. Here, T is inherited from the tall plant, and t is inherited from the dwarf plant.
The heterozygous tall (Tt) plant is crossed with a homozygous dwarf (tt) plant.
Tt ✖ tt
First-generation:-

| T| t
---|---|---
t| Tt| tt
t| tt| Tt

Thus the ratio of progeny will be 1:1.

Additional information:-
Alleles are the different forms of genes. Diploid organisms consist of two pairs of alleles in each of the genes. One of the two alleles come from each parent. A heterozygous pair is formed due to the combination of two different alleles. One allele is inherited from each parent, among which one is dominant and the other one is called recessive. The dominant trait is expressed in the phenotype and suppresses the recessive trait.

Note:
-There are certain conditions in which the dominant allele does not completely dominate the recessive allele. They are called incomplete dominance.
-While in codominance, no allele dominates the phenotypes.