(3)

When a tall true breeding garden pea plant is crossed with a dwarf true breeding garden pea plant and the ${\mathrm{F}}_1$ plants were selfed the resulting genotypes were in the ratio of 1 : 2 : 1, i.e., Tall homozygous : Tall heterozygous : Dwarf
It can be illustrated as given below:

[IMAGE 18]

Phenotypic ratio: 3 : 1 :: Tall : Dwarf
Genotypic ratio: 1 : 2 : 1 :: TT : Tt : tt

(2)

The phenomenon of expression of both the alleles in a heterozygote is called co-dominance. The alleles which do not show dominance-recessive relationship and are able to express themselves independently when present together are called co-dominant alleles. As a result, the heterozygous condition has a phenotype different from either of homozygous genotypes, e.g., alleles for blood group A ($I^A$) and for blood group B ($I^B$) are codominant so that when they come together in an individual, they produce blood group AB.

(1)

Genes are the units of inheritance and contain the information that is required to express a particular trait in an organism. Alternating forms of a single gene which code for a pair of contrasting traits are known as alleles. For example, two alleles determine the height of pea plant (tall and dwarf).

(1)

(1)

The man has blood group A, thus its genotype can either be $I^A{I}^A$ or $I^A{I}^O$. Similarly, woman can either have $I^B{I}^B$, or $I^B{I}^O$ genotype. Thus, their offspring can have any of the blood groups A $\left(I^A{I}^A\text{ or }{I}^A{I}^O\right), B \left(I^B{I}^B\text{ or }{I}^B{I}^O\right), \text{AB }\left(I^A{I}^B\right) \text{or }O \left(I^O{I}^O\right)$.