Question

(a) State and explain the law of dominance as proposed by Mendel.
(b) How would phenotypes of monohybrid F₁ and F₂ progeny showing incomplete dominance in Snapdragon and co-dominance in human blood group be different from Mendelian monohybrid F₁ and F₂ progeny? Explain.

Answer 1

(a) According to Mendel's law of dominance, characters are controlled by discrete units called 'factors'. These 'factors' occur in pairs, where one member of the pair dominates the other in a dissimilar pair.
The Mendel's law of dominance was given on the basis of results of monohybrid crosses where, in F₁ generation, only one of the parental characters is expressed, and in F₂ generation, both are expressed.
This can be understood as follows:

Here, for the height of a plant, tallness is a dominant trait, while dwarfness is recessive trait. If a pure tall plant (TT) is crossed with a short plant (tt), then F₁ generation will be all tall but their genotype will be heterozygous (Tt) and on selfing of F₁ generation, both tall and short plants will arise in F₂ generation in the phenotypic ratio of 3 : 1 and genotypic ratio of 1 : 2 : 1.

(b) As shown above in Mendelian monohybrid cross, F₁ generation is all tall and F₂ generation has both tall and dwarf plants.
In case of Snapdragon, incomplete dominance is observed. Here, dominant trait is not completely dominant over recessive trait and both produce an intermediate phenotype in heterozygous condition, that is, none of them express their typical phenotype when present together.

When a true breeding plant producing red flowers (RR) was crossed with a pure breeding plant producing white flowers (rr), the F₁progeny so obtained produced pink-coloured flowers (Rr). When the F₁ progeny was selfed, the F₂ progeny so obtained produced red-, pink- and white-coloured flowers in the ratio 1 : 2 : 1.

In case of co-dominance, both alleles express their phenotype independently in the heterozygous condition. In human blood group, multiple allelism is observed, that is, more than two contrasting forms of a gene are present.
The ABO blood group is controlled by 3 alleles: I^A, I^B, and i.  The alleles I^A and I^B are completely dominant over i but when I^A and I^Bare present together, they both express themselves and behave as co-dominant alleles.

Genotype	Blood Group
IAIA, IAi	A
IBIB, IBi	B
IAIB	AB
ii	O