Question

Explain Mendel’s monohybrid progeny with the help of any one cross.

Answer 1

1. In this experiment, Mendel brought about the cross between two pea plants with only one pair of contrasting characters. This type of cross is called a monohybrid cross.
2. Tall pea plants and dwarf pea plant were used in this cross. Hence, this is parent generation (P₁). Mendel referred to the tall and dwarf plants as dominant and recessive respectively. The tall plant was referred to as dominant because all the plants in the next generation were tall.
3. The dwarf plant was referred to as recessive because this characteristic did not appear in next generation at all.
4. Though all the plants in F₁ generation were tall, they contained the factor responsible for dwarfness. i.e. though the phenotype of F₁ plants is tall, their genotype is mixed.
5. Genotype of F₁ tall plants is Tt and it produces two types of gametes, T and t. Thus, based on this, we can say that in case of tall plants of F₁ and P₁ generations, though they show similar phenotype, their genotypes are different. Mendel further continued this experiment and brought about self-fertilization in F₁ plants from which a second filial (F₂) generation was produced.
6. According to the data collected by Mendel, out of 929 pea plants, 705 were tall and 224 were short. Thus, the phenotypic ratio of these plants is 3(tall):1(dwarf) and genotypic ratio is 1(TT):2(Tt):1(tt).
7. Thus, it can be inferred that in the F₂ generation, phenotypically there are two types of plants whereas genotypically there are three types. These types are shown in the following table.
   

   F2 Pure dominant TT - tall plants	Homozygous
   F2 Pure recessive (tt) - dwarf plants	Homozygous
   F2 Hybrid plants (Tt) - tall plants	Heterozygous

Mendel’s experiment of the Monohybrid Cross
Parental Generation P1
Phenotype	Tall	Dwarf
Genotype	TT	tt
Gametes	T	t
First Filial Generation F1		Tt
	(Phenotype: Tall)
Parental Generation P2	Selfing in F1
Phenotype	Tall	Tall
Genotype	Tt	Tt
Gametes	T t	T t
First Filial Generation F2	Male gamete/Female gamete	T	T
	T	TT Tall	Tt Tall
	t	Tt Tall	tt Dwarf