Question

In a typical monohybrid cross, F${}_2$ population ratio is written as 3: 1 for phenotype but expressed as 1: 2: 1 for genotype. Explain with the help of an example.

Answer 1

A cross between pure breeding tall (TT) and dwarf (tt) varieties gives all heterozygous tall (Tt) progeny in $F_1$ generation. Selfing of $F_1$ hybrid gives tall and dwarf $F_2$ progeny in 3: 1 ratio respectively. Since allele for tallness (T) is dominant over that of dwarfism (t); heterozygous dominant genotype (Tt) gives "tall" phenotype and dwarfism is masked from being expressed. Both homozygous dominant (TT) and heterozygous dominant (Tt) exhibit "tall" phenotype while homozygous recessive genotype gives dwarf phenotype; hence phenotypic ratio is 3 tall: 1 dwarf but the genotypic ratio is 1 TT: 2 Tt: 1 tt.

Parent generation TT x tt

$F_1$ generation Tt

Tt Tt	T	t
T	TT (tall)	Tt (tall)
t	Tt (tall)	tt (dwarf)