Lesson 2 of 11 · 10 min
Inheritance of one gene: dominance and segregation
NCERT § "Inheritance of One Gene"
Your grandfather crosses a true-breeding tall pea plant with a true-breeding dwarf one. Every single F1 plant comes up tall — not a hint of the dwarf parent. Where did that trait go?
The lesson in notes
In short
In a monohybrid cross of tall with dwarf pea, all F₁ plants were tall. Self-pollinating F₁ gave F₂ plants in a 3 tall : 1 dwarf ratio, and the dwarf plants were as dwarf as the original parent, with no blending.
Mendel said a stable 'factor' passes each character from parent to offspring; these factors are now called genes. Alternative forms of one gene are alleles, such as T and t.
A plant with two identical alleles (TT or tt) is homozygous; one with two different alleles (Tt) is heterozygous. Tt is a hybrid that looks tall, so genotype and phenotype are not the same thing.
The F₂ genotypic ratio is 1 TT : 2 Tt : 1 tt, while the phenotypic ratio is 3 tall : 1 dwarf. The Punnett square, drawn by Reginald C. Punnett, is used to work these out.
A test cross crosses an organism of unknown genotype showing the dominant trait with the recessive parent. A 1:1 result shows it was heterozygous; all-dominant offspring show it was homozygous.
Law of dominance: characters are controlled by paired factors, and in a dissimilar pair one factor is expressed (dominant) while the other is masked (recessive). It explains why only one parental form shows in F₁ and both show in F₂.
Law of segregation: the two alleles of a pair do not blend; they separate during gamete formation so each gamete gets only one of them. A homozygote makes one kind of gamete, a heterozygote two kinds in equal numbers.