Principles of Inheritance and Variation — BIOLOGY STD 12 Science — Question

Mendel conducted his experiments on pea plant. He selected 7 allelic traits of pea for his experiment. Both the traits in each pair are alleles to each other e.g. tall and dwarf, red and white, yellow and green etc. Mendel studied the transmission of seven pairs of allelic traits from parents to offsprings one by one and obtained the same results each time.
For example, when pure red flowered pea plant crossed with pure white flowered pea plant only red flowered plants were produced in F_{1} generation when red flowered plants of F 1 generation were self-crossed, Mendel obtained red and white flowered plants in 3:1 in F₂ generation. In other words, 75% red flowered and 25% white flowered progenies produced.

rr = White flowered progeny
Therefore phenotypic ratio = 3:1
Mendel explained law of dominance on the basis of this hybridization. According to this law one factor (gene) of the pair factors hides the expression, of other factor e.g. Red colour of allele in F, generation is dominant upon white colour allele therefore all plants of F_{1} generation are red flowered. Hence this trait (Red Colour) which expresses in F₁ generation due to dominance. Mendel called it dominant trait and the other trait (white colour) which does not express in F₁ gen or is masked by dominant trait, Mendel called it recessive trait. Dominant trait is symbolised by capital letter and recessive trait by small letter as shown in diagram.