Question

Name the following:
A cross between two parents having one pair of contrasting characters.

Answer 1

Hint: Mendel described the cross to determine genotypes. Through his experiments on pea plants, he described the principle of inheritance. A pair of contrasting characters are known as alleles. In this pair, one character is dominant and the other is recessive.

Complete answer: Mendel performed a cross between parents having one pair of contrasting characters called the monohybrid cross. The parents differed in a single pair of contrasting characteristics. For example, the height of the plant. A cross made in between a homozygous tall (TT) and a homozygous short (tt), where tall is dominant over short.
F1 generation: The offspring (TT X tt ) is Tt (tall). The F2 generation is obtained by the heterozygous cross between Tt X Tt.

	T	t
T	TT (tall)	Tt (tall)
t	Tt (tall)	Tt (short)

The phenotypic ratio obtained is 3:1.
The genotypic ratio obtained is 1:2:1.
The monohybrid cross is used to determine the dominant allele. Through this, he put forward the theory of dominance. It states that one factor out of the two is dominant over the other. The other factor is called recessive. The dominant factor is expressed in the first generation and the recessive trait is only expressed in the F2 generation.

Note: In some cases, especially the colour of the flower, one gene is incompletely dominant over the other. It leads to the expression of both the characters resulting in a different variety. A cross between purple and white flowers yielded pink flowers. The phenotypic and genotypic ratio in case of incomplete dominance is 1:2:1.