Inheritance of a single trait

Inheritance of a single trait

Monohybrids cross

A monohybrid cross is a cross between two individuals hybrid (heterozygous) for one particular character. Mendel derived the Law of Segregation from these crosses.

Parental generation

Mendel started with a cross between a true-breeding (Homozygous) dominant variety (e.g. Round seeded plant) and a recessive variety (e.g. Wrinkled seeded plant). These two varieties he referred to as parental generation (P₁).

Gametes of the parent generation 

The pure breeding round seeded plant has both the dominant alleles i.e. RR and wrinkled seeded plants have both recessive alleles i.e. rr, therefore, gametes produced by these plants have dominant allele and recessive allele respectively.  

F1 progeny

The first filial generation showed all dominant phenotypes but hybrid for the seed shape. Mendel self-crossed the individuals of the F1 generation.

Gametes of F1 generation

The F1 individuals were Heterozygous dominant, Therefore, they process two types of gametes i.e. with a dominant allele R and recessive allele r.

F2 progeny

In the Second filial generation, a 3:1 ratio was observed. Three out of four were dominant while one was found to have recessive character. The genotypic ratio was 1:2:1 i.e. one homozygous dominant, two heterozygous dominant, and one recessive.

Law of segregation

The law of segregation states that the two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes. Thus, an egg or a sperm gets only one of the two alleles that are present in the diploid cells.

This law is applicable to the genes present on the same locus (alleles). Alleles are located on the homologous chromosomes. This law is also called as “law of Purity”.

Explanation:

According to the law of dominance or the law of purity of gametes, in a heterozygous condition, the dominance and recessive alleles remain together without mixing with each other. The alleles separate or segregate from each other during gametogenesis, so that each gamete receives only one allele, whether it is dominant or recessive.

The law of segregation can be easily demonstrated in a monohybrid cross.