Law of dominance

The Law of Dominance is one of the fundamental principles of inheritance proposed by Gregor Mendel, the father of modern genetics, based on his experiments with pea plants in the 19th century. This law describes the behavior of alleles, which are different forms of a gene that can exist at a specific locus (location) on a chromosome. The Law of Dominance can be summarized as follows:

1. Dominant and Recessive Alleles:

Mendel’s experiments involved studying traits controlled by single genes with two contrasting alleles.

He observed that one allele in a pair of alleles could mask or suppress the expression of the other allele.

The allele that masks the expression of the other allele is called the “dominant allele.”

The allele whose expression is suppressed in the presence of the dominant allele is called the “recessive allele.”

2. Phenotype Expression:

The dominant allele determines the phenotype (observable trait) of an organism when it is present.

The recessive allele only determines the phenotype when it is present in a homozygous state (i.e., when an organism has two copies of the recessive allele).

3. Homozygous and Heterozygous Genotypes:

Organisms can have one of three possible genotypes for a particular trait:

Homozygous Dominant (e.g., AA): Both alleles are dominant, resulting in the expression of the dominant trait.

Heterozygous (e.g., Aa): One dominant allele and one recessive allele are present. The dominant trait is expressed.

Homozygous Recessive (e.g., aa): Both alleles are recessive, and the recessive trait is expressed.

4. Mendel’s Observations:

Mendel observed that in monohybrid crosses (crosses involving one trait), the F₁ generation (first filial generation) always exhibited the dominant trait.

In the F₂ generation (second filial generation), he observed a consistent 3:1 phenotypic ratio, where three-fourths of the offspring displayed the dominant trait, and one-fourth displayed the recessive trait.

5. Genetic Symbols:

Geneticists represent dominant alleles with uppercase letters (e.g., A) and recessive alleles with lowercase letters (e.g., a).

Homozygous dominant individuals are denoted as AA, heterozygous individuals as Aa, and homozygous recessive individuals as aa.

Example:

In Mendel’s experiments, he studied the trait of flower color in pea plants.

Purple flower color (P) was dominant, and white flower color (p) was recessive.

When he crossed a homozygous dominant plant (PP) with a homozygous recessive plant (pp), all the F₁ offspring had purple flowers (Pp).

In the F₂ generation, he observed a 3:1 ratio, with three-fourths having purple flowers (PP or Pp) and one-fourth having white flowers (pp).