How do recessive alleles express themselves in organisms?

Recessive alleles express themselves in organisms when two copies of the same recessive allele are present.

In genetics, an organism's traits are determined by the combination of alleles it inherits from its parents. Each parent contributes one allele for each trait to their offspring. These alleles can be either dominant or recessive. Dominant alleles are those that express themselves even if only one copy is present. On the other hand, recessive alleles require two copies, one from each parent, to express themselves.

For example, let's consider the trait for eye colour. The allele for brown eyes is dominant, while the allele for blue eyes is recessive. If an individual inherits a brown eye allele from one parent and a blue eye allele from the other, they will have brown eyes because the brown eye allele is dominant. However, if the individual inherits two blue eye alleles, one from each parent, they will have blue eyes because they have two copies of the recessive allele.

This is explained by Mendel's law of segregation, which states that the two alleles for each trait separate during the formation of gametes (eggs or sperm). Each gamete then carries only one allele for each trait. During fertilisation, the egg and sperm combine to form a new organism with two alleles for each trait, one from each parent.

In summary, recessive alleles express themselves in organisms when two copies of the same recessive allele are present. This is a fundamental concept in genetics and is crucial for understanding how traits are inherited from parents to offspring.