Describe a recessive allele as an allele that is only expressed when there is no dominant allele of the gene present in the genotype

Recessive Allele: Expression in the Absence of Dominant Alleles

A recessive allele is a specific type of allele that is expressed in the phenotype of an organism only when two copies of that allele are present in the genotype, meaning there is no dominant allele for that gene. When a dominant allele is present, it masks the expression of the recessive allele.

1. Definition of Recessive Allele

• Recessive Allele: An allele that does not manifest its trait in the presence of a dominant allele. It is typically denoted by a lowercase letter (e.g., “a” for a recessive trait).

2. Expression of Recessive Alleles

The expression of a recessive allele occurs under specific conditions:

• Homozygous Recessive (aa): For a recessive trait to be expressed, an individual must have two copies of the recessive allele. For example, if “a” represents the allele for white flowers in pea plants, only individuals with the genotype aa will exhibit white flowers.
• Heterozygous (Aa): In a heterozygous individual, where one dominant allele (A) and one recessive allele (a) are present, the dominant trait will be expressed, and the recessive trait will not be visible.

3. Examples of Recessive Alleles

• Pea Plant Flower Color: In Mendel’s experiments, white flower color (p) is recessive to purple flower color (P). Therefore, only plants with the genotype pp will have white flowers; those with genotypes PP or Pp will display purple flowers.
• Human Traits: Certain traits in humans, such as blue eyes or attached earlobes, are often controlled by recessive alleles. For instance, if “b” represents blue eyes and “B” represents brown eyes, only individuals with the genotype bb will have blue eyes.

4. Importance of Recessive Alleles

Understanding recessive alleles is important for several reasons:

• Genetic Variation: Recessive alleles contribute to genetic diversity within populations. They can remain hidden in heterozygous individuals but may reappear in future generations when two carriers breed.
• Genetic Disorders: Many genetic disorders are caused by recessive alleles. For example, cystic fibrosis results from being homozygous recessive (ff) for a specific gene; individuals who are heterozygous (Ff) do not show symptoms but can pass on the recessive allele to their offspring.