Recessive allele

Definition

A recessive allele is a version of a gene that does not manifest its trait in the presence of a dominant allele. In Mendelian inheritance, for an organism to express a trait associated with a recessive allele, it must possess two copies of that allele, one inherited from each parent. This concept is fundamental in understanding how traits are passed down through generations and how genetic variation is represented in populations.

5 Must Know Facts For Your Next Test

1. Recessive alleles are represented by lowercase letters, while dominant alleles use uppercase letters, making it easy to distinguish between them in genetic notation.
2. An individual must inherit two recessive alleles (one from each parent) to express the trait linked to that recessive allele, resulting in a homozygous recessive genotype.
3. In cases where an individual has one dominant and one recessive allele (heterozygous), the dominant trait will be expressed, effectively masking the presence of the recessive allele.
4. Recessive alleles can persist in a population even if they are not expressed, as they can be carried by heterozygous individuals without affecting their phenotype.
5. Examples of traits influenced by recessive alleles include cystic fibrosis and sickle cell anemia, where two copies of the recessive allele are required for the disorder to manifest.

Review Questions

• How does the presence of a dominant allele affect the expression of a recessive allele in an organism?
  • When a dominant allele is present alongside a recessive allele in an organism's genotype, the dominant allele will determine the organism's phenotype, effectively masking the trait associated with the recessive allele. This means that even if an individual carries a recessive allele, it will not be expressed unless both copies of the gene are recessive. Therefore, only homozygous recessive individuals will show traits related to recessive alleles.
• Discuss the implications of recessive alleles on genetic diversity within a population.
  • Recessive alleles contribute to genetic diversity within a population by allowing for a wider range of genotypic combinations. Heterozygous individuals carry both dominant and recessive alleles, which can mask recessive traits while still allowing those alleles to be passed on to future generations. This means that even rare or harmful recessive alleles can persist in the gene pool, leading to greater variability in traits among offspring and influencing overall population adaptation and evolution.
• Evaluate the role of recessive alleles in genetic disorders and how they impact inheritance patterns within families.
  • Recessive alleles play a critical role in genetic disorders because many conditions arise from mutations in genes that are inherited in a recessive manner. For example, if both parents are carriers of a recessive allele for a disorder like cystic fibrosis, there is a 25% chance with each pregnancy that their child will inherit both copies of that allele and express the disease. Understanding these inheritance patterns helps families assess their risk for genetic disorders and informs strategies for genetic counseling, population screening, and research into therapeutic interventions.

Related terms

dominant allele: A dominant allele is a variant of a gene that can mask the expression of a recessive allele when both are present in an organism.

homozygous: An individual is homozygous when they have two identical alleles for a particular gene, whether dominant or recessive.

genotype: The genotype is the genetic makeup of an organism, including the specific alleles inherited from its parents.