Dominant allele

Definition

A dominant allele is a variant of a gene that expresses its trait even when only one copy is present in an organism's genotype. This means that if an individual has at least one dominant allele for a specific trait, that trait will be observed in their phenotype, overshadowing any recessive alleles that may also be present. Understanding dominant alleles is crucial for grasping fundamental genetic concepts like inheritance patterns and the expression of traits.

5 Must Know Facts For Your Next Test

1. In genetic notation, dominant alleles are often represented by uppercase letters (e.g., 'A'), while recessive alleles are denoted by lowercase letters (e.g., 'a').
2. Dominant alleles can lead to the expression of certain traits, such as brown eyes or the ability to roll one's tongue, even if only one copy is inherited from a parent.
3. When considering genetic crosses, the presence of dominant alleles can significantly influence the phenotypic ratio among offspring.
4. A carrier of a recessive trait may possess one dominant allele and one recessive allele, but will not express the recessive trait themselves.
5. The concept of dominant and recessive alleles is foundational to Mendelian genetics and helps explain how traits are passed down through generations.

Review Questions

• How does the presence of a dominant allele affect the phenotype of an organism?
  • The presence of a dominant allele affects an organism's phenotype by determining which trait will be expressed. If an organism has at least one dominant allele for a particular trait, that trait will be visibly manifested, regardless of whether any recessive alleles are also present. This means that traits governed by dominant alleles can overshadow those controlled by recessive alleles, making it crucial for understanding inheritance patterns.
• Compare and contrast homozygous and heterozygous genotypes in relation to dominant alleles.
  • Homozygous genotypes consist of two identical alleles for a specific gene, which could either both be dominant or both be recessive. In contrast, heterozygous genotypes have one dominant allele and one recessive allele. In the case of a heterozygous genotype, the presence of the dominant allele will determine the phenotype. Therefore, while homozygous individuals may express a trait associated with either only dominant or only recessive alleles, heterozygous individuals will always express the trait associated with the dominant allele.
• Evaluate how understanding dominant alleles can enhance our approach to genetic counseling and predicting inheritance patterns.
  • Understanding dominant alleles is essential in genetic counseling because it allows counselors to predict how traits may be inherited within families. By analyzing the presence of dominant and recessive alleles in parents, genetic counselors can estimate the likelihood of certain traits appearing in offspring. This knowledge helps families make informed decisions about health risks and potential genetic disorders, enabling proactive measures for managing inherited conditions. Additionally, this understanding aids researchers in exploring complex genetic interactions and how they might influence various traits across generations.

Related terms

recessive allele: A recessive allele is a variant of a gene that only expresses its trait when two copies are present in an organism's genotype, meaning it can be masked by a dominant allele.

homozygous: An organism is homozygous when it has two identical alleles for a specific gene, which can either be both dominant or both recessive.

heterozygous: An organism is heterozygous when it has two different alleles for a specific gene, with one being dominant and the other recessive.