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 organism has at least one dominant allele for a specific trait, the associated characteristic will be displayed, overshadowing the effects of any recessive alleles present.
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Dominant alleles are represented by uppercase letters, while recessive alleles are usually represented by lowercase letters in genetic notation.
In a monohybrid cross, the presence of a dominant allele can completely mask the expression of a recessive allele, leading to a phenotypic ratio in offspring of 3:1 in a typical Mendelian inheritance pattern.
Codominance occurs when two different dominant alleles are expressed equally in the phenotype, as seen in blood type AB where both A and B alleles are dominant.
Incomplete dominance is a situation where neither allele is completely dominant, resulting in a blended phenotype, such as pink flowers from red and white parent plants.
The concept of dominance is key to understanding Mendel's laws of inheritance, particularly his law of segregation, which states that alleles separate during gamete formation.
Review Questions
How do dominant alleles affect phenotypic expression in organisms?
Dominant alleles significantly influence phenotypic expression since they can manifest traits even when only one copy is present in the genotype. For example, in a monohybrid cross involving a trait controlled by one gene with one dominant and one recessive allele, the dominant trait will be expressed in the phenotype regardless of whether the organism has one or two copies of the dominant allele. This means that traits associated with dominant alleles can overshadow those linked to recessive alleles.
Compare and contrast codominance and incomplete dominance concerning dominant alleles.
In codominance, both alleles are fully expressed in the phenotype when present together, as seen in blood type AB where both A and B alleles contribute equally. In contrast, incomplete dominance results in a blended phenotype where neither allele is fully expressed; for example, crossing red and white flowers can produce pink offspring. While both codominance and incomplete dominance involve interactions between dominant and recessive alleles, they differ in how those dominant traits manifest in the phenotype.
Evaluate how Mendel's laws of inheritance relate to the concept of dominant alleles and their role in genetic variation.
Mendel's laws of inheritance, particularly the law of segregation and the law of independent assortment, provide a framework for understanding how dominant alleles contribute to genetic variation. The law of segregation states that during gamete formation, alleles segregate so that each gamete carries only one allele for each gene. This means that when a dominant allele is present, it can dominate the phenotype in offspring regardless of other alleles. The law of independent assortment further explains how different traits are inherited independently from one another, allowing for diverse combinations of dominant and recessive alleles to shape genetic diversity within populations.
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 the characteristic will not appear if a dominant allele is also present.
The genotype refers to the genetic constitution of an organism, specifically the alleles present for a given trait, which determine the potential characteristics expressed.
The phenotype is the observable physical or biochemical characteristics of an organism, determined by both genetic makeup (genotype) and environmental influences.