A dominant allele is a variant of a gene that expresses its trait in the phenotype even when only one copy is present in the organism. This means that if an individual has at least one dominant allele, that trait will be visible, overshadowing any recessive alleles they may also possess. Understanding dominant alleles is crucial for grasping the basic principles of heredity outlined by Mendel, which laid the groundwork for modern genetics.
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Dominant alleles are represented by uppercase letters (e.g., 'A') while recessive alleles are typically shown in lowercase (e.g., 'a').
An individual with a genotype of 'AA' or 'Aa' will display the dominant phenotype due to the presence of at least one dominant allele.
Mendel's experiments with pea plants demonstrated how dominant and recessive alleles interact during inheritance, leading to predictable patterns of trait expression.
In a Punnett square, dominant alleles are placed on top and side, illustrating how they can dominate over recessive alleles in offspring.
Dominant alleles can arise from mutations in genes and may lead to specific traits or disorders that are more likely to manifest if at least one copy is present.
Review Questions
How does the presence of a dominant allele influence an organism's phenotype?
A dominant allele influences an organism's phenotype by ensuring that its associated trait is expressed whenever at least one copy is present. This means that individuals with either one or two copies of a dominant allele will show the characteristic associated with that allele. For instance, in Mendel's pea plants, a plant with the genotype 'Aa' would exhibit the trait controlled by the dominant allele, overriding any effect from the recessive allele 'a'.
Discuss how Mendel's experiments contributed to our understanding of dominant and recessive alleles.
Mendel's experiments with pea plants established foundational principles of genetics, demonstrating how traits are inherited through dominant and recessive alleles. By carefully observing the offspring of cross-pollinated plants, he found consistent patterns in trait inheritance. His results showed that dominant alleles would mask recessive ones in heterozygous individuals, leading to predictable ratios in progeny. This work laid the groundwork for genetic science and our understanding of how traits are passed down through generations.
Evaluate the role of dominant alleles in genetic variation and evolutionary processes.
Dominant alleles play a significant role in genetic variation and evolutionary processes by influencing which traits are favored or selected in a population. When a dominant allele confers an advantageous trait, it is more likely to be passed on to future generations, potentially leading to rapid changes in a population's characteristics. Furthermore, understanding how these alleles interact with environmental factors can provide insights into evolutionary pressures, helping to explain biodiversity and adaptation within species over time.
A recessive allele is a variant of a gene that must be present in two copies to express its trait in the phenotype, meaning it is masked by a dominant allele.
genotype: The genotype refers to the genetic makeup of an individual, including the specific alleles they carry, whether dominant or recessive.
phenotype: The phenotype is the observable expression of traits in an organism, determined by the interaction of its genotype with the environment.