In the field of
genetics, understanding the concept of dominance is crucial to comprehending how traits are inherited from one generation to the next. Dominance refers to the relationship between alleles, which are different versions of a gene. When an organism has two different alleles for a particular trait, one allele may exert influence over the other in determining the organism's appearance or phenotype. This allele is known as the dominant allele.
What is a Dominant Allele?
A
dominant allele is one that expresses its trait even when only one copy is present in the genotype. In contrast, a recessive allele only expresses its trait when two copies are present. When an individual inherits a dominant allele from one parent and a recessive allele from the other, the dominant allele will mask the expression of the recessive allele. This means the phenotype will exhibit the trait associated with the dominant allele.
How is Dominance Represented Genetically?
In genetic notation, dominant alleles are typically represented by uppercase letters, while recessive alleles are represented by lowercase letters. For instance, in the case of a simple Mendelian trait like flower color in peas, the allele for purple flowers (P) is dominant to the allele for white flowers (p). Therefore, both genotypes PP and Pp will result in purple flowers, showcasing how the dominant allele masks the recessive one.Are All Dominant Alleles Completely Dominant?
Not all dominant alleles are completely dominant. In some cases, dominance can be
incomplete or exhibit
codominance. In incomplete dominance, the heterozygous phenotype is an intermediate between the two homozygous phenotypes. For example, in some plants, a cross between a red-flowered plant and a white-flowered plant may result in pink flowers. In codominance, both alleles are fully expressed, as seen in the AB blood type in humans, where both A and B alleles are equally expressed.
Does Dominance Affect Frequency in a Population?
Interestingly, dominance does not necessarily correlate with the frequency of an allele in a population. A dominant allele is not inherently more common than a recessive one. The frequency of an allele in a population is influenced by various factors, such as
natural selection, genetic drift, and mutation rates. For example, the allele for polydactyly (having extra fingers or toes) is dominant, yet it is rare in human populations.
How does Dominance Relate to Genetic Disorders?
Dominance also plays a role in genetic disorders. Some disorders are caused by dominant alleles, known as
autosomal dominant disorders. These disorders require only one copy of the defective allele to manifest the disease. Examples include Huntington's disease and Marfan syndrome. Conversely,
autosomal recessive disorders require two copies of the defective allele for the disease to be expressed, such as cystic fibrosis and sickle cell anemia.
Can Dominance Change Over Time?
Dominance can be influenced by evolutionary forces over time. In certain environments, a previously recessive allele may become advantageous, and
selection pressure could increase its frequency. This shift might eventually lead to changes in the dominance relationship between alleles. Additionally, as new mutations arise, they can alter existing patterns of dominance.
Conclusion
The concept of dominance is a fundamental aspect of genetics that helps us understand how traits are inherited and expressed in living organisms. It is important to recognize that dominance does not imply superiority or increased frequency, but rather describes the interaction between alleles. As genetic research continues to advance, our understanding of dominance and its implications in various biological contexts will undoubtedly deepen.
