MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I.Fedoniuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Fundamentals of Human Genetics

Multiple Allelism

In G. Mendel's experiments, genes existed in only two forms: dominant and recessive. However, most genes are represented not by two, but by a greater number of alleles. In addition to the main alleles (dominant and recessive), intermediate alleles also exist. A series of alleles (three or more) of a single Gene is referred to as multiple alleles, and the phenomenon itself as multiple allelism. Multiple alleles arise As a result of repeated Mutations of the same chromosome locus. While the genotype of a diploid Organism contains only two alleles of a given gene, their number in a population is practically unlimited. A characteristic feature of interactions among multiple alleles is that they can be arranged in a hierarchical sequence in which each member is dominant to all subsequent ones and recessive to the preceding ones. For instance, in rabbits, solid dark coloration is determined by the dominant allele A, whereas homozygous recessive animals (aa) are white. Yet, several other alleles of this gene exist, each producing its own distinct phenotype in the homozygous state: chinchilla (a^cha^ch) and Himalayan (a^ha^h). Chinchilla rabbits are gray, while Himalayan rabbits are white, except for pigmented tips on their ears, tail, feet, and Nose. The entire series of alleles can be written as the dominance hierarchy A>a^ch>a^h>a. A dark coat color will be observed in rabbits with the genotypes AA, Aa^ch, Aa^h, and Aa; chinchilla color in those with a^cha^ch, a^cha^h, and a^cha; and Himalayan color in those with a^ha^h and a^ha.

Significance: multiple allelism enriches the gene pool of a population, enhancing its genotypic and phenotypic polymorphism, which is of great evolutionary importance.