Multiple allelism

Etiology and Mechanism of Occurrence

The appearance of multiple alleles is based on mutations. Most genes in an organism have a complex structure and consist of thousands of nucleotides. This creates a colossal basis for genetic variability.

Mutations represent the substitution of original nucleotides or a change in their number. Since such structural changes can occur in completely different regions (loci) of the same gene, new unique allele variants accumulate in the population over time. This process shapes the diversity of the species' gene pool and has critical evolutionary significance, allowing populations to adapt to changing conditions.

Key Inheritance Rules

To understand multiple allelism, it is necessary to strictly distinguish between the genetics of an individual organism and the genetics of a population as a whole:

1. At the individual level: Any normal diploid organism receives genetic material from two parents. Consequently, its genotype can contain only two alleles from the entire existing series — one inherited from the father, the other from the mother.
2. At the population level: The full diversity of variants of a given gene is present within the entire species. Different individuals carry different paired combinations of these alleles, forming the common gene pool.

For convenience, alleles of a single gene are usually designated by a single letter of the Latin alphabet, adding various superscripts to it (for example: $A^1, A^2, A^3, A^4$ and so on). Dominant wild-type traits are designated by uppercase letters, and recessive mutations by lowercase letters (for example, B is gray body, and b is black body; V is normal wings, and v is vestigial).

Classic Example: Coat Color in Rabbits

The most illustrative example of multiple allelism is the inheritance of coat color in rabbits. This trait is governed by a whole series of alleles with varying degrees of phenotypic effect, forming a complex dominance hierarchy: $C > c^{ch} = c^h > c$.

• $C$ (agouti) — wild type, providing solid dark coloration. This is an absolutely dominant gene.
• $c^{ch}$ (chinchilla) — provides solid gray (light or dark) coloration.
• $c^h$ (Himalayan) — pointed coloration: the body is white, but the tips of the ears, tail, paws, and nose are black.
• $c$ (albino) — completely white coloration. This is an absolutely recessive gene.

Interaction within the series: Intermediate alleles ($c^{ch}$ and $c^h$) behave differently depending on their genotypic "neighbor". Relative to the dominant wild type ($C$), they appear recessive. Relative to the albinism gene ($c$), they act as dominant. However, when meeting each other in the same genotype, the $c^{ch}$ and $c^h$ genes interact codominantly, meaning they are expressed jointly without fully suppressing each other.

Human Example: Blood Groups

In humans, the most well-known manifestation of multiple allelism is the inheritance of ABO blood groups. The genes responsible for this trait are represented in the population in three forms:

• $I^A$
• $I^B$
• $I^0$

At the same time, genes $I^A$ and $I^B$ are dominant relative to the recessive gene $I^0$. Any human carries only two alleles out of these three, and it is their combination that determines the final blood group.

Class and exam questions

What else to study on this topic

• Allele Interaction: Codominance using $c^{ch}$ and $c^h$ as an Example
• Molecular Mechanisms of Gene Mutations (Substitution and Frameshift)
• Evolutionary Significance of Gene Pool Polymorphism
• Inheritance of Recessive Mutations (Using Body Color and Wing Shape as Examples)

Other topics in «Patterns of Inheritance (Nuclear Genes)»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.