Genetics and Fundamentals of Selection - M.P. Myhun - 2008

CHAPTER IV. Variability and Its Genetic Foundations

4.3. The Phenomenon of Multiple Allelomorphism

METABOLISM/2.html">THE CONCEPT OF the Gene as an indivisible unit of recombination, mutation, and function was scientifically valid in the early Stages of Genetics. Later, it became clear that the gene is a complex unit of heredity. Numerous studies have shown that the same gene can mutate and assume various states. This is clearly demonstrated by fur coloration in rabbits. It has been established that full coloration in rabbits is a dominant trait represented by black color C. It dominates over the Himalayan (ermine) fur coloration of the ChCh genotype, in which the extremities, tail, Nose, and ears are black while the rest of the body is white. When crossed with an albino rabbit, both fully dark and Himalayan fur colorations dominate over the pure white coloration of the cc genotype. When a Himalayan rabbit is crossed with a white (albino) one, the F₁ generation is Himalayan, and the F₂ generation exhibits a segregation ratio of 3 Himalayan : 1 white (typical of monohybrid inheritance).

Thus, each pair of genes in this series of multiple alleles behaves in segregation like a single allelic pair. If they were non-allelic, segregation corresponding to dihybrid or polyhybrid crosses should be observed. However, this does not occur. Testing other mutant genes of this series reveals monohybrid crossing in all cases. The first member of the C gene dominates over all others, while c^a (albino) is recessive to all members of this allelic series. Other members of this series exhibit intermediate coat pigmentation (traits).

Consequently, the fur color gene in rabbits manifests in three alternative states, and in general, such states for certain genes can number in the dozens.

Any gene A can mutate to states a¹, a², a³...aⁿ, and gene B to states b¹, b², b³...bⁿ.

We already know that genes influencing The Development of the same trait and localized at homologous loci of homologous Chromosomes are called allelic.

Since the same gene can exist in many different states that together form a series of multiple alleles, this phenomenon is termed multiple allelomorphism.

It should be noted that under normal conditions, only a single allele from the series of multiple alleles can be expressed at a chromosome locus. This phenomenon has been discovered in higher animals (cattle, rabbits, mice, guinea pigs, Drosophila) and plants, such as corn, tobacco, and peas. In humans, it is exemplified by Blood Groups.

The widespread occurrence of this phenomenon is due to several reasons.

1. Multiple allelomorphism increases the reserve of mutational Variability in evolution.

2. It broadens the range of combinatorial variability.

3. Multiple allelomorphism indicates that gene mutation is not the ultimate unit of hereditary variability. The gene has a complex Structure.

The Study of multiple alleles has demonstrated that the gene is a divisible unit of heredity (as discussed in Section 2.2).