Genetics - A. V. Sivolob 2008

Formal genetics: patterns of trait inheritance
Deviations from Mendelian segregations
Causes of statistically significant deviations from Mendelian segregation ratios

It should be noted that, as follows from the cytological basis of heredity described in Chapter 1, Mendelian segregation can in principle only occur in species with normal sexual reproduction, diploidy in both sexes, and without meiotic abnormalities.

However, even when these conditions are met, DEVIATIONS FROM MENDELIAN Ratios are quite common. This does not mean that Mendelian principles are violated; rather, they are modulated by additional effects. Three such effects—non-allelic Gene interaction, linkage of gene groups within the same chromosome, and the Location of a gene on sex Chromosomes—are discussed in separate subsections. Other causes include the following:

1. Lethality of certain allele combinations. For example, when crossing yellow (heterozygous) mice, the segregation ratio of yellow to black (recessive homozygotes) is always 2 : 1. A similar result is observed when crossing platinum foxes. Examination of pregnant females reveals that a quarter of the embryos die before birth—specifically, homozygotes for the dominant allele, which explains the deviation from the expected 3 : 1 segregation ratio (one of the four genotypic classes is missing). Thus, in this example, the dominant allele is truly dominant only with respect to coat color, whereas regarding viability, the same allele acts as a recessive. This case, where a single gene influences multiple traits simultaneously (coat color and viability), is a classic example of pleiotropy.

2. Certain alleles are phenotypically expressed only in a fraction of the organisms that carry them in their genotype. The concepts of penetrance and expressivity are used to describe such variable gene manifestation. Penetrance is measured as the proportion of individuals showing a specific phenotype among all those with the identical genotype. Some human Hereditary diseases, such as Gout, exhibit incomplete penetrance, meaning that not all individuals carrying the mutation develop the disease. Expressivity reflects the degree or severity of phenotypic manifestation of a mutant gene. For instance, Drosophila flies homozygous for the mutant eyeless allele can have varying numbers of eye facets—ranging from a normal count to their complete absence. The capacity of a genotype to manifest differently depending on external conditions reflects its norm of reaction, which is the range of phenotypic plasticity within which traits can vary in response to environmental fluctuations during development.

3. Some traits are not expressed throughout an Organism's entire life but appear only at a specific stage, meaning the segregation pattern can be age-dependent. For example, certain human hereditary syndromes manifest only at an advanced age (e.g., Alzheimer's disease, Huntington's disease).

4. Genomic Imprinting is another phenomenon that leads to deviations from expected Mendelian ratios. It is based on Epigenetic Inheritance—the transmission from parents not just of DNA, but of Chromatin carrying specific chemical markers that determine the structural state of a region and, consequently, The activity of certain genes (see Chapter 6). As a result of this effect, the expression of a given gene may depend entirely on which parent contributed the chromosome.

5. In nature, many species exhibit Sexual Dimorphism, and sex plays a major role in the phenotypic expression of a wide range of traits (see the subsection on sex linkage, as well as Chapter 6).

In fact, all the effects mentioned above can ultimately be reduced to gene interactions within the complex functional System of the genome, although such interactions are not always easy to describe, as it is often necessary to account for a multitude of interacting elements.