Genomic mutations

Haploidy: reduction of the genetic set

All genomic mutations are based on quantitative shifts. Haploidy is characterized by a reduction in the standard chromosome number by exactly half. As a result, somatic cells of the organism receive a single set ($n$), which under normal physiological conditions is typical exclusively for germ cells.

In the natural environment, this state is a normal part of the life cycle of bacteria, lower fungi, and various unicellular algae. In the animal kingdom, haploidy can be induced artificially by applying specific stimulation of sperm or egg development.

Phenotypically, organisms with a haploid set are noticeably smaller than their counterparts. They are completely sterile, and all present recessive genes begin to actively manifest at the trait level.

Polyploidy: proportional increase in chromosomes

Polyploidy is a mutation in which the total number of chromosomes increases in exact multiples of the basic haploid set (resulting in $3n$, $4n$, $5n$ sets, and so on).

Features in different organisms:

• In plants: This mutation is extremely widespread among cultivated species, such as wheat, rice, oats, potatoes, beets, pears, and apples. The main advantages of such plants are significantly greater vegetative mass and a 20–50% higher yield compared to diploid predecessors.
• In animals: Extremely rare. Polyploidy is typical only for certain hermaphrodites (e.g., earthworms), as well as isolated species of amphibians, fish, crustaceans, and insects.
• In humans: Polyploidy (specifically triploidy and tetraploidy) is completely incompatible with life. Such genetic disorders are detected only upon examination of biological material after spontaneous abortions.

Main causes (etiology) of polyploidy:

1. Disruption of normal chromosome segregation during mitosis.
2. Pathological changes in meiosis leading to the formation of gametes with an unreduced (full) chromosome number.
3. Physical fusion of cells from various somatic tissues or directly of their nuclei.

Aneuploidy and human syndromes

Aneuploidy differs from polyploidy in that the change in chromosome number is not a multiple of the haploid set. The primary mechanism underlying this pathology is the failure of chromosome segregation during meiosis.

There are three main forms of aneuploidy:

• Monosomy: loss of a single chromosome from a specific pair.
• Nullisomy: complete loss of both chromosomes forming a pair.
• Polysomy: addition of one or more individual chromosomes to the standard karyotype.

In clinical practice, several classic examples of human aneuploidies lead to severe congenital syndromes:

1. Sex chromosome abnormalities:
2. Turner syndrome (karyotype 45, X0). This is a classic sex chromosome monosomy and the only monosomy compatible with human life.
3. Klinefelter syndrome (karyotype 47, XXY). This is a sex chromosome polysomy.
4. Trisomy X syndrome, or Triple-X (karyotype 47, XXX). Also belongs to the polysomies.

1. Autosomal abnormalities:
2. Down syndrome (karyotype 47, 21+). This is a polysomy (trisomy) characterized by an extra 21st chromosome in the karyotype.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of impaired chromosome segregation during mitosis
• Features of meiotic pathologies and the formation of unreduced gametes
• Methods of artificial stimulation of egg development to induce haploidy in animals
• Cellular processes during the fusion of somatic tissues and their nuclei

Other topics in the "Variability and its forms" section

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