Variability in genetics

Classification of forms of variability

Genetics distinguishes two fundamentally different forms of variability, depending on whether the organism's DNA is affected.

• Non-hereditary (modification or phenotypic). It arises under the influence of environmental factors. It does not alter the genotype and is always strictly limited by the norm of reaction.
• Hereditary (genotypic). It is always a consequence of changes in the genetic apparatus and is transmitted to offspring.

Genotypic variability, in turn, is divided into two subtypes:

1. Combinative variability (gene recombination) — the structure of the genes themselves remains the same, but new combinations of them arise.
2. Mutational variability (mutations) — a persistent change in the structure of the genetic material occurs, leading to the appearance of new phenotypes.

Mechanisms of combinative variability

Genetic diversity during sexual reproduction is ensured by three key processes. All of them are aimed at recombining paternal and maternal alleles.

• Crossing over. This is the exchange of segments between homologous chromosomes, which occurs at the pachytene stage in prophase I of meiosis. Chiasma formation can occur in different regions and involve varying amounts of genetic material. Single and double exchanges are distinguished (the latter can involve two, three, or even four chromatids of a bivalent). Important condition: the mechanism is effective only in heterozygotes (Aa).
• Independent assortment of chromosomes. In the equatorial plane of the spindle apparatus, bivalents are arranged completely at random. In anaphase I of meiosis, non-homologous chromosomes diverge to the poles, forming unique combinations in the haploid set of future gametes.
• Fertilization. The random meeting and fusion of different types of sex cells.

Role of meiosis and fertilization

The main task of meiosis is the formation of sex cells with a haploid set of single-stranded chromosomes. This result is achieved because DNA replication occurs only once, while division follows twice. The relative autonomy of chromosomes allows for the regular distribution of information and efficient recombination of genetic material.

During fertilization (both cross- and self-fertilization), the spermatozoon introduces its haploid set into the egg. The nuclei fuse, and a diploid zygote nucleus is formed. In it, each linkage group is represented again in double — by paternal and maternal chromosomes. Thus, meiosis and fertilization guarantee the preservation of the species karyotype across generations, while structural changes in the genome are most often associated with disruptions of meiotic processes specifically.

Variability and the modern synthesis of evolution

Biological evolution (evolutio — unrolling) is an irreversible process of nature's development having an adaptive direction. Its global outcome is the emergence of new species and the transformation of the biosphere.

The modern synthetic theory of evolution (STE) combined classical Darwinism with data from genetics, ecology, and paleontology. A crucial step in this direction was made by S.S. Chetverikov in 1926. He proved that natural populations are saturated with mutant recessive genes, "like sponges are saturated with water." These mutations are masked in the heterozygous state by dominant alleles.

Main postulates of the STE:

• The elementary unit of evolution is the population, not an individual organism.
• The elementary evolutionary material is hereditary variability (mutational and combinative).
• The elementary evolutionary phenomenon is a stable change in the genotypic composition of a population.
• Speciation (the emergence of reproductive isolation mechanisms) is considered as the central qualitative stage of evolution.

Class and exam questions

What else to study on this topic

• Stages of prophase I of meiosis (pachytene and bivalent formation)
• Types of crossing over: single and multiple exchanges
• Differences between meiosis and mitosis
• Mechanisms of reproductive isolation in speciation
• Population polymorphism and species criteria within the framework of STE

Other topics in the "Additional Topics" section

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