Interaction of allelic genes
Allelic genes are variants of the same gene occupying identical loci on homologous chromosomes. Several types of their interaction are distinguished.
- Complete dominance — the dominant allele completely suppresses the recessive one; heterozygotes are phenotypically identical to dominant homozygotes.
- Incomplete dominance — heterozygotes develop an intermediate trait (e.g., pink flowers from crossing red and white ones).
- Sex-linked dominance — the manifestation of the allele depends on sex: in sheep, the gene for horns is dominant in males and recessive in females.
- Codominance — both alleles in a heterozygote are expressed independently and simultaneously (AB0 blood group system).
- Interallelic complementation — protein subunits encoded by different "broken" alleles compensate for each other's defects (bacterial β-galactosidase).
- Overdominance — the heterozygote surpasses both homozygotes in viability; a classic example is resistance to malaria in heterozygotes for sickle-cell anemia.
- Allelic exclusion — only one allele is expressed in a cell; an example is the inactivation of one X chromosome in female mammals, the formation of Barr bodies, and the mosaic coat color of calico cats.
- Lethal alleles — lead to the death of the organism. Recessive lethals manifest only in homozygotes (absence of chlorophyll in plants), while dominant ones are lethal in the homozygote, and heterozygotes are carriers (yellow coat color in mice).
Interaction of non-allelic genes
Traits can be controlled by multiple genes at different loci.
- Polygeny — a quantitative trait is determined by multiple equivalent genes. Non-cumulative: a single dominant allele is sufficient (presence of awns in wheat). Cumulative: the severity of the trait is proportional to the number of dominant alleles (human skin pigmentation).
- Epistasis — suppression of one gene by another non-allelic one. Dominant epistasis: a suppressor gene in the dominant state blocks the hypostatic gene (pumpkin color). Recessive epistasis: suppression due to homozygosity for the recessive allele of the suppressor gene. Double recessive epistasis — the trait appears only in the presence of dominant alleles at both loci.
- Complementarity — a new trait appears only with the simultaneous presence of dominant alleles of two or more non-allelic genes (synthesis of purple pigment from two colorless precursors).
Penetrance and Expressivity
Expressivity — the degree of phenotypic expression of an allele in a particular individual. Penetrance — the proportion of individuals in a population in whom the allele is phenotypically expressed relative to all carriers of that allele. These indicators explain why the same genotype can produce different manifestations of a trait.
Combinative variability
Associated with new combinations of existing genes. Sources:
- Crossing over — exchange of segments of homologous chromosomes in prophase I of meiosis, forming new allele combinations.
- Independent assortment of chromosomes in anaphase I of meiosis — each gamete receives a unique set of maternal and paternal chromosomes.
- Random fertilization — any gamete can fuse with any other, which drastically increases offspring diversity.
Mutational variability
According to mutation theory, mutations are sudden, discrete, inheritable, non-directed, and can occur repeatedly. Based on the scale of genetic material affected, they are distinguished into:
- Gene (point) — substitutions, deletions, or insertions of individual nucleotides.
- Chromosomal — changes in chromosome structure: deletions, inversions, duplications, translocations.
- Genomic — changes in chromosome number: polyploidy, monosomy, trisomy.
By cell type, mutations are divided into generative (in germ cells, passed to offspring) and somatic (in body cells, not inherited). Causes include physical, chemical, and biological mutagens; artificial induction of mutations for selection is possible.
Modificational variability
Non-hereditary phenotypic changes under environmental influence without altering the genotype. The key concept is the reaction norm: the limits of trait variability for a given genotype (e.g., a cow's milk yield is determined by genotype, but the specific volume is determined by feeding and care). Morphoses — sharp non-hereditary changes caused by extreme factors (change in coat color of an ermine rabbit upon local cooling). Phenocopies arise under the action of teratogens (alcohol, certain drugs, rubella virus) and mimic hereditary defects — for example, congenital deafness.
Methods for studying human genetics
- Genealogical method — construction and analysis of pedigrees to determine the type of inheritance: autosomal dominant (trait in every generation), autosomal recessive (skipped generations, birth of affected children to healthy parents), sex-linked.
- Twin method — comparison of monozygotic and dizygotic twins. High concordance in monozygotes indicates the leading role of the genotype, while discordance indicates the contribution of the environment. The heritability coefficient of the trait is calculated.
- Cytogenetic method — microscopic study of the karyotype. Blood lymphocytes are stimulated to divide, stopped at metaphase, chromosomes are photographed, and a karyogram is compiled. The method detects chromosomal diseases — Down, Klinefelter, Turner syndromes.
Key points
- Allelic genes interact via complete and incomplete dominance, codominance, overdominance, interallelic complementation, allelic exclusion, and lethal alleles.
- Non-allelic genes interact through polygeny, epistasis, and complementarity.
- Penetrance and expressivity describe the frequency and strength of allele expression.
- Combinative variability is ensured by crossing over, independent chromosome assortment, and random fertilization.
- Mutations are divided into gene, chromosomal, and genomic; and into generative and somatic.
- Modificational variability is limited by the reaction norm and is not inherited.
- Teratogens cause phenocopies that mimic hereditary diseases.
- The main human genetics methods are the genealogical, twin, and cytogenetic methods.