Complementarity
In complementary interaction, the simultaneous presence of two dominant non-allelic genes in the genotype leads to the development of an entirely new trait. At the same time, each dominant gene individually (if the second one is recessive) can manifest its effect independently.
The essence of the mechanism lies in the fact that the trait is formed as a result of a multi-step process, where each stage is controlled by a separate enzyme. The genes literally complement each other's work.
Examples of complementarity:
- Comb shape in chickens. Crossing individuals with pea-combs and rose-combs produces F1 hybrids with a new form — walnut-comb. In the F2 generation, classic Mendelian segregation of 9:3:3:1 is observed.
- Coat color in mice. Genes for pigment synthesis and its distribution along the hair shaft interact. When crossing dihybrids, the segregation in F2 changes to 9:3:4 (where 4 parts are white coat, as the absence of pigment masks its distribution).
- Hearing in humans. Normal hearing requires two dominant genes: one is responsible for the development of the cochlea of the inner ear, and the second for the auditory nerve. In dihybrid parents, the segregation into hearing and deaf children is 9:7.
- Vision in humans. The development of the visual analyzer requires three unlinked dominant genes (for the lens, retina, and optic nerve).
Epistasis (Epistatic interaction)
In epistasis, one of the non-allelic genes suppresses the action of another. The gene that suppresses is called epistatic (suppressor or inhibitor), and the one being suppressed is called hypostatic.
Depending on which allele acts as the suppressor, epistasis is divided into two types:
- Dominant epistasis. The dominant allele possesses epistatic action.
- Horse coat color inheritance: the gene for premature greying suppresses the genes for black and chestnut coats. When crossing dihybrids, the segregation is 12:3:1 (12 parts are grey horses).
- Chicken plumage color: the inhibitor gene suppresses the black color gene. Segregation in F2 is 13:3 (13 parts are white chickens).
- Recessive epistasis. Epistatic action manifests only in the recessive homozygous state.
- The Bombay phenotype in humans: the H gene is required for the synthesis of the blood group antigen precursor. If a person has the hh genotype (recessive homozygote), antigens A and B are not formed, even if alleles $I^A$ or $I^B$ are present in the genotype. Phenotypically, such individuals always present with blood group I (0). At the same time, the hidden gene can be transmitted to descendants (for example, a child with blood group IV can be born if the epistatic effect is lifted by a dominant allele from the second parent).
Polygenic inheritance
Polygenic inheritance is a type of interaction in which several different genes are responsible for the formation of the same trait, enhancing or weakening it. Usually, polymeric genes control quantitative traits: height, weight, intellectual abilities, and pigmentation intensity.
The degree of expression of a trait depends on the total number of dominant alleles in the genotype — this is the so-called additive (cumulative) effect. The more dominant alleles, the stronger the trait is expressed. Since genes perform the same function, they are usually designated by a single Latin letter with different numerical indices (e.g., $S_1, S_2, S_3$).
Human example: Skin pigmentation is controlled by three pairs of polygenic genes. A person with the darkest skin color has a genotype consisting only of dominant alleles ($S_1S_1S_2S_2S_3S_3$), while a person with white skin has only recessive ones ($s_1s_1s_2s_2s_3s_3$). When mulatto individuals (trihybrids) intermarry, the probability of having a child with extreme phenotypic variants (completely white or completely black) is only 1/64, since the possible allele combinations are $4^3 = 64$.
Briefly: answers to topical questions
What enzyme is encoded by the H gene and is absent in the Bombay phenotype?
The sources do not name the specific enzyme encoded by the H gene. It is indicated that the H gene is responsible for the synthesis of the precursor of antigens A and B.
In the Bombay phenotype in recessive homozygotes (hh), this precursor is not synthesized, so antigens A and B are not formed, even if alleles $I^A$ or $I^B$ are present in the genotype. Phenotypically, blood group I (0) develops.
The "CEP" rule: Complementarity — genes work as a Team (complement each other). Epistasis — Egoist (one gene suppresses another). Polygenic inheritance — Plus (the effect of dominant alleles is summed up).
Class and exam questions
What is the main difference between complementarity and epistasis?
In complementarity, two genes jointly form a new unique trait that did not exist when they existed separately. In epistasis, one gene does not participate in creating a new trait, but simply blocks the work of another gene.
What phenotype ratio is characteristic of dominant epistasis?
Depending on the specific suppression mechanism, the offspring of dihybrids (F2) may show 12:3:1 segregation (as in horses) or 13:3 (as in chicken feather coloration).
Why are genes designated by the same letter in polygenic inheritance?
Since several non-allelic genes encode the development of the same quantitative trait and act unidirectionally (enhancing the effect), they are designated by the same letter with different numerical indices for ease of counting.
What else to study on this topic
- Molecular mechanisms of hearing and vision formation in humans
- Detailed analysis of crosses in the inheritance of comb shape in chickens
- Biochemistry of the Bombay phenotype (the role of the H gene)
- Problems on calculating segregation ratios (9:3:4, 12:3:1, 13:3) in gene interactions
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