Essence of the phenomenon and classical example
Under complete dominance, the action of the dominant gene (denoted by a capital letter A) leaves no chance for the expression of the recessive gene (a). This means that organisms with genotypes AA (homozygous for the dominant trait) and Aa (heterozygous) will look completely identical.
The classical example used to study this pattern is pea seed color. The dominant allele determines yellow color, and the recessive allele determines green. In the offspring population from heterozygous parents, the following genotypes are formed:
- AA — yellow peas;
- Aa — yellow peas (the dominant gene completely suppressed the recessive one);
- aa — green peas.
As a result, the genotypic segregation is strictly 1 : 2 : 1 (one part AA, two parts Aa, and one part aa). However, since heterozygotes are visually indistinguishable from dominant homozygotes, the phenotypic segregation appears as 3 : 1 — for every three yellow peas, there is only one green pea.
Dominant human traits: appearance
In humans, a vast number of morphological features are inherited precisely according to the principle of complete dominance. They can be divided into several large groups that shape our external appearance.
Eyes and face Dominant genes determine brown eye color and large eyes, Mongoloid facial structure, straight palpebral fissures, and the presence of an epicanthus (overhanging upper eyelid). Interestingly, myopia (nearsightedness) is also a dominant trait relative to normal vision. On the face, dimples in the cheeks, wide ears, bushy eyebrows, and a long, straight, and wide chin are inherited dominantly. A thick and drooping lower lip suppresses the normal lip gene.
Oral cavity The ability to roll the tongue into a tube or bend it backward are genetically determined dominant skills. This also includes protruding teeth and jaws, the presence of a gap between incisors (diastema), and even a rare feature — the presence of teeth in an infant at birth.
Skin and hair Dark and thick skin dominates over white and thin skin. The presence of freckles and elliptical patterns on the fingertips also predominate (the recessive trait is circular patterns). In hair characteristics, curly hair, hair growth along the midline of the forehead (the so-called "widow's peak"), and the presence of a white lock of hair above the forehead are dominant. Early greying (as early as age 25) completely suppresses late greying (after age 40). Baldness is dominant in men and recessive in women.
Limbs Right-handedness stably dominates over left-handedness. In addition, polydactyly (having six or seven fingers on a hand) is a dominant trait, whereas our usual five-fingered norm is recessive.
Physiological and biochemical traits
In addition to appearance, complete dominance determines the most important internal characteristics of the organism.
In the ABO blood group system, antigens A and B are dominant, completely suppressing the absence of antigens — the zero (first) blood group. The Rhesus factor works similarly: the presence of the antigen (Rh+) dominates over its absence (Rh-).
Genetics also influences musical and vocal data. Absolute pitch (perfect pitch) is a dominant trait. As for the voice, in women soprano dominates over alto, and in men bass predominates over tenor. Normal hearing completely suppresses genes for hereditary deafness.
Briefly: answers to topical questions
How does complete dominance differ from incomplete dominance and codominance?
The main difference lies in the degree of suppression of the recessive allele and the phenotypic expression of the trait in heterozygotes.
| Inheritance pattern | Mechanism of allele action | Heterozygote phenotype |
|---|---|---|
| Complete dominance | The dominant allele completely suppresses the recessive one | Only the dominant trait is expressed |
| Incomplete dominance | The dominant gene does not completely suppress the recessive one | An intermediate trait is formed |
| Codominance | Each of the alleles manifests its effect independently | A new trait arises |
What hereditary diseases and syndromes, besides polydactyly, are inherited in humans in an autosomal dominant manner?
In addition to polydactyly, the following single-gene monogenic diseases and syndromes are inherited in humans in an autosomal dominant manner:
- Achondroplasia (Achondroplasia) — an autosomal dominant form of congenital dwarfism; caused by a defect in the FGFR3 gene.
- Marfan syndrome — a hereditary metabolic disease with systemic involvement of connective tissue.
- Osteogenesis imperfecta (Osteogenesis Imperfecta) — a hereditary skeletal disorder.
- Huntington's chorea.
- Myotonic dystrophy.
- Hereditary spherocytosis.
- Neurofibromatosis type 1.
- Colorectal polyposis.
- Familial hypercholesterolemia.
To easily remember the 3:1 phenotypic segregation using the pea example, imagine a traffic light showing three yellow signals (dominant) and only one green signal (recessive).
Class and exam questions
How to externally distinguish a heterozygote from a dominant homozygote?
Under complete dominance—not at all. The heterozygote (Aa) will phenotypically look identical to the dominant homozygote (AA), as the recessive trait is completely masked.
Which eye color is dominant?
Brown eye color dominates over blue. The presence of the dominant gene ensures pigment production, while blue eyes are the result of the expression of a recessive allele.
Is it true that nearsightedness is transmitted as a dominant trait?
Yes, myopia (nearsightedness) is a dominant manifestation, and this gene completely suppresses the gene for normal visual acuity.
What else to study on this topic
- Molecular basis of inheritance of the ABO blood group system
- Genetics of polydactyly: why the dominant trait is rare
- Formation of the epicanthus and the Mongoloid eye type
- Inheritance of the Rhesus factor and Rhesus incompatibility
- Physiological causes of bass dominance over tenor
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