Biochemical basis of pigmentation
Hair and skin color are closely related to the body's metabolism. Normal metabolism of dietary proteins involves the stepwise transformation of amino acids. First, proteins are hydrolyzed into phenylalanine, which is converted into tyrosine by the enzyme phenylalanine hydroxylase. Then, tyrosine is converted into the final pigment, melanin, with the participation of the enzyme tyrosinase.
In genetic disorders such as phenylketonuria, a biochemical block of phenylalanine hydroxylase occurs. This leads to a deficiency of tyrosine and, consequently, a shortage of melanin. Patients with such enzymopathies often have light hair and skin pigmentation, demonstrating the direct impact of enzymatic defects on external traits.
Polygenic inheritance of color and structure
A trait such as hair color belongs to human polygenic traits. In polygenic inheritance, multiple non-allelic genes affect the same trait, enhancing its expression.
In addition to hair color, the polygenic action of genes determines:
- growth
- skin color
- eye color
- blood pressure level
- mental abilities
- muscle strength
The interaction of non-allelic genes in this case is quantitative in nature, forming a wide range of phenotypic variations in the population.
Y-linked and monogenic inheritance
Certain features of hair growth are transmitted strictly through the male line via the Y chromosome. This is the holandric (Y-linked) inheritance pattern:
- The genes are localized on the Y chromosome.
- The trait is transmitted from father to all sons and only to sons.
- Daughters never inherit the trait due to the absence of the Y chromosome.
- Penetrance for males is 100%.
Examples of traits: hypertrichosis of the auricles and excessive hair growth on the middle phalanges of the hand fingers.
Furthermore, hair anomalies can be part of complex genetic syndromes. For example, in Ellis-van Creveld syndrome (autosomal recessive inheritance pattern, mutation of the EVC gene on chromosome 4p16), symptomatic disturbances in the growth and structure of nails, hair, and teeth are observed, along with symmetric limb shortening and polydactyly.
Holandric — strictly in bare (bald) males (from father to sons); Polygenic — a POOL of multiple genes for a single trait.
Class and exam questions
Which enzymes are involved in the synthesis of hair and skin pigment?
Normal metabolism involves phenylalanine hydroxylase (converts phenylalanine to tyrosine) and tyrosinase (converts tyrosine to melanin).
How is hypertrichosis of the auricles inherited?
Hypertrichosis of the auricles is inherited in a holandric (Y-linked) manner: the trait is transmitted exclusively from father to sons.
What type of gene interaction determines hair color?
Hair color is a polygenic trait controlled by several non-allelic genes that mutually enhance each other's expression.
What else to study on this topic
- Population calculations of heterozygote and allele frequencies in phenylketonuria
- Chromosomal mechanisms of sex determination and types of heterogamety
- Complementary gene interaction and the Bombay phenotype
- Gene position effect using the Rhesus factor as an example
- Smith-Lemli-Opitz syndrome and other congenital enzymopathies
- Critical periods of embryonic development and the action of teratogens
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