Mechanism and experiments on Drosophila
The specific nature of sex-linked inheritance is due to chromosome structure. The X chromosome contains a large region that has no homologous counterpart on the Y chromosome. As a result, male individuals with an XY karyotype turn out to be hemizygous — genes in this unique region of the X chromosome are present in them in a single copy. Consequently, they always manifest in the phenotype, even if they are recessive (there is no second allele capable of suppressing them).
The pattern was discovered in experiments on Drosophila melanogaster. Thomas Morgan studied eye pigmentation in these insects, where the wild-type red-eye gene ($W$) dominates over the white-eye gene ($w$). When crossing a homozygous red-eyed female ($X^WX^W$) with a white-eyed male ($X^wY$), all offspring of the first generation turned out red-eyed. However, when the first-generation flies were intercrossed, differentiation occurred in the second generation: white-eyed males appeared, but not a single white-eyed female was found. This convincingly proved that the locus determining eye color is located exclusively on the X chromosome, and complete sex linkage occurs when genes are localized in non-homologous regions of chromosomes.
X-linked inheritance in humans
The human X chromosome contains over 1,000 genes, many of which are critical for the normal functioning of the body. The inheritance of such traits deviates from classical Mendelian laws because the X chromosome regularly passes from one sex to the other: daughters always receive one of their X chromosomes from their father, while sons receive their only X chromosome from their mother.
X-linked recessive traits: Manifest predominantly in males due to their hemizygosity. Classical examples of pathologies:
- Hemophilia (a blood clotting disorder).
- Color blindness (daltonism).
- Duchenne muscular dystrophy.
- Morris syndrome (testicular feminization associated with an androgen receptor defect).
X-linked dominant traits: Occur less frequently, transmitted to both males and females. These include follicular keratosis, vitamin D-resistant rickets (phosphate diabetes), and dental enamel hypoplasia.
Criss-cross inheritance
One of the striking features of sex-linked genes is criss-cross inheritance. This is a type of genetic transmission in which sons inherit a phenotypic trait from their mothers, and daughters inherit it from their fathers.
The mechanism operates in two ways:
- X-linked recessive type: The trait (or disease) is transmitted from a carrier mother to her sons.
- X-linked dominant type: The trait is transmitted from an affected father to all his daughters, since they invariably receive the affected X chromosome from him.
X chromosome pathology map
Mutations in the X chromosome cause a wide range of diseases, which can be systematized by the affected systems:
- Musculoskeletal and cutaneous: Duchenne myopathy, spinal and bulbar muscular atrophy (Kennedy's disease), ichthyosis, chondrodysplasia punctata, hypohidrotic ectodermal dysplasia.
- Hematological and immune: Hemophilia A and B, immunodeficiency with hyper-IgM, one of the forms of agammaglobulinemia.
- Metabolic: G6PD deficiency, Lesch-Nyhan syndrome.
- Neurological: Fragile X syndrome.
- Visual: Color blindness, retinitis pigmentosa.
Holandric (Y-linked) inheritance
The Y chromosome carries genetic information divided into two categories: genes also present on the X chromosome (responsible, for example, for total color blindness, xeroderma pigmentosum, or hemorrhagic diathesis), and unique genes lacking homologues.
The inheritance of traits encoded exclusively on the Y chromosome is called holandric. Its main feature is the transmission of the trait strictly through the male line: from father to absolutely all his sons. Females cannot develop such traits.
Unique genes of the Y chromosome control testis differentiation (spermatogenesis factor), the expression of histocompatibility antigens, and even tooth size. Examples of holandric phenotypic traits and pathologies:
- Hypertrichosis: Excessive hair growth on the auricle, manifesting in adulthood.
- Syndactyly: Formation of webbing between the second and third toes.
- Gonadal dysgenesis: Associated with a mutation in the key SRY (TDF) gene.
Briefly: answers to topical questions
What is the physiological role and function of the SRY (TDF) gene on the Y chromosome?
The SRY gene encodes the regulatory factor TDF (Testis-Determining Factor), which directs the differentiation of initially bipotential gonads along the male pathway. It is essential for testis differentiation and male development during embryogenesis.
To memorize criss-cross inheritance, imagine the letter "X" (crossing). The upper ends of the letter are the parents, the lower ends are the children. A recessive trait "flows" diagonally from mother to son, and a dominant trait from father to daughter.
Class and exam questions
Why do men suffer from X-linked recessive diseases more often?
Males have only one X chromosome (XY karyotype), meaning they are hemizygous for the genes located on it. If this chromosome carries a recessive mutation, the organism lacks a second, "healthy" allele to compensate for it, so the pathology will inevitably manifest.
What is hemizygosity?
This is a state in which a gene (and the trait it controls) is present in the organism's genotype in a single copy, without a pair. In humans, males are hemizygous for genes on the non-homologous regions of the X and Y chromosomes.
Are holandric traits transmitted to daughters?
Never. Holandric traits are controlled by unique regions of the Y chromosome, which is transmitted exclusively from father to sons. Females lack the Y chromosome.
What else to study on this topic
- The mechanism of action of the SRY (TDF) gene and the development of gonadal dysgenesis.
- Biochemical foundations of Lesch-Nyhan syndrome and G6PD deficiency.
- Molecular pathogenesis of Morris syndrome (androgen receptor defect).
- Genetics of fragile X syndrome and associated neurological disorders.
- Differences in the inheritance mechanisms of hemophilia types A and B.
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