Medical Genetics - V. M. Zaporozhan 2005

Monogenic Disorders
Clinical Features and Genetics of Certain Monogenic Disorders
Sex-Linked Inheritance - Y-Chromosome-Linked Inheritance

The Y chromosome is the smallest human chromosome, containing roughly 2–3% of the human haploid genome. Pseudoautosomal regions are located at the tips of both the short and long arms, where genetic material can be exchanged with the X chromosome during Meiosis. Consequently, the genes in these regions are inherited like autosomal genes (partial sex-linkage). This phenomenon explains certain hereditary disorders that exhibit X-linkage in some families and Y-linkage in others (known as "mixed inheritance"). Partially sex-linked disorders include total color blindness (complete inability to distinguish colors, resulting in black-and-white Vision) and certain genodermatoses. In pedigrees, it can be challenging to distinguish partial sex-linkage from autosomal inheritance.

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Fig. 6.28. Hypertrichosis of the pinna

The major portion of the Y chromosome does not participate in recombination, representing the haploid segment of The Human Genome. It comprises a large heterochromatic region, which varies in length among different males, and a euchromatic region. The key genes within this region determine sex (SRY), height (GCY), and control gametogenesis (AZF).

More than 40 genes have been mapped to the Y chromosome, of which only 7 are known to cause hereditary disorders, including retinitis pigmentosa, sex-differentiation disorders, azoospermia, Dyschondrosteosis, and Gonadoblastoma. It was previously hypothesized that Y-linked inheritance might account for a form of ichthyosis ("porcupine man" Skin), hypertrichosis of the pinna, and Syndactyly of the toes. Subsequent studies confirmed holandric inheritance exclusively for hypertrichosis of the pinna (Fig. 6.28).

Y-linked traits occur solely in males, are transmitted from a father to all his sons, and are never passed on to daughters. Pedigree characteristics are detailed in section 4.2.3. Mutations in the Y-chromosomal region responsible for Spermatogenesis are not inherited because affected individuals are infertile. Modern assisted reproductive technologies, such as In vitro Fertilization, enable such men to have offspring, but all their sons will carry the exact same mutation.