Genetics - A. V. Sivolob 2008

Genetics of Multicellular Eukaryotes
Genetics of Sex
Trait Inheritance and Sex

The Inheritance of Traits influenced by sex has distinct characteristics (see also the core principles of trait inheritance discussed in Chapter 3). Typically, these traits are divided into three groups: sex-linked traits, sex-limited traits, and sex-influenced traits.

Sex-linked traits are those whose genes are located on the sex Chromosomes X (Z) or Y (W) (these genes are not necessarily related to sexual characteristics). The X (Z) and Y (W) sex chromosomes differ significantly in their Gene complements. An individual of the homogametic sex can be either homozygous or heterozygous for a given gene. The heterogametic sex is hemizygous for most genes, meaning the genotype contains only a single copy of the gene—either on the X (Z) or the Y (W) chromosome. Consequently, in the homogametic sex, a recessive trait is manifested only when the gene is in the homozygous state, whereas in the heterogametic sex, it is always expressed.

Examples of X-linked traits in humans include hemophilia and color blindness, while Y-linked traits include hypertrichosis (hairiness of the auricle) and webbed toes. Traits linked to the X chromosome are inherited via a criss-cross pattern (see Chapter 3). Traits linked to the Y chromosome are transmitted exclusively through the male line and are referred to as holandric.

Although the X and Y chromosomes of mammals differ significantly in their gene content, they share homologous regions in the distal parts of their short and long arms, known as pseudoautosomal regions. The genes located in these regions (approximately 10) are formally sex-linked (housed within the sex chromosomes), yet they exhibit an inheritance pattern identical to that of autosomal genes.

Sex-limited traits are characteristics whose genes are located on autosomes but are expressed in only one of the sexes. Examples include egg production in birds and milk yield in mammals. Both male and female individuals possess the genes for these traits, but their phenotypic expression is possible only in females, as males lack the necessary physiological Organs.

Sex-influenced traits are controlled by genes located on autosomes that can be expressed in both sexes, though their mode of inheritance (recessive or dominant) depends on sex. For instance, pattern baldness is a sex-influenced trait in humans. The allele responsible for partial baldness in men is dominant; thus, the trait manifests with just a single copy present. In women, however, the phenotypic expression of this trait requires two copies of the allele in the genotype, meaning the exact same allele behaves recessively. The expression of sex-influenced genes is governed by hormonal status, resulting in heterozygotes of different sexes exhibiting different phenotypes. Hornedness and polledness in sheep are inherited in a similar manner.

It is important to note that most genes determining sex-specific phenotypes are located not on sex chromosomes, but on autosomes. The traits they govern (Primary and secondary sexual characteristics) are precisely sex-limited or sex-influenced traits. Their expression is regulated by the appropriate balance of male and Female Sex Hormones.