Describe the inheritance of sex in humans with reference to X and Y chromosomes

The inheritance of sex in humans is determined by the presence of X and Y chromosomes, which are the sex chromosomes that dictate biological sex.

Chromosomal Basis of Sex Determination

• XX and XY Systems: Humans typically have two sex chromosomes. Females possess two X chromosomes (XX), while males have one X and one Y chromosome (XY) . The combination of these chromosomes at fertilization determines the biological sex of the offspring.
• Role of the Y Chromosome: The Y chromosome carries the SRY gene (Sex-determining Region Y), which triggers male development. In its presence, the embryo develops testes and produces male hormones, leading to male characteristics. In contrast, the absence of a Y chromosome results in female development .

Inheritance Patterns

• Father to Son Transmission: Males pass their Y chromosome to their sons, which means that traits linked to the Y chromosome can only be inherited from father to son. This is known as Y-linked inheritance, and it is relatively rare as there are fewer genes located on the Y chromosome .
• Father to Daughter Transmission: Fathers pass their X chromosome to all their daughters. Consequently, if a father has an X-linked trait (like hemophilia), all his daughters will be carriers of that trait, but none will express it if they inherit a normal X from their mother .
• Mother to Offspring Transmission: Mothers contribute one X chromosome to each child. Sons inherit their only X from their mother, making them susceptible to X-linked recessive traits if they receive a mutated allele. Daughters can inherit one X from each parent, which means they may be carriers if they receive a normal allele from their father .

Implications of Sex-Linked Inheritance

• X-Linked Traits: Many genetic disorders are linked to genes on the X chromosome. Since males have only one X chromosome (hemizygous), they are more likely to express recessive traits associated with this chromosome, such as color blindness or hemophilia. Females, having two X chromosomes, would need two copies of a recessive allele to express these conditions .
• Carrier Status: Females can be carriers of X-linked conditions without expressing them if they have one normal allele. This carrier status allows them to potentially pass on these conditions to their sons, who would express the trait if they inherit the affected allele