D3.2.11 Sex determination in humans and inheritance of genes on sex chromosomes Notes

The X Chromosome Is Packed with Genes, While the Y Chromosome Specializes in Male Development

1. Humans have 23 pairs of chromosomes, with the 23rd pair determining sex:
   1. Females: Have two X chromosomes (XX), which carry about 900 genes essential for survival and development.
   2. Males: Have one X and one Y chromosome (XY).
      1. The Y chromosome is much smaller, carrying only 55 genes, most of which are specific to male development.

The Sperm Determines Whether the Offspring Is Male or Female

1. The sex of a zygote is determined during fertilization:
   1. Sperm carrying an X chromosome → Offspring develops as XX (female).
   2. Sperm carrying a Y chromosome → Offspring develops as XY (male).

The Role of the SRY Gene

1. The SRY gene on the Y chromosome acts as a switch for male development by triggering the formation of testes.
2. In the absence of the SRY gene, the default pathway leads to female development.

Errors in Meiosis Can Cause Sex Chromosome Abnormalities

1. Mistakes during the formation of gametes can lead to unusual combinations of sex chromosomes, such as:
   1. Klinefelter’s Syndrome (XXY): Individuals develop as males but may have reduced fertility and some female characteristics.
   2. Turner’s Syndrome (XO): Individuals develop as females but may have underdeveloped reproductive systems and shorter stature.

The X Chromosome Carries Many Essential Genes, Making Males More Vulnerable to X-Linked Disorders

1. The X chromosome carries genes unrelated to sex determination, which are critical for both males and females.
2. However, males are more vulnerable to X-linked disorders because they have only one X chromosome and lack a backup.