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.

X-Linked Traits Follow Distinct Inheritance Patterns

1. X-linked traits behave differently in males and females due to the structure of sex chromosomes.

The Fruit Fly (Drosophila) Helped Uncover X-Linked Inheritance Patterns

1. Geneticist Thomas Morgan used fruit flies to study sex-linked inheritance. He discovered that:
   1. Males inherit their X chromosome from their mother, determining traits like eye color.
   2. Reciprocal crosses revealed that eye color was an X-linked trait.

Understanding Sex Chromosomes Is Key to Solving Genetic Mysteries

1. From explaining why certain disorders are more common in males to tackling fertility issues, understanding sex chromosomes has real-world implications:
   1. Medical Genetics: Helps diagnose and treat chromosomal abnormalities.
   2. Ethical Considerations: Raises questions about genetic testing and its societal impact.

Key Takeaways

1. The sex of a human offspring is determined by the sperm, which carries either an X or a Y chromosome.
2. The X chromosome carries many essential genes, while the Y chromosome is specialized for male development.
3. Sex-linked inheritance patterns differ between males and females due to the structure of the sex chromosomes.