16.08 Sex Linkage

Sex Chromosomes and Sex-Linked Genes

• Sex Chromosomes in Humans:
  • Females: XX (two X chromosomes)
  • Males: XY (one X and one Y chromosome)
  • Y Chromosome: Short, with few genes.
  • X Chromosome: Longer, with many essential genes not found on the Y chromosome.
  • Sex Linkage: When a gene is located on a sex chromosome, typically the X chromosome in humans. Since males only have one X chromosome, any allele on the X will affect their phenotype directly.

Example: Haemophilia (Blood Clotting Disorder)

• Haemophilia Gene on the X Chromosome:
• Alleles:
  • F (dominant): Codes for normal factor VIII, allowing blood to clot normally.
  • f (recessive): Codes for a lack of factor VIII, causing haemophilia (blood does not clot normally).
• Female Genotypes and Phenotypes:
  • X^F X^F: Normal blood clotting
  • X^F X^F: Normal blood clotting (carrier)
  • X^fX^f: Haemophilia
• Male Genotypes and Phenotypes:
  • X^FY: Normal blood clotting
  • X^fY: Haemophilia

Example Cross: Carrier Female (X^FX^f x Normal Male (X^FY)

1. Parental Genotypes and Gametes:

• Mother X^FX^f: Gametes: X^F or X^f
• Father X^FY): Gametes: X^F or Y

1. Punnett Square:

3. Predicted Offspring Ratio:

• 50% females with normal clotting: X^FX^F and X^FX^f) (carrier)
• 25% males with normal clotting: X^FY
• 25% males with haemophilia: X^fY

Key Points About Sex-Linked Inheritance

• Inheritance Pattern: Since males have only one X chromosome, they are more likely to express recessive X-linked traits (e.g., haemophilia, red-green color blindness).
• Male Inheritance: Sons inherit their X chromosome from their mother; a boy cannot inherit X-linked traits like haemophilia from his father.

Example Question Applications

1. Red-Green Color Blindness:

• Alleles: Suggested symbols could be (C^N) for normal vision and (C^c) for color blindness.
• Example Cross: Heterozygous female (X^NX^c) and normal male (X^NY).
  • Predicted Probability for Color-Blind Son: 25% chance for X^cY, resulting in a color-blind male.

1. Tortoiseshell Coat Color in Cats:

• Alleles: (C^O) for orange fur and (C^B) for black fur (codominant).
• Genetic Explanation for Male Cats: Male cats (XY) cannot be tortoiseshell (orange and black) because they have only one X chromosome, allowing only one color allele (either (C^O) or (C^B)).

Example Cross: Orange Male (X^OY) x Tortoiseshell Female (X^OX^B)

1. Parental Genotypes and Gametes:

• Female (X^OX^B): Gametes: X^O or X^B
• Male (X^OY): Gametes: X^O or Y

1. Punnett Square:

3. Predicted Offspring Phenotypes:

25% black male (X^BY)

25% orange female (X^OX^O)

25% tortoiseshell female (X^OX^B)

25% orange male (X^OY)