Sex Linkage

Sex Chromosomes:

• Females: Have two X chromosomes (XX).
• Males: Have one X chromosome and one Y chromosome (XY).

Examples of Sex-Linked Traits in Humans:

1. Red-green colour blindness: Inability to distinguish between red and green.
2. Haemophilia: Inability to form blood clots properly.

Features of Sex-Linked Traits:

• Genes on X Chromosome:

  • Most genes related to sex-linked traits are located on the X chromosome, as the Y chromosome has very few genes.
  • This makes sex-linked traits more likely to appear in males.

• Why Sex-Linked Traits are More Common in Males:

  • Males only need one recessive gene on their single X chromosome to express the trait (e.g., colour blindness).
  • Females need two recessive genes (one on each X chromosome) to express the trait.

• Carrier Females:

  • Females with one normal allele and one abnormal allele (heterozygous) do not express the trait but are carriers.
  • Carriers can pass the trait to their offspring.

Example: Genotypes for Colour Vision

• N: Represents the normal allele for vision.
• n: Represents the allele for colour blindness (abnormal allele).

Example Cross: Carrier Female (XN Xn) x Normal Vision Male (XN Y)

Let's determine the possible genotypes and phenotypes for the offspring of a carrier female (XN Xn) and a normal vision male (XN Y).

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Step 1: Gametes

• Female (XN Xn) produces two types of eggs: XN and Xn.
• Male (XN Y) produces two types of sperm: XN and Y.

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Step 2: Punnett Square

	XN (Sperm)	Y (Sperm)
XN (Egg)	XN XN (Normal Female)	XN Y (Normal Male)
Xn (Egg)	XN Xn (Carrier Female)	Xn Y (Colour Blind Male)

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Step 3: Results

Genotypes:

• XN XN: Female with normal vision.

• XN Xn: Carrier female (normal vision).

• XN Y: Male with normal vision.

• Xn Y: Male with colour blindness. Phenotypes:

• Normal Vision Female: 25% (1 out of 4).

• Carrier Female (Normal Vision): 25% (1 out of 4).

• Normal Vision Male: 25% (1 out of 4).

• Colour Blind Male: 25% (1 out of 4).

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