Sex-Linked Inheritance

• This is a form of inheritance where the gene/allele for the characteristic being studied is on the X chromosome.
• Diseases such as colour-blindness and haemophilia are inherited this way, and are more common in males than females.

• Alleles, Genotypes and Phenotypes for Haemophilia (Blood-Clotting Inability)
  X_H - allele for normal blood clotting
  X_h - allele for haemophilia
  
  X_HX_H - genotype of normal female
  X_hX_h - genotype of haemophiliac female
  X_HX_h - genotype of carrier female (with normal blood-clotting ability, but who can pass the defective gene to her children)
  
  X_HY - genotype of normal male
  X_hY - genotype of haemophiliac male

• Example 1 - Haemophilia
  A man with normal blood-clotting ability (X_HY) marries a woman who is a haemophiliac (X_hX_h). The possible phenotypes of their children are...

	XH	Y
Xh	XHXh	XhY
Xh	XHXh	XhY

Possible genotypes = 1 X_HX_h : 1 X_hY
Possible phenotypes = 1 carrier female : 1 haemophiliac male

• Example 2 - Haemophilia
  A normal woman (X_HX_H) marries a haemophiliac man (X_hY). The possible genotypes and phenotypes of the children are...

	XH	XH
Xhh	XHXh	XHXh
Y	XHY	XHY

Possible genotypes = 1 X_HX_h : 1X_HY
Possible phenotypes = 1 carrier female : 1 normal male

• Alleles, Genotypes and Phenotypes for Colour-Blindness
  Xc - allele for normal colour vision
  X_c - allele for colour-blindness
  
  Xc Xc - genotype of normal female
  X_c X_c - genotype of colour-blind female
  Xc X_c - genotype of carrier female (with normal colour vision, but who can pass the defective gene to her children)
  Xc Y - genotype of normal male
  X_c Y - genotype of colour-blind male

• Example 3 - Colour-Blindness
  A male with normal vision (XcY) and a colour-blind female (X_c X_c) have children. The possible genotypes and phenotypes of the children are...

	Xc	Y
Xc	XcXc	XcY
Xc	XcXc	XcY

Possible genotypes = 1 Xc X_c : 1 X_c Y
Possible phenotypes = 1 carrier female : 1 colour-blind male

• Example 4 - Colour-Blindness
  A carrier female (Xc X_c ) marries a normal-visioned male (Xc Y). The possible genotypes and phenotypes of the children are...

	Xc	Xc
Xc	XcXc	XcXc
Y	XcY	XcY

Possible genotypes = 1 XcXc: 1 XcX_c:1 XcY : 1 X_cY
Possible phenotypes =1 normal female:1 carrier female:1 normal male :1 colour-blind male