SEX LINKED INHERITANCE

a. X- linked dominant (X-LD) inheritance:
 Uncommon, but disorders, manifest in the heterozygous female as well as in the male,
with the mutant allele on his single X chromosome.
 An excess of affected females may be seen in families with X-LD disorders.
 A mosaic pattern demonstrated in affected heterozygous females for some X-LD
disorders.
 Both the daughters & the sons of an affected female with X-LD disorder, have 1 in 2
(50%) chance of being affected.
 Affected male transmit the trait to the daughters but none to his sons.
 Examples: Vitamin D-resistant rickets, Incontinentia pigmenti.
b. X-linked recessive(X-LR) inheritance:
 It usually manifests only in males.
 A male with a mutant allele on his single X is called hemizygous.
 A male can’t transmit X-linked trait to his son.
 X-linked recessive diseases are transmitted by healthy heterozygous female carriers to
male.


 Trait is transmitted to male grandchild through the obligate carrier daughters. This
type of pedigree is referred to as a ‘diagonal or a knight’s move’ pattern of transmission.
 In hetrozygous females, variable expression seen.
 A male transmits his X to his daughters & Y to his sons.
 For an affected male with a normal female: all daughters will be obligate carrier, but
none of the sons will be affected.
 For a carrier female with a normal male, each son has a 1 in 2 (50%) chance of being
affected & each daughter has 1 in 2 (50%) chance of being obligate carrier.
 Examples: Haemophilia, DMD (Duchenne Muscular Dystrophy).
c. Y-linked inheritance / Holandric inheritance:
 Y- linked or holandric inheritance implies that only males are affected.
 Affected male transmits Y –linked trait to all his sons, but none to the daughters.