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Sex - Linkage in Humans
Humans have many X - linked traits of which some like haemophilia and colour blindness
are recessive while others like hypophosphatemic or vitamin D resistant rickets are
dominant. X - linked dominant is a trait which is determined by an X linked dominant
gene, while X - linked recessive is a trait that is determined by an X - linked recessive
gene. Their patterns of inheritance are very diferent from each other.
X - linked recessive inheritance: Experimental matings are not practically possible in
humans. Mode of inheritance of human traits can be traced through pedigrees.
Genetics of Haemophilia: Haemophilia is a rare X - linked recessive trait. Haemophiliac’s
blood fails to clot properly after an injury, because it has either a reduction or malfunction
or complete absence of blood clotting factors. It is a serious hereditary disease because
a haemophiliac may bleed to death even from minor cuts. Haemophilia is of three types:
A, B and C. Haemophilia A and B are non - allelic recessive sex - linked, but haemophilia
C is an autosomal recessive trait. 80% haemophiliacs, sufer from haemophilia A due
to abnormality of factor VIII, about 20% sufer from haemophilia B due to disturbance
in factor IX, but less than 1% sufer from haemophilia C due to reduction in factor XI.
Being X - linked recessives, haemophilia A and B afect men more than women, but
haemophilia C afects both the sexes equally because it is autosomal. Chances for a
man to be afected by haemophilia A and B are greater than a woman. A woman can
sufer from haemophilia A or B only when she is homozygous for the recessive allele,
but a man with just one recessive allele will display the trait. Haemophilia A and B
zigzag from maternal grandfather through a carrier daughter to a grandson. It never
passes direct from father to son. Gene for normal is H. Gene for haemophilia A is h.
In generation I of this pedigree (Fig. 22.27) a man (I - 2) sufering from haemophilia A
marries a normal woman (I - 1). He passes haemophilia gene to his daughter (II - 2)
through his X chromosome. He cannot pass this gene to his son (II - 3) because the son
receives only Y chromosome from him. His daughter (II - 2) also receives another X but
with normal dominant allele from her mother (I - 1).
The daughter looks phenotypically normal, but she is heterozygous and a carrier for
the recessive gene. When she marries a normal man (II - 1) she passes her father’s
trait to one of her two sons (HI - 4) who inherits grandfather’s X from her. The single
recessive allele for haemophilia expresses successfully in the hemizygous son because
his Y chromosome does not carry its counterpart. The other son (III - 3) is normal as heinherits grand mother’s X with normal gene, One daughter (III - 1) with both normal X
is normal, but the other daughter (III - 2) is carrier like her mother.
Activity: Cases of Haemophilia A are reported in Queen Victoria’s family. Pedigree of
Queen Victoria’s family (Fig. 22.28) indicates that Queen Victoria was a carrier mother,
because she gave birth to an afected son Prince Leopold. Prince Leopold passed on
this recessive X - linked trait in typical zigzag fashion through his carrier daughter
(III -
1) to his grandson Rupert (IV - 1). Assign genotype to each individual. Can you explain
how Alexis (IV - 3) became haemophiliac