89
PRINCIPLES OF INHERITANCE AND VARIATION
Haemophilia : This sex linked recessive disease, which shows its
transmission from unaffected carrier female to some of the male progeny
has been widely studied. In this disease, a single protein that is a part
of the cascade of proteins involved in the clotting of blood is affected.
Due to this, in an affected individual a simple cut will result in non-stop
bleeding. The heterozygous female (carrier) for haemophilia may transmit
the disease to sons. The possibility of a female becoming a haemophilic
is extremely rare because mother of such a female has to be at least
carrier and the father should be haemophilic (unviable in the later stage
of life). The family pedigree of Queen Victoria shows a number of
haemophilic descendents as she was a carrier of the disease.
Sickle-cell anaemia : This is an autosome linked recessive trait that can
be transmitted from parents to the offspring when both the partners are
carrier for the gene (or heterozygous). The disease is controlled by a single
pair of allele, Hb
A
and Hb
S
. Out of the three possible genotypes only
homozygous individuals for Hb
S
(Hb
S
Hb
S
) show the diseased phenotype.
Heterozygous (Hb
A
Hb
S
) individuals appear apparently unaffected but they
are carrier of the disease as there is 50 per cent probability of transmission
of the mutant gene to the progeny, thus exhibiting sickle-cell trait (Figure
5.15). The defect is caused by the substitution of Glutamic acid (Glu) by
Figure 5.15 Micrograph of the red blood cells and the amino acid composition of the relevant portion
of
b
-chain of haemoglobin: (a) From a normal individual; (b) From an individual with
sickle-cell anaemia
2015-16
99
PRINCIPLES OF INHERITANCE AND VARIATION
Haemophilia
: This sex linked recessive diseas
e,
which shows its
transmission from unaffected carrier female to some of the male progeny
has been widely studied. In this disease, a single protein that is a part
of the cascade of
pr
oteins involved in the clottin
g
of blood is affected.
Due to this, in an affected individual a simple cut will result in non-stop
bleeding. The heterozygous female (carrier) for haemophilia may transmit
the disease to sons. The possibility of a female becoming a haemophilic
is extremely rare because mother of such a female has to be at least
carrier and the father should be haemophilic (unviable in the later stage
of l
haem
Sick
be t
carr
pair
homo
Hete
are
of the
5.15
Fi
gu
r
e
5.
15
Mi
cr
og
ra
ph
of the r
ed blood cells and the amino acid co
mp
osition of the r
elevant
po
rtion
of
b
-chain of haemoglobi
b
n
: (a) From a normal individual; (b) From an individual wit
h
si
ck
le
-c
el
l
anaemi
a
2015-1
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88
99
rrier and the father should be haemophilic (unviable in the later stage
life). The family pedigree of Queen Victoria shows a number of
emophilic descendents as she was a carrier of the disease.
ckle-cell anaemi
a
:
This is an autosome linked recessive trait that can
transmitted from parents to the offspring when both the partners are
rrier for the gene (or heterozygous). The disease is controlled by a single
ir of allele, Hb
A
a
nd
H
b
A
S
. Out of the three possible genotypes only
mo
zy
go
us individuals for Hb
S
(Hb
S
S
Hb
S
) show the diseased phenot
yp
e.
terozy
go
us (Hb
A
Hb
Hb
Hb
S
) individuals appear apparently unaffected but they
carrier of the disease as there is 50 per cent probabili
ty
of transmission
he mutant gene to the progeny, thus exhibiting sickle-cell trait (Figure
15). The defect is caused by the substitution of Glutamic acid (Glu) by