Cambridge International A Level Biology Revision Guide

Chapter 16: Inherited change
Sex linkage
The X chromosome contains many different genes. (You
can see some of these in Figure 16.4, page 366.) One of
them is a gene that codes for the production of a protein
needed for blood clotting, called factor VIII. There are
two alleles of this gene, the dominant one, H, producing
normal factor VIII, and the recessive one, h, resulting in a
lack of factor VIII. The recessive allele causes the disease
haemophilia, in which the blood fails to clot properly.
The fact that the gene for haemophilia is on the X
chromosome, and not on an autosome, affects the way that
it is inherited. Females, who have two X chromosomes,
have two copies of the gene. Males have only one
X chromosome, and so have only one copy of the gene.
Therefore, the possible genotypes for men and women are
different. They are shown in Figure 16.17.
The factor VIII gene is said to be sex linked. A
sex-linked gene is one that is found on a part of the X
chromosome not matched by the Y, and therefore not
found on the Y chromosome.
Genotypes including sex-linked genes are always
represented by symbols that show that they are on an
X chromosome. Thus the genotype of a woman who has
the allele H on one of her X chromosomes, and the allele
h on the other, is written as X H X h .
You can draw genetic diagrams to show how sex-linked
genes are inherited, in exactly the same way as for other
genes. For example, the following diagram shows the
children that could be born to a couple where the man
does not have haemophilia, while the woman is a carrier
for the disease.
Parental phenotypes normal man carrier
woman
Parental genotypes X H Y X H X h
Gametes X H or Y X H or X h
Offspring genotypes and phenotypes:
Gametes from woman
Gametes
from man
X H Y
X H
X H X H
normal
female
X H Y
normal male
X h
X H X h
carrier
female
X h Y
haemophiliac
male
Each time this couple has a child, there is a 0.25
probability that it will be a normal girl, a 0.25 probability
that it will be a normal boy, a 0.25 probability that it will
be a carrier girl, and a 0.25 probability that it will be a boy
with haemophilia.
Females
H H
X H X H
normal
Males
H
X H Y
normal
Figure 16.17 The possible genotypes and phenotypes for
haemophilia.
QUESTIONS
H h h h
X H X h X h X h
normal
lethal
(carrier) (zygote usually
does not develop)
h
X h Y
haemophilia
16.13 Can a man with haemophilia pass on the disease to:
a his son?
b his grandson?
16.14 One of the genes for colour vision in humans is found
on the X chromosome but not on the Y chromosome.
The dominant allele of this gene gives normal colour
vision, whereas a recessive allele produces red–
green colour blindness.
a Choose suitable symbols for these alleles, and
then write down all of the possible genotypes for
a man and for a woman.
b A couple who both have normal colour vision
have a child with colour blindness. Explain how
this may happen, and state what the sex of the
colour-blind child must be.
c Is it possible for a colour-blind girl to be born?
Explain your answer.
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