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Sex Linkage and Lethal
Genes
Sex determination
Sex in mammals is determined by two chromosomes, known
as the sex chromosomes or heterosomes.
The X chromosome is
larger and contains about
2000 genes, whereas the
Y chromosome contains
fewer than 100.
In humans:

Females (XX) are the homogametic sex.

Males (XY) are the heterogametic sex.
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© Boardworks Ltd 2009
Sex linkage
Genes are located on the sex chromosomes are described as
sex linked. The study of their inheritance involves examining
both the sex of the offspring and the genetic trait of interest.
X-linked diseases
Haemophilia
Duchenne muscular
dystrophy
Red–green colour blindness
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Y-linked diseases
Rare and debatable!
It is argued that there is little
room on the Y chromosome
for anything other than genes
controlling testes formation
and function.
© Boardworks Ltd 2009
Sex linkage: X linkage
X linkage is more common because:

the X chromosome is larger

part of it does not have a homologous section on the Y
chromosome, therefore only one allele of a gene will be
present and so will always be expressed.
homologous regions
do not carry sex-determining genes
non-homologous regions
carry sex-determining genes
and other genes
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Recognising a sex-linked trait
 Usually more males than females are affected.
 If a woman has the trait, all her sons have it.
 The characteristic often appears to skip a generation
from grandfather to grandson.
 For a female to be affected her father must have the
characteristic and her mother must at least be a
carrier.
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© Boardworks Ltd 2009
Example 1: Red-green colour blindness
The allele for red-green colour vision is carried on
the X chromosome.
There is a recessive allele that causes red-green
colour blindness.
Females inherit 2 alleles for colour vision, one on
each X chromosome. If they inherit one faulty allele,
they are unaffected.
Males only inherit one allele from their mother. If
they inherit the faulty allele, they will suffer from redgreen colour blindness.
Notation
• For sex-linked traits, it is important to include
the sex chromosome in the genotype. The
allele for the gene is written as a superscript.
E.g. XCXC is a healthy female
XcY is a colour blind male etc.
Questions
• Draw genetic crosses for the following
parents:
– Colour blind male and healthy female
– Healthy male and colour blind female
• Why are males more frequently affected than
females?
Example 2: Haemophilia
Blood clotting disorder.
Caused by a recessive X-linked allele.
More common in males than females.
If H = normal allele for blood clotting, and h = allele for
haemophilia, to what phenotypes do the following
phenotypes correspond?
XHXH = ……………………………
XHXh = ……………………………
XhXh = ……………………………
XHY = ……………………………
XhY = ………………………………
Example 3: Tortoiseshell cats
All tortoiseshell cats are female
Allele for coat colour is carried on
the X chromosome.
If a female cat inherits a black
allele and a ginger allele she will
be tortoiseshell (XBXG)
A male cat can only inherit one of
these alleles so he will be either
black or ginger. (XBY or XGY)
A little bit extra…
The reason why: X Chromosome Inactivation
Early in a female’s embryonic development, one of the X chromosomes
is inactivated. This supercoils to form a Barr Body.
This occurs randomly in each cell, so all the cells that descend from that
cell will have either their maternal or paternal X chromosome switched
off.
Tortoiseshell cats will have a unique pattern depending upon which
chromosome is switched off, as one carries the black gene and one
carries the ginger gene.
Something else:
Lethal Genes
Some genes when expressed in the homozygous condition can
be fatal for the developing embryo, e.g. the Manx cat.
The tailless allele is dominant over the gene that produces a tail.
T = no tail
t = tail
If the cat inherits a heterozygous allele combination the gene will
be expressed and the cat will not have a tail.
Tt = no tail
If the cat inherits a homozygous dominant allele combination,
the foetus will not survive.
TT = death
Questions
1. The genotype and phenotype of certain cat coat colours are
shown in the table:
Phenotype
Genotype
Male
Female
Black
XgY
XgXg
Ginger
XGY
XGXG
Tortoiseshell
-
XGXg
A breeder has a number of black, ginger, and tortoiseshell cats.
a) If she wants all the female kittens to be tortoiseshell, what are
the genotypes and phenotypes of the parents she should use?
b) What will be the phenotypes of the male kittens in the crosses
you have suggested?
2. In a certain species of mouse, colour of fur is sex linked. Yellow fur
(Y) is dominant to grey fur (y).
Two yellow mice are mated. The ratio of their offspring is always 3:1
yellow: grey. Use a genetic diagram to explain why.