Sex linked genes
Why are some characteristics ‘sex
linked’?
• The X-chromosome 23
carries 1098 genes
• Over 100(recessive) genes
for genetic disorders have
now been mapped to the
X-chromosome 23.
• The human Xchromosome is much
smaller, and only carries a
few genes
Sex-linked inheritance
Red-green colour blindness
• test......
Genetics of colour blindness
• 0.4% of women are colour
blind
• 7% of men are colour blind
• In colour blindness, green
or red cones work poorly, or
not at all…
• Genes for green and red
cones are close to each
other on the X-chromosome
• Why is ‘blue blindness’
rare?
Haemophilia
• Recessive, sex linked
disease which causes
abnormal blood clotting
• Individuals are at risk of
severe bleeding
• Many die from brain
haemorrhages
• Type A: Lack of Factor VII:
80% of cases of haemophilia
• Type B: Lack of Factor IX:
20% of cases
Haemophilia in the Royal families of
Europe
• Queen Victoria has 9 children (4 sons, 2 daughters). She
was a carrier for the Haemophilia gene
• 1 of 4 sons developed haemohilia (died at age 30)
• 2 of 5 daughers were carriers (married and moved to
Spain, Russia, Germany)
Diseases linked to the 23rd chromosome
More than 100 sex-linked genetic disorders have
been linked to the X-chromosome of
chromosome 23 in humans, including:
• Haemophilia
• Duchenne Muscular dystrophy
• Colourblindness
• Melanoma
• X-linked severe combined immunodeifciency
• X-inactivation centre
Why are sex-linked diseases more
common in males?
1. Males have only 1 X chromosome; thus all X-linked
alleles will be phenotypically expressed in males,
even if they are recessive…
2. Females must have BOTH copies of the allele to be
recessive, in order to express the gene phenotypically
3. Thus the recessive phenotype of a sex-linked genetic
disorder is much more common in males
4. Sex-linked genes can move from fathers to their
daughter, where the daughters will be unaffected
carriers, but pass the gene to their sons…
Inherited Diseases
Autosomal Dominant
Disorders
Neurofibromatosis
Huntington’s Disease
Autosomal dominant disease:
Huntingdon’s Disease
Can we test for Huntingdon’s Disease?
• Pre-natal testing (amniocentesis or chorionic
villus sampling)
• Genetic testing after birth
Autosomal Recessive
Disorders
Tay Sachs
Cystic
Fibrosis
Genetic Diseases
Inherited diseases
• Phenylketonuria
• Sickle Cell disease
• Cystic fibrosis
Cystic Fibrosis
Phenylketonuria
Sickle Cell Disease
Inherited diseases
Genetic Testing (1)
• How to read a chromosome
Genetic Testing (2)
There are various types of genetic testing:
1. Prenatal
2. Newborn Screening (e.g. newborn test for PKU)
3. Carrier diagnosis for couples with a family
history of disease
4. Late-onset disorders can be tested at any time
5. DNA fingerprinting
Genetic Testing and Counseling
Diagnosis of carriers
– Biochemical (i.e. Tay-Sachs)
– Genetic (i.e. Huntington’s disease)
– Ethical considerations
Fetal diagnosis
– Amniocentesis
– Chorionic villi sampling
Chorionic villi sampling
Chorionic villus
sampling (CVS) is
the removal of a
small piece of tissue
(chorionic villi)
from the uterus
during early
pregnancy to
screen the baby for
genetic defects.
A son with cystic fibrosis
(autosomal recessive) is born to
a couple who appear to be
normal. What are the chances
that any child born to this
couple will have cystic fibrosis?
And the Answer is…..
25%
In humans the allele for short fingers
is dominant over that for long
fingers. If a person with short fingers
who had one parent with long
fingers reproduces with a person
having long fingers, what are the
chances of each child having short
fingers?
And the Answer is…..
50%
Homework: Complete the remain questions
on the genetics problem set and do your
genetics mini project.
Combining multiplication and addition rules
to solve complex problems
• An organism with the genotype BbDD is
mated with one with the genotype BBDd.
Assuming independent assortment of
these two genes, what is the probability
that you will get a BBDD offspring?
• ¼ BBDD