Inherited Diseases
Cystic Fibrosis:
A disease of the lungs and digestive system. Very thick mucus is produced and
blocks the airways and intestines. Patients have difficulty breathing and absorbing
food. Blocked airways are also easily infected.
Caused by a recessive gene: you need two genes to show symptoms. If you have just
one gene you are a carrier. You don not have the disease but you can pass on the
genes to your children.
cc = Cystic fibrosis
CC = Normal
Cc = Carrier
Huntington’s Disease:
This disease affects the nervous system. It affects people in middle age. Movement
starts to become jerky and clumsy eventually the person will need a wheel chair and
will not be able to feed or dress themselves.
Caused by a dominant gene. You only need to inherit a gene from one of your parents.
Hh = Huntington’s
hh = Normal
HH = Huntington’s
Sickle Cell Anaemia:
This disease affects your red blood cells. It means that you can not carry enough
oxygen around your body. The cells are a different shape and can block veins and
arteries. People with Sickle cell anaemia often die at a young age.
Sickle cell anaemia is not common in Britain. It is very common in countries where
you find malaria. This is because carriers of sickle cell are protected against malaria.
Caused by a recessive gene. You need both genes to be sick:
ss = Sickle Cell Anaemia
SS = Normal
Ss = Carrier
GENETIC DISORDERS
Family tree questions
1. Genetic counsellors draw family trees, using symbols to show the sex of a person, and whether
they are affected or are known to be a carrier. Look at figure 1, which shows the family tree
of two children with cystic fibrosis.
a) What is the sex of children 10 and 11?
b) Why must both parents 6 and 7 be carriers of cystic fibrosis?
Figure 1
1
Key
(Grand parents)
2
3
4
unaffected male
unaffected female
(Parents)
affected male
5
6
7
8
9
10
11
affected female
(Children)
male carrier
12
13
14
female carrier
c) Use the square to show if child 14 could be a carrier of cystic fibrosis:
Remember:
cc = Cystic fibrosis
CC = Normal
Cc = Carrier
C
C
c
c
2. Figure 2 shows the family tree of a family affected by Huntington’s disease, which is a carried
by a dominant gene.
a) Label the grand parents, parents and children.
b) How do you know that the affected grandfather has Huntington’s disease?
Figure 2
H
h
h
h
c) If H is the gene for Huntington’s disease and h is the normal gene, label the genes of the
children.
(HINT: Use the square to show how the genes were passed from the parents to the children)
Figure 3 shows another family tree. Clive was born with cystic fibrosis. Diane is pregnant again.
a) How do they know that Jeffrey and Diane are both carriers of cystic fibrosis?
b) Could George be a carrier?
c) What is the chance that the foetus has inherited cystic fibrosis?
(HINT: Use the square to show how the genes were passed from the parents to the children)
Figure 3
Jeffrey
Diane
?
George
Clive