INHERITED
DISEASES
The
rules of inheritance as explained earlier can be used
to explain certain inherited diseases where a single
gene determines whether the disease will be present
in the offspring or not. Many human features or inherited
diseases are not so simple, eg asthma is the result
of the interaction of many genes.
One
common inherited disease determined by a single gene
is thalassemia. Thalassemia is an inherited condition
affecting the production of red blood cells. Thalassemia
is common amongst Singaporeans; about 3% of our local
population are carriers of the thalassemia gene
A
carrier is someone who is perfectly healthy but he/she
carries the thalassemia gene. A carrier can pass on
the disease to his/her offspring. To illustrate this,
suppose a perfectly healthy couple are both carriers
of the disease. What is the probability that their
child would inherit thalassemia? We use T to denote
the thalassemia gene and N to denote the absence of
the thalassemia gene. Since a carrier is perfectly
healthy, he/she must be TN.
|
| |
|
Women
|
|
|
Man
|
|
T |
N |
| T |
TT |
TN |
| N |
NT |
NN |
|
We
see here that there are four possibilities for the
offspring. It can be either TT, TN, NT or NN. If the
resulting offspring is TT, then the child would be
born with thalassemia. If the offspring is either
TN or NT, then the offspring will not be born with
thalassemia. However, the child would be a carrier
of the disease and he/she could pass this on to his/future
offspring. If the offspring is NN, then the child
is completely free from thalassemia - he/she will
not be born with thalassemia and he/she would also
not be a carrier of the disease and his/her offspring
will never be born with thalassemia.
Consequently,
we see that in this case there is a 25% chance of
the child being born with the disease when both parents
are perfectly healthy.
