THE GENETICS OF DOWN'S SYNDROME

There are three types of Down's syndrome, briefly described below.

Standard (regular or primary) trisomy 21

About 94% of all people with Down's syndrome fall into this group. This type of Down's syndrome is always an accident of nature. It can happen to anyone, and there is no known reason why it occurs. The parents have normal chromosomes, but the baby has three rather than two chromosomes of the number 21 pair.

Translocation

This type of Down's syndrome occurs in about 3% of babies with Down's syndrome. Part of the number 21 chromosome becomes attached to another chromosome, so that both parts of the new large chromosome move together as one. In about half of the people who have Translocation Down's syndrome, the translocation is a one-off occurrence and does not mean that it will happen again in any future pregnancies.

In the other half, however, it occurs because one or other parent, though having a normal balanced chromosomal make-up, has one of the number 21's "stuck on" to another chromosome. In this case the chance of another child with Down's syndrome being conceived is quite high. Blood tests can be carried out to find out more detailed information.

Mosaicism

This type of Down's syndrome is also rare. Only about 2-3% of people with Down's syndrome fall into this category. In Mosaic Down's syndrome the cells with the extra 21 chromosome are mixed with other "normal" cells. In other words, only a certain percentage of the cells are affected.

Further information

Down's Syndrome Association

Produced with the support of Sun Life