Post by Daniella on May 26, 2006 10:28:32 GMT
Appendix II
The genetics of Down’s syndrome
There are three types of Down’s syndrome:
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 bay has three rather than two chromosome 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 chromosomes 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 of the 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.
The genetics of Down’s syndrome
There are three types of Down’s syndrome:
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 bay has three rather than two chromosome 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 chromosomes 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 of the 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.