In our genetic make-up a piece of one of the chromosomes from pair number 7 has detatched itself and reattached itself to one of the chromosomes from pair number 21. This is unusual and is called Translocation.

This has not affected our make-up. We have the correct amount of genetic material in our cells. It is not in the correct position, but this does not seem to matter. (The genetic material is scrambled up inside the nucleus and is not actually in the neat rows shown in the diagram above.)

The problem comes in reproduction. Cells split in half to make sperm and ovum cells. One from each pair of chromosomes goes into each sperm cell or ovum. This includes one of the chromosomes from pair number 7 and one of the chromosomes from pair number 21. If there is too much genetic material or too little, the sperm or ovum will be a dud. There are four possible permutations of the good and bad chromosomes from pairs 7 and 21.

So two of the permutations are abnormal and two are normal. The chances of abnormality are 50%. Within the permutations giving a normal amount of genetic material, one will carry the translocation, one will not; so for normal children born to a parent with the translocation, the chance of them carrying the translocation - i.e. being a carrier is 50%.

What does this mean?

This translocation means that 50% of the sperm or ovum we produce are likely to result in an abnormal pregnancy - i.e. a miscarriage. In general you can expect that 50% of the pregnancies of you or your partner will miscarry. This seems to occur around the tenth to twelth week of pregnancy. This will happen irrespective of whether you are a male or a female. It is equally likely.

To put this into perspective, the rest of the world has a miscarriage rate of 20% - about two pregnancies in every ten will miscarry. With one of the partners having the translocation, five pregnancies in every ten can be expected to miscarry.

There is a small chance that an abnormal pregnancy will not abort naturally. This has not yet happened in our family history so the chances are probably very small. Any live child born with the genetic abnormality would be likely to have multiple handicaps and have retardation in mental development. It is important therefore that our pregnancies are screened carefully. If an abnormal pregnancy was detected that did not abort naturally, most doctors would recommend aborting it.

Who's got it?

The translocation can be traced back to grandfather Legge. Some of his decendants have it, and can be traced on the family tree. Out of 28 pregnancies known about, 14 were abnormal, by chance exactly the 50% as is expected from the chromosome pairing probabilities. The ratio of carriers to non-carriers is not so exact. In our family tree of descendants from Edwin Legge, there are 13 people of whom 9 are carriers. This is a bit higher than would be expected from the probabilities.

What can be done?

The best thing to do is to screen each pregnancy as it occurs. If the pregnancy is abnormal, it is most likely that it will abort naturally. In the case of an abnormal pregnancy proceeding further than expected, it can be aborted. The earlier an abnormal pregnancy can be detected, the less traumatic the abortion, if it is needed.

Fortunately the science of genetic screening is developing very rapidly. In the past few years, a new technique of genetic screening has been developed called Corionic Villi biopsy (CVS) which can give a genetic analysis of an embryo at about 12 weeks. The results are available in about 48 hours. (see the article on Gene Screening in The Economist July 14 1984 - it's also come a long way since then.)

Another method of genetic screening is amniocentesis, which is carried out at about 16 weeks. This is a slightly more complex operation and takes longer to get the results - up to four weeks.

With either of these screening techniques there is a small chance that the test itself could cause a miscarriage to the foetus, even if it is normal. This is because the test involves taking a small piece of tissue from the placenta (CVS) or taking a smaple of fluid from the amniotic sac. The risk is higher with CVS, around 5 to 6% probability that after the procedure the foetus will abort. With amniocentesis the risk is less than 1 in 200. But earlier in pregnancy, miscarriage is more likely in any case. 1 in 5 pregnancies miscarry anyway.

The details of these screening techniques can be explained to you in more detail by your own GP and by a Genetic Counsellor. You should see them in any case when you get married or plan to have children.

The process of having children is slightly harder work for the 7-21 club. But membership does have its privileges. CVS is an expensive and complex analysis and is not given to most pregnant women. It can show up a whole range of genetic abnormalities, and the screening process will be relatively thorough. Just show your 7-21 membership card at the hospital door for a full genetic check-up! And it also tells you whether it's a boy or a girl. Amaze your friends by predicting the sex of your baby at 12 weeks!!! (None of those wasted presents the wrong colour..) Mind you, you don't have to know the sex if you don't want to.

So don't be too discouraged. With modern medicine and genetic care, the trials and tribulations of being a 7-21 member is reduced to careful checks during pregnancy and hopefully a minimal amount of disruption to your life. It will, unfortunately mean miscarriages - that's our curse - but hopefully nothing worse. Normal children have been born to all the previous generations of translocation carriers and the worse that has happened to them is that they've ended up carriers like us.