Genes are inherited in linear groups called chromosomes. In humans there are 46 chromosomes arranged as 23 pairs, and contained within them is all the information to make a human being. Each individual has a unique genetic constitution brought about through a special kind of cell division known as meiosis. Of the 46 chromosomes, 22 pairs are known as autosomes and the twenty-third pair are the sex chromosomes, which determine the sex of the individual. Each one of a pair of autosomes is very much like the other, but the sex chromosomes are very different. There are two kinds, the X and the Y. The X is larger, and when an offspring carries a pair of X-chromosomes it is female; when it carries one X- and one Y-chromosome it is male.
There are certain conditions that are inherited, some of which are sex-linked, notably haemophilia (the bleeding disease) and colour blindness. Haemophilia is an X-chromosome linked recessive condition which affects the blood’s ability to clot. Females carry the disease but only very rarely suffer from it, the condition appearing only in males. Queen Victoria of England was a carrier of haemophilia which was passed, through her daughters, to most of the European royal families. Today the condition can be controlled by giving sufferers the blood factors they lack. A man suffering from haemophilia transmits the disease on his X-chromosomes, which means that any female offspring will almost certainly become carriers. There is a 50 per cent chance that such a woman will pass the X-chromosome containing the affected gene to her male offspring, who will as a result suffer from the disease. A female who receives the defective chromosome will not suffer from haemophilia because, unlike the man, she possesses a second ‘healthy’ X-chromosome. A woman who is known to be a carrier of haemophilia can undergo special screening during pregnancy to determine whether her child has been affected by the disease. This is done using a method known as amniocentesis which involves drawing off some of the amniotic fluid that protects the unborn child and examining the genetic constituents of foetal cells in the fluid. Amniocentesis is also used to check for other kinds of inherited conditions such as Down’s syndrome or mongolism, in which there is a duplicate of chromosome number 21 in every cell, resulting in mental retardation. The same test can also be used to screen for certain types of muscular dystrophy. There are in fact many genetically causedwhich affect people. This may cause great distress to people who know that members of their family suffer from them and that they may pass the disease on to their own children.
Today, doctors called genetic counsellors can assess the probability of parents having children with such disorders. The techniques involved include obtaining information from the parents’ genotypes, and more simply taking a detailed family history. They may look at family photographs because some of the disorders have subtle unmistakable facial or physical characteristics. The counsellor can provide advice on the chances of certain inherited conditions occuring in a pregnancy. The couple can then decide whether they will accept the risk. If possible the mother will have a screening test during pregnancy or abandon having children at all.