Almost all people who have inherited Kallmann's syndrome are male. If your brother, father or grandfather also has or had Kallmann's syndrome, then you almost certainly have acquired the disease by inheriting the defective Kallmann gene from one of your parents, most probably your mother. To understand how Kallmann's syndrome could be passed on from generation to generation, we must first understand the basic principles of genetics.

You may have already heard of or read about DNA fingerprinting and genetic engineering, but probably wondered what these terms actually mean. Much of your body is made up of thousands of millions of microscopic cells, all of which have a central nucleus, each containing tiny thread-like "packages" of information known as chromosomes (see Figure 15a). Chromosomes are made up of even smaller units called genes containing copies of a code describing every one of our physical characteristics and features, such as the colour of our skin, hair and eyes and even the likelihood of us developing particular diseases. Each one of these genes consists of a specific segment or sequence of the highly complex "building block" of life, the DNA molecule. Each one of your cells (except your sperm or egg cells) has an exact copy of 23 pairs of chromosomes and unless you have an identical twin, your genetic code is unique to you. This forms the basis of "DNA fingerprinting" and "genetic engineering" which are used, amongst other things, for catching criminals. Samples of blood, saliva, semen or hair, all of which contain DNA, can be taken from a crime scene and these can then be used to confirm that a particular suspect was present at the scene.

One of the 23 chromosome pairs is the pair of sex chromosomes. As the name suggests, the sex chromosomes determine a person's sex. A male has one X and one Y sex chromosome and the female, two X chromosomes. The remaining 22 pairs of chromosomes are known as the autosomes. Only sperm or egg cells have less than 23 pairs of chromosomes; they have exactly half this number or 23 unpaired chromosomes (see Figure 15b). This is because when an egg is fertilised by a single sperm cell, both parents' genes are brought together and the total number of pairs of chromosomes in the fertilised egg cell adds up to the usual 23.

Except in very unusual circumstances, the mother's unfertilised egg cell always has one X sex chromosome, whereas the father's sperm cell can either have an X or a Y sex chromosome. When the egg cell is fertilised by the sperm cell, the sex of the baby is established, XY for boys and XX for girls.

Occasionally, an abnormal gene is inherited from one of the parents. In inherited Kallmann's syndrome, the abnormal or Kallmann gene is located on the "X" sex chromosome (Figure 16a). The Kallmann gene is therefore defined as being X-linked. If a male, who always gets his single X chromosome from his mother, inherits the Kallmann gene, he will develop X-linked Kallmann's syndrome

However, this does not mean that a female, who gets one X chromosome from her father and another from her mother, will also develop X-linked Kallmann's syndrome if she inherits the abnormal gene. This is because the Kallmann gene works in such a way that it is only "switched on" if there is no "healthy" second X chromosome to cancel it out. In other words, X-linked Kallmann's syndrome will only become apparent if the baby's sex chromosome pair is XY; i.e. X-linked Kallmann's syndrome affects males only. It is extremely unlikely that a female with Kallmann's syndrome has the X-linked form, because this can only be the case if both of her parents also have the Kallmann gene. A woman can be a carrier of the X-linked Kallman gene and still be perfectly healthy, but there is a risk of her passing it on to her own children.

If you are either a man with X-linked Kallmann's syndrome or a woman carrier of the Kallmann gene, Figure 16b applies to you. In this case, neither of your parents have the disease but your mother is a carrier, having inherited the gene from her own family. Your mother's egg cell could either have contained a healthy X chromosome or the defective Kallmann gene linked to an abnormal X chromosome. Your father's sperm cell would have contained either a healthy X or Y chromosome. There was a 50% chance that any child your parents had would have been completely healthy with no sign of the Kallmann gene at all, a 25% chance that a daughter would also have become a carrier and a 25% probability that their son would have inherited the Kallmann gene and would later have developed X-linked Kallmann's syndrome.

Figure 16c will only apply to you if you are a female carrier. In this case, your mother is perfectly healthy but your father has X-linked Kallmann's syndrome. Obviously, your father must first have been treated successfully for his infertility. If you have a brother or brothers, they are healthy and will not develop Kallmann's syndrome. However, you and all of your sisters are carriers, the implications of which have already been explained above and illustrated in Figure 16b.

In addition to X-linked Kallmann's syndrome, there is also a less common inherited forms of the disease which affects men and women equally.

Apart from the x-chromosome there are genes on other chromosomes that are known to, or are thought to cause Kallmann's and other types of HH. There is some, but a lot of active research in this area.

Currently there are at least 5 different genes on 4 different chromosomes that are thought to be involved in the development of inherited KS and HH.

There is no clear picture on the inheritance of Kallmann's and HH at present, but research is being done in this area.