During fertilisation the egg cell merges with the sperm cell and a single cell results with - if all goes well - 46 chromosomes: 23 from the mother and 23 from the father. In the egg cell the 23rd chromosome is by definition an X; a sperm cell, however, has either an X or a Y. It is therefore the sperm cell that determines the gender of the embryo: if it adds an X, the embryo will be XX (a girl), and otherwise it will be XY (a boy).
We inherit our genes through the chromosomes, so logically we also have two copies of these. We refer to the two variants of the same gene as alleles.
If a particular characteristic or disorder is linked to a single gene, it is referred to as monogenic. If multiple genes are responsible we call it multigenic and if environmental factors also have an impact it is called multifactorial.
Monogenic, Mendelian inheritance
The fact that the number of chromosomes is reduced by half in sex cells explains why we inherit some characteristics from one of our parents and not others.
It is a matter of chance which of the two recombined chromosomes in the pair ends up in the sex cell from which we are formed as an embryo. For example, if one parent has one healthy and one mutant allele of a particular gene, there is a 50-50 chance that the affected allele will find its way into the fertilised egg. If the other parent only has two healthy cells for the same gene, there is a one in two chance that the embryo will have one affected allele.
As soon as fertilisation has occurred, genetic transmission occurs according to specific rules. During the nineteenth century this was discovered through cross-breeding peas and flowers by an Austrian monk named Mendel - which is why it is called Mendelian inheritance.
Two alleles (two versions of a single gene) may be identical: if so the mother and the father pass on the same instruction for a particular characteristic. They may, however, be different. In these cases we have two versions in our DNA instruction manual. In this situation there are certain characteristics that are more likely to be expressed, while others are less likely to be expressed. The former are referred to as 'dominant' and the latter are 'recessive'.
The two gene variants are collectively known as the genotype. This determines the characteristics displayed by the descendant, i.e. his or her phenotype. As a descendant, you will only express the recessive characteristic if you receive a recessive variant of the gene from both parents. Otherwise the dominant variant of the gene will be expressed: if it is inherited this characteristic is always expressed.
If your father has black hair and your mother is blond, you are very likely to have black hair. That is because black hair is dominant. If, however, your father also has a blond allele (alongside the dominant black one), he can still father a blond child with your mother, if the fertilised sperm cell happens to contain the recessive allele. In that case the child has received two blond alleles. What is more the child may be blond even if neither of the parents has blond hair. In that case both parents are carriers of the recessive gene and both of them pass it on by chance.
Genetic variations that cause diseases can also be dominant or recessive. That is why we distinguish between three types of inherited disorders:<top>
In this case the abnormality is on one of the 22 autosomes (body-defining chromosomes) and it tends to take precedence over the healthy allele. If the affected parent passes on the affected allele, it will also take precedence over the healthy allele that the embryo may have received from the other parent.
In short, the risk of inheriting a dominant disorder is fifty percent in every pregnancy. Whenever the abnormality is present, the disorder will occur.
Known autosomal dominant disorders include Huntington's disease, Steinert's myotonic dystrophy and Marfan syndrome.
Once again the abnormality is situated on one of the 22 autosomes but in this case the healthy allele takes precedence over the affected one.
The only way the disorder can be passed on is if both parents are carriers and both happen to pass on the recessive gene (just as in our example of a blond child with two dark-haired parents).
The risk of passing on an autosomal recessive disorder is therefore 25 percent. If the descendant carries only one mutation he or she will not develop the disorder.
The commonest autosomal recessive disorder is cystic fibrosis. A number of metabolic diseases are also inherited in this way.
A sex-linked inherited disorder is caused by a genetic mutation on the X chromosome. These are also inherited in a dominant or recessive pattern. There are also disorders associated with the Y chromosome, but these are inherited less frequently. That is because they are often associated with male infertility. <top>
In X-linked dominant disorders, both men and women are almost always affected.
In X-linked recessive disorders, girls have a reduced chance of inheriting the condition because they have two X chromosomes, one of which is always inactive. This deactivation usually occurs at random, so they are usually not affected. Boys, however, have only one X chromosome. If they inherit an affected allele from their mother, even if it is recessive, they will have the disorder. This explains why some X-linked disorders or characteristics occur much more frequently or almost exclusively in boys. Well-known examples of this include colour blindness and haemophilia (a blood clotting disease).
The risk that a boy will inherit an X-linked disorder from his mother is fifty percent. He cannot pass on the condition to his son, because a son will by definition inherit his Y chromosome. If he carries the disorder, however, he will pass it on to his daughter. She will in turn become a carrier.