Patterns of inheritance of disease

We have two copies of every gene (except for the genes on the X and Y chromosomes). One copy is inherited from our father and the other from our mother. The two copies of a gene may be exactly the same or slightly different versions, called alleles. The genes for eye colour and hair colour are alleles accounting for the variations that are seen in eye and hair colour. Most characteristics and diseases have a complex genetic and environmental basis. There are a few traits and diseases that result from different alleles of a single gene that are well understood. Alleles can be dominant or recessive. A dominant allele is one that influences the phenotype even if it is present in only one copy. This means that if you have one mutant gene that causes the disease, you will develop the disease. An example of an autosomal dominant disease is Huntingtons disease. A recessive allele will only effect the phenotype if its counterpart allele on the other chromosome becomes inactivated or lost. This means that individuals that have one copy of a mutant gene will not develop the disease but will be carriers - they have a 50% chance of passing the mutant gene to their offspring. If both parents are carriers, there is a 25% chance that their offspring will inherit two recessive alleles and therefore develop the disease. Cystic fibrosis is an example of an autosomal recessive disease. Mutations on the X chromosome show a third and fourth pattern of inheritance, namely sex-linked dominant and sex-linked recessive diseases. X chromosomal genes are different from the autosomal genes because the male only carries one copy of the X chromosome that he inherited from his mother. A recessive allele does not influence the phenotype in females if it is present in only one copy, because there is a second copy to block it. Of course there is no second gene copy in males and therefore the recessive allele will appear. An example of an X-linked recessive trait is colour blindness. Sex linked dominant traits are quite rare. Most diseases are not so simple. Cystic fibrosis can result from a number of different mutations in the same gene causing a range of different effects in individuals while Alzheimers patients can show an identical outcome as a result of mutations in several different genes. Some diseases are multi-factorial requiring mutations to develop in several different genes (heart disease). The phenomenon of imprinting determines which of a pair of genes, either the one from the mother or the father, will be activated. In some cases, an individual may have the mutated gene and not develop the disease. Penetrance is a measure of the likelihood that a given mutation will cause a disease.