The essay concentrates on the single gene defects, looking initially at the types of mutations which effect genes and then moving on to the three major groups of these disorders; autosomal recessive, autosomal dominant and sex-linked. Each of these groups accompanied by at least one example and within the autosomal recessive disorders, haemoglobinopathies are discussed. There is then a brief discussion about the molecular basis of some cancers and the other groups of human molecular diseases.
Human Molecular Diseases.
Human molecular diseases cover a vast range of disorders that are caused by genetic mutations. There are many different types of mutations yet they cause disease in two main ways; affecting the quantity or the function of the gene product. Much about molecular diseases is still unknown due to the complexity of such diseases and the effects of other factors such as environmental factors. The field is however developing quickly with ever growing knowledge of gene structure and functions, from projects such as the Human Genome project. .
Single Gene Defects.
A large proportion of human molecular diseases are caused by single gene defects'. A single gene defect arises from a mutation in either a single allele or in both the alleles from a pair. Over four thousand of such defects have been found, these can be separated into three main categories depending on the affect of the mutation. The categories are; autosomal recessive, autosomal dominant and sex-linked. Autosomal' refers to mutations affecting the non-sex chromosomes (1-22). Where as sex-linked defects arise from mutations which occur on the sex chromosomes. Autosomal conditions affect males and females at equal frequencies, where as sex-linked conditions affect the sexes differently. A recessive' condition means that there is no expression of the disorder unless both alleles in the pair are affected. Where as a dominant' condition only requires a mutation in one of the alleles of the pair for the disorder to be expressed.