Genes and Disease

We develop disorders because of the genes that we are given, and what we give our genes. Genes consist of a number of molecules of DNA (deoxyribonucleic acid) which is a structure made up of strings of chemicals called nucleotides. There are four nucleotides adenine, guanine, thymine and cytosine. The order in which these occur in the strand of DNA is the so-called genetic code. Genes are of variable length depending on the protein that they code for. They are contained on chromosomes, of which a human cell has 46 - 2 of which are so called sex chromosomes: X X in a female and X Y in a male. The other 44 comprise 22 pairs. Under normal circumstances we inherit one of each pair of chromosomes from each of our parents. The chromosomes contain about 100,000 genes, consisting of thousands of millions of nucleotides. The sequence of the human genome has recently been elucidated, giving rise to the possibility that individual gene defects responsible for individual disorders might be identified, and more targeted treatments be produced. We all have different genes (with the exception of identical twins) with a great degree of variability - which is why we are all different. It is the variability in the genes that conveys different susceptibility to different diseases, while what we expose ourselves to may determine whether or not the disease develops. With some conditions the presence of the defective gene means that we will definitely develop the disorder (that for Huntington's chorea, for example, described in the brain section) while others require environmental triggers, which we may not meet. For some conditions both copies of the gene need to be defective before it develops a (a mode of transmission called autosomal recessive) while others require only one defective copy (and so-called autosomal dominant). Other disorders involve many genes - called polygenic - and may require exposure to a number of environmental factors. We don't only reap what we sow, we also reap what we’re sown.

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