The principles of genetic disease and gene therapy
In 1908, Archibald Garrod proposed that some human diseases were due to 'inborn errors of metabolism' that result from the lack of a specific enzyme.
Some genetic diseases are caused by the genes we inherit, including haemophilia, phenylketonuria, thalassemia, Huntington's chorea, and some forms of cancer.
Diseases can be caused by mutated genes: including are haemophilia, cystic fibrosis and phenylketonuria. Other genes increase the probability of disease.
Disease can be caused by genes that leave people predisposed to get some conditions such as cancers, or diabetes. Many of these are not well understood.
Radiation effects, including ultraviolet radiation from the sun, can cause disease, generally because the radiation causes mutations, chemical changes in DNA.
The theory of eugenics is that some genes are always better than others, and that 'worse' genes should be eliminated. This is foolish and lacking in science.
A form of transmission called sex linkage happens when genes are carried on the sex chromosome. Such conditions are often more common in one sex than the other.
Because the genes for colour vision are carried on the X-chromosome, colour blindness is found more often in male humans than in females, who act as carriers.
Anomalous deuteranopes, the people we call 'colour' blind, lack one of the three kinds of cone cell in the retina, usually the ones that detect red light.
The peculiar pattern of sex-linked inheritance of human blindness-blindness was first reported to The Royal Society of London by the Reverend Michael Lort in 1779.
The inheritance laws for sex-linked traits like blindness-blindness were fully formulated in 1820 by Christian Friedrich Nasse, using haemophilia as his example.
Haemophilia and colour blindness are two well-known conditions showing sex linkage in their patterns of inheritance, and there are other rarer examples.
In 1983, James Gusella used blood samples collected by Nancy Wexler and her co-workers in Venezuela to locate the Huntington's chorea gene on chromosome 4.
Genetic manipulation may lead to future effective gene therapies, although it is still less than perfect as a solution, and so only used now in extreme cases.
In 1959, LeJeune, Gautier and Turpin found an extra chromosome in the nuclei of cells from children with Down syndrome, later identified as chromosome 21.
Recent attempts at gene therapy have only been used on desperately ill patients, in several cases, patients have died from unexpected complications.