Is there a gene for it?

 
Among the tests that are given to newborn babies in this country is one for a substance called phenylketone in their urine. This reveals the presence of what turned out to be a recessive single gene disorder (called phenylketonurea, or PKU). The baby is unable to make an enzyme, phenylalanine hydroxylase, which converts phenylalanine to tyrosine. Untreated, this used to lead to behavioral problems and severe mental retardation. The solution is to ensure that the individual's diet is free from phenylalanine.

Some individuals cope quite well as they get older. Although genes produce individual proteins, they do not act in isolation. The individual who develops (the phenotype), is the end product of all the genes and all the biochemical pathways they give rise to.

In addition, the mature nervous system does not appear to be so susceptible to its ill-effects. However, mothers with PKU have to be careful when they are pregnant, even though the baby has a normal (dominant) allele from its father.

At any given point on the DNA, a base pair of a gene on each chromosome may be slightly different, that is, there are different alleles or polymorphisms. Mendel showed that, in given pair, one may be dominant, and the other recessive. An individual may have any one of the four combinations of alleles. In three of these it is the dominant gene which is expressed.

Note that, in the population, there may be a number of alleles, of which any individual has two, and their dominance is relative. The dominant and recessive alleles may not be so in other pairings. Complicating the situation, in some cases dominance may not be so obvious.

Recessive conditions like phenylketonuria occur when an individual has two copies of the recessive gene.

Where an allele is dominant, there is a three out of four chance of being affected.

Geneticists are cataloguing long lists of so-called 'single-gene disorders' some due to recessive, other to dominant genes. Most of them are very rare. The vast majority of human variation, including illnesses, is polygenetic, that is, due to the interactions of many genes.

A simple example is height, which is influenced by a number of different genetic factors (including sex difference). It is also influenced by the environment in which one has grown up.

Some genes are not linked directly with physical features but are concerned with timing. The proteins produced by some, such as the homeobox genes, control the expression of other genes. Thus identical genes in various parts of the genome may have widely differing effects depending on the genes they control. The situation in inheritance is even more complicated. An allele which was recessive in the parent may be dominant to the changed allele in the child. The genes which interact with others may do so quite differently in the offspring, a phenomenon known as epistasis, and the genes involved may be on different chromosomes.

One has to be wary of reading about the influence of genes too uncritically - particularly newspaper articles, but also books by geneticists.

In Steve Jones' In The Blood there is the remarkable statement that "some genes do quite different jobs in the two sexes." The example quoted is a gene that is implicated in premature balding in men, and polycystic ovaries in women. A little thought shows that it may be an influence on both processes in both sexes.^* Men, though, are unlikely to suffer from polycystic ovaries, while, for women, premature balding may be masked by other factors.

Bibliography:
Jones, S., (1996) In The Blood: God, Genes and Destiny, London: Harper Collins

* In fact the gene codes for the enzyme 5 alpha reductase, which we will be returning to later.

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