Human Genetics , Examples of known single-factor


The "one gene-one character" effects described above illustrate very clearly the Mendelian* principles of inheritance, but they are the exceptions rather than the rule. Rarely do single genes control one trait. Colour in sweet peas (Lathyrus spp.) 1s controlled by two pairs of genes, CC and RR. Gene C controls the production of the colour base and gene R the enzyme which acts on it to make a colour. The recessive cc will produce no colour base and rr will have no enzyme, CCrr and ccRR combinations will thus be unable to produce coloured flowers.

At least six factors operate to produce coat colour in mice. In man, eight of the chemical changes involved in blood clotting are known to be under genetic control so that several genes are responsible for coagulation; absence of any one of them may lead to a blood-clotting disease such as haemophilia.

When one gene only is responsible for an important, physiological change, its absence or modification will have serious consequences. Therefore most known instances of single-factor inheritance in man are associated with ratheer freakish abnormalities. These are usually rare conditions, e.g. occurring once in 10,000 to 100,000 individuals, but there are a great number of different kinds of genetic abnormality.

Examples of known single-factor inheritance involving a dominant gene in man are white forelock, woolly hair, one form of night-blindness, one form of brachydactyly in which the fingers are abnormally short owing to the fusion of two phalanges and achondroplastic dwarfism in which the limb bones fail to grow. Recessive single factors are known to control, for example, red hair, inability to taste phenylthiourea, red-green colour vision, and one form of albinismm which is the absence of pigment from the eyes, hair and skin.

In the white-skinned races, although skin and hair colour are not controlled by single gene pairs, the number of alleles involved is probably quite small. If one assumes that for hair pigmentation there are only three genes, B,, B, and B3, B producing a light pigmentation and B, a heavy pigmentation, then it is possible to predict five possible phenotypes for hair colour as follows: blond (B,B,), light brown (B,B,), medium brown (B,B, or B,B,) dark brown (B,B,) and black (B ,B,).

In experimental animals or plants, the type of inheritance and the genetic constitution can often be established by breeding together the off'spring of the F generation, or by back-crossing one of the F, individuals with the mother and or father and producing numbers of offspring large enough to give results that have statistical significance. These methods are obviously not applicable to man and our knowledge of human genetics comes mainly from detailed analyses of the pedigrees of families, particularly those showing abnormal traits such as albinism, from statistical analysis of large numbers of individuals from different families for characteristics such as sex ratio, intelligence, susceptibility to disease, etc., and from individual studies of identical twins.

Despite the scarcity of evidence for single-factor inheritance in humans, there is plenty of evidence to suggest genetic control of many physiological, physical and mental characteristics.

Body height, eye colour, hair colour and texture, susceptibility to certain diseases, and facial characteristics are all genetically controlled but in a more complex way than that described for mice.


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