Incomplete Penetrance of Genes

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Incomplete Penetrance of Genes

Incomplete Penetrance of Genes
Some genes in homozygous as well as in heterozygous conditions fail to provide complete (cent per cent) phenotypic expression of them. Such genes are called to have incomplete penetrance.
Examples of Incomplete Penetrance
I. Polydactyly in man is thought to be produced by a dominant gene P. The normal condition with five digits on each limb is produced by the recessive genotype (pp). Some heterozygous individuals (Pp) are not polydactylus and therefore has a penetrance of less than 70%.
2. In man, the tendency to develop diabetes mellitus (a condition in which there is an excess of sugar in the blood) is controlled by certain genes.

However, not everyone carrying the gents for diabetes actually develops the condition, for the genes have incomplete penetrance.
Effects of environment on penetrance- The environmental factors and genetical background have some definite effect on the degree of penetrance of a gene. Accordingly, the percentage of penetrance of a given gene may be altered by changing the conditions of temperature, moisture, nutrition and so forth, under which the organism develops. For example, when various twins which carry genes for diabetes mellitus are studied, it is found that the disease appears only in those cases which ate more carbohydrate foods (starch and sugars).

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