Penetrance anD exPreSSivity
Genes express themselves by producing visible phenotypic effect. Generally, the structural genes express themselves in all the
individuals that carry them. The ability of gene to express itself in an individual that is carrying it, is called penetrance.
all the factors in Mendelian experiments were able to express themselves completely, producing the expected phenotypes. but
cases are noticed where structural genes failed to produce expected phenotypic ratio. On this basis penetrance can be complete or
incomplete.
complete penetrance
In complete penetrance, a dominant gene expresses itself in all the organisms in which it is present producing the expected phenotype
while the recessive gene produces the associated phenotype in all those organisms in which it is present in homozygous condition.
examples of complete penetrance
(i) In pea plants the red and white flower colour, tall and dwarf character, the seed colour and shape all exhibit complete
penetrance.
(ii) In Drosophila, the recessive gene for wing character (vestigial wings) has complete penetrance.
(iii) In guinea pigs the gene for coat colour exhibits complete penetrance.
incomplete penetrance
Some genes when in homozygous condition fail to produce the phenotypic expression in all the organisms in which these are
present. In these cases degree of penetrance is usually expressed as the percentage of individuals that actually express the phenotype
determined by a particular genotype.
examples of incomplete penetrance
(i) In man, genes for polydactyly, blue sclerotic and diabetes mellitus all exhibit incomplete penetrance. The gene for blue
sclerotic syndrome (produces sclerosis of blue colour) has about 90% penetrance, i.e., it develops in about 90% persons,
possessing it. Similarly, gene for polydactyly has 70% penetrance.
(ii) a gene in Lima beans causing partial chlorophyll deficiency in cotyledonary leaves has 10 per cent penetrance.
expressivity
The expressivity is the ability of a gene to express itself uniformly in all the individuals. The genes may have uniform or variable
expressivity.
uniform expressivity
a gene that expresses itself uniformly in all the individuals that carry it, has uniform expressivity. In Mendel’s experiments, all the
factors had uniform expressivity.