variable expressivity
The genes which do not have similar expression in the individuals possessing them, have variable expressivity. The expressivity
of a gene may be influenced by the environment or by the genetic make up of the individual. The presence of complementary,
supplementary, modifier or suppressor genes in the genotype of an individual change the expression of the gene in question.
examples
- The gene producing chlorophyll deficiency in Lima bean (Phaseolus lunatus) exhibits variable expressivity in addition to
incomplete penetrance. In some seedlings, the complete cotyledonary leaves are without chlorophyll, in some only the leaftip
lacks the chlorophyll, while in others the margins of the leaves have no chlorophyll. - In man, gene for polydactyly has variable expressivity and incomplete penetrance. Polydactyly is due to the dominant gene
P, the normal condition with five digits in each limb is due to the genotype pp. It has been noticed that some heterozygous
(Pp) persons are not polydactylous. Moreover, polydactylous persons mostly have an extra digit in only one of the hands
or feet.
Why do phenotypes show differences in penetrance and expressivity?
• research has shown that variable phenotypes can be caused by a number of factors, including modifier genes, environmental
factors, allelic variation, complex genetic and environmental interactions.
• In most cases in which a particular genotype is inherited, it is not fully known why the same allele can cause subtly
different or profoundly different phenotype. In some cases, however, there is genetic evidence that modifier genes influence
phenotypic variation.
• Modifier genes can affect penetrance, dominance and expressivity. a genetic modifier, when expressed, is able to alter the
expression of another gene. Modifier genes can affect transcription and alter the immediate gene transcript expression, or
they can affect phenotypes at other levels of organisation by altering phenotypes at the cellular or organismal level.
• Some modifiers can increase the proportion of individuals affected by a disease-causing allele by decreasing the threshold
for trait expression. Other modifiers may also shift the trait distribution, or the range of disease phenotypes, which causes
more individuals carrying a disease-causing allele to express a more (or less) severe disease phenotype. This process
results in variable expressivity.
• Many human diseases also vary according to genetic and environmental factors, which can lead to both incomplete
penetrance and variable expressivity. In fact, for most diseases, variable expressivity of the disease phenotype is the norm
among individuals who carry the same disease-causing allele or alleles, though the causes are not always clear. Thus, the
identification of additional modifier genes will help scientists better understand the nature of a wide range of human
diseases.
New MCQs
- 9 : 3 : 3 : 1 ratio is replaced by 9 : 7 ratio due to
(a) complementary gene
(b) hypostatic gene
(c) supplementary gene
(d) epistatic gene. - When both alleles express their effect on being present
together, the phenomenon is called
(a) dominance
(b) codominance
(c) pseudodominance
(d) amphidominance.
3. In Mirabilis jalapa, when red colour flowers were crossed with
white flowers, pink colour flowers were obtained. It was due
to
(a) epistasis (b) complete dominance
(c) crossing over (d) incomplete dominance.
4. The phenomenon, in which an allele of one gene suppresses
the activity of an allele of another gene, is known as
(a) epistasis (b) dominance
(c) suppression (d) inactivation.
5. Complementary genes were first studied by , in 1906,
in case of flower colour of .
(a) bateson and Punnet, Lathyrus odoratus