2. Variable expressivity:
When the phenotypic expression differs in different carriers of a gene is known as variable expressivity, e.g.
In lima bean a dominant gene responsible for chlorophyll deficiency in seedling leaves exhibit variable expressivity.
In some seedlings it leads to chlorophyll deficiency on leaf tips, in other on leaf margins and in some cases entire cotyledonary leaf.
Expressivity is also influenced by environmental conditions in some cases. The epistatic gene interaction also affects penetrance and expressivity of genes.
Variable expressivity may be due to the following reasons.
1) Modifying effect of the genetic background or modifying genes.
2) The effect of various environmental factors on the gene expression.
A single gene with variable expressivity produce a number of phenotypes often producing a quantitative type of variation in the concerned characters, e.g. Recessive gene producing partial chlorophyll deficiency in the cotyledonary leaves of lima beans.
Cotyledonary leaves of seedlings homozygous for this entire area of cotyledonary leaves. In most other variable part of cotyledonary leaves lacking chlorophyll while the leaves of other seedlings show chlorophyll deficiency only at the margins or at the tip.