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  Section: Genetics » Lethality and Interaction of Genes
 
 
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Penetrance and expressivity

 
     
 
Content
Lethality and Interaction of Genes
Lethality
Interaction of genes
Abbreviated genotypic ratio
Two gene pairs affecting same character
Epistasis
Complementary genes
Duplicate genes
Additional interactions involving two gene pairs
Interactions between more than two gene pairs
Modifiers, suppressors and pleiotropic genes
Meiotic drive, segregation distortion and selfish genes
Penetrance and expressivity


In this section, we have emphasized that a gene can not determine the phenotype in isolation, but its effect on phenotype is controlled both by other genes and by the environment. These factors controlling the expression of a gene are complex and their precise nature can not be established easily. In order to describe this situation, the two useful terms often used in genetics are penetrance and expressivity.

Penetrance is defined as the percentage of individuals with a given genotype, which exhibit the phenotype associated with this genotype. For instance, all individuals with the genotypes AA or Aa, may not express the dominant phenotype, due to -either the presence of modifiers, suppressors, epistatic genes, etc. or due to the modifying effect of the environment. If only 80% of individuals with AA or Aa express dominant phenotype, it will be described as 80% penetrance of gene (or allele) 'A' Expressivity on the other hand, describes the degree or extent to which a given genotype is expressed phenotypically in a particular individual. This may also be due to lack of full expression due to rest of the genome and the environment. For instance, the red eye colour in Drosophila flies may show different shades of colour (within the norm of reaction) suggesting variable expression of wild allele ‘W’- in WW or Ww flies. The distinction between penetrance and expressivity is illustrated in Figure 3.3, and variable expression of coat colour in dogs is shown in Figure 3.4.

 
Effects of penetrance and expressivity on a hypothetical trait 'pigment intensity'. In each row all individuals have same genotype. (Redrawn from Suzuki et at., 1986).
Fig. 3.3. Effects of penetrance and expressivity on a hypothetical trait 'pigment intensity'. In each row all individuals have same genotype. (Redrawn from Suzuki et at., 1986).

In human pedigree analysis, variable penetrance and expressivity often create problems. For instance, if an allele causing a disease does not have 100% penetrance, an individual's phenotype in the pedigree may be disease free, but the individual may still carry the disease genotype leading to wrong classification (Fig. 3.5).

Four grades of piebald spotting in dogs attributed to variable expressivity. (Redrawn from Little, 1957).
Fig. 3.4. Four grades of piebald spotting in dogs attributed to variable expressivity. (Redrawn from Little, 1957).
 
Lack of penetranee of a dominant gene in an individual Q included fn a pedigree (the gene is passed on to a part of the progeny).
Fig. 3.5. Lack of penetranee of a dominant gene in an individual Q included fn a pedigree (the gene is passed on to a part of the progeny).







 
     
 
 
     




     
 
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