In genetics, expressivity is the degree to which a phenotype is expressed by individuals having a particular genotype. Alternatively, it may refer to the expression of a particular gene by individuals having a certain phenotype. Expressivity is related to the intensity of a given phenotype; it differs from penetrance, which refers to the proportion of individuals with a particular genotype that share the same phenotype.

Variable expressivity

Variable expressivity refers to the phenomenon by which individuals with a shared genotype exhibit varying phenotypes. This can be further described as a spectrum of associated traits that can range in size, colour, intensity, and so forth. Variable expressivity can be seen in plants and animals, such as differences in hair colour, leaf size, and severity of diseases.

Mechanisms influencing expressivity

thumb|318x318px|Figure 1. This figure illustrates some factors influencing gene expressivity including cis-regulatory elements, trans-acting factors, environmental factors, and DNA modification.

This variation in expression can be affected by modifier genes, epigenetic factors or the environment.

  1. Modifier genes can alter the expression of other genes in either an additive or multiplicative way.
  2. Epigenetic factors are heritable changes in the chromatin accessibility that affect the gene expression. Epigenetic factors can include:
  3. Cis-regulatory elements, which are regions of non-coding DNA that regulate transcription of genes, such as promoters or enhancers.
  4. Trans-regulatory elements, which are regulatory proteins, such as transcription factors (TFs) that bind to DNA to regulate gene expression.
  5. Histone modifications, which regulate the accessibility of chromatin for gene transcription.
  6. Chromatin variants, which are different states of chromatin.
  7. Genomic imprinting, which determines whether some genes inherited from the mother and father get expressed.
  8. The expressivity of a gene can be influenced by the environmental conditions. For example, pigmentation in the fur of Himalayan rabbits is determined by the C gene, the activity of which is dependent on temperature. During rearing of genetically identical rabbits, if a rabbit’s fur reaches a temperature higher than 35°C, the fur will develop as white. If a rabbit’s fur stays at a temperature between 15 and 25°C, the fur will develop as black.

Variable expressivity in plants and animals

Plants

Expressivity is commonly seen in plants and can be regulated by complex interactions between the environment, hormonal signalling, and genetics. An example of expressivity in plants caused by a rare gene is the variation in the number of branches. Initially identified in sorghum plants, this rare gene is called the Sorghum bicolor Axillary Branched Mutant (SbABM). Over several years of studies on SbABM in the rabi sorghum plant, researchers found that the progeny of the plants ranged from having 0 to 33 branches, even though they all had the same SbABM genotype.

Clinical application

thumb|200x200px|Figure 3. Example of Cleft lip seen as a result of the Van der Woude syndrome. |alt=thumb|201x201px|Figure 2. Individuals with Marfan syndrome usually have fingers that are longer than those that do not have the syndrome. The extremity of difference in finger length is a result of variable expressivity.|alt=|leftSome common syndromes that involved phenotypic variability due to expressivity include: Marfan syndrome, Van der Woude syndrome, and neurofibromatosis.

The characteristics of Marfan syndrome widely vary among individuals. The syndrome affects connective tissue in the body and has a spectrum of symptoms ranging from mild bone and joint involvement to severe newborn forms and cardiovascular disease. This diversity in symptoms is a result of variable expressivity of the FBN1 gene found on chromosome 15 (see figure 2). The gene product is involved in the proper assembly of microfibrils, which are structures found in connective tissues to provide support and elasticity. These varying levels were not associated with either sex or age. Lower levels of mRNA expression were associated with a higher risk for ectopia lentis, the displacement of the crystalline lens of the eye, and pectus deformity, an abnormality of the chest muscle, indicating that variation in expression could be due to levels of expressivity and not genotype. Carriers of the rare allele can also have pits near the centre of the lower lip which may appear to be wet due to the presence of salivary glands.

Neurofibromatosis (NF1), also known as Von Recklinghausen disease, is a genetic disorder that is caused by a rare mutation in the neurofibromin gene (NF1) on chromosome 17. This loss of function mutation in the tumor suppressor gene can cause tumors on the nerves called neurofibromas. These appear as small bumps under the skin. It is stipulated that the phenotypic variation is a result of genetic modifiers.

See also

  • Anticipation
  • Pleiotropy
  • Mendelian inheritance
  • Genetic heterogeneity
  • Haploinsufficiency

References

Further reading