Gene expression and environment

The growth and the development of an organism not only depend on the internal factors in the body but also depend on external factors such as the environment. We know that genes interact with environmental factors. Environment-gene interaction plays an important role in evolution and variation. Phenotypic changes and development often require environmental factors along with the genetic components and thus involve tight regulatory mechanisms. The genes encode certain products which directly or indirectly depend on certain environmental factors. Also, certain environmental factors induce changes in the DNA sequence. Thus gene-environment interaction plays a very important role in the development of an organism. Sunlight and UV rays sometimes harm the genetic components. On a contrary, some conditions require adequate exposure to sunlight. Thus, it depends on the health condition and the type of the genetic component. Here, the environmental factor involves the sunlight. The genetic component involves the gene sequence getting affected.

Penetrance and expressivity:

Image: Penetrance and Expressivity

Certain individuals show a phenotype which may not necessarily match with its respective genotype. The heterozygous gene mostly expresses similar to the homozygous dominant gene. The frequency with which the homozygous dominant gene or the heterozygous dominant gene expresses itself is known as penetrance. So, penetrance is nothing but the frequency of the gene expression (mostly the dominant one). The presence of epistatic or other genes also contributes to the penetrance. Thus, penetrance not only depends on the genotype but also depends on the environment. A complete penetrance is always 100%. It occurs in three main conditions. The first condition involves the expression of one phenotype by all the homozygous recessives. The second condition involves the expression of the other phenotype by all the homozygous dominants. The third condition involves all alike heterozygotes. For example, the individuals in a population express the phenotype associated with the dominant mutant allele. Such a condition involves 100% penetrance since the expected phenotype gets expressed completely. Example of complete penetrance includes human ABO blood group system.

Now, we clearly get to known the concept of incomplete penetrance from the idea of complete penetrance. The expression of the phenotype associated with the genotype is less than 100%. Hence, the condition is known as incomplete penetrance. The expected phenotypic expression is less than 100%. In some of the cases, many genes altogether show a reduced penetrance. Example of reduced penetrance involves a condition known as Brachydactyly. It is a condition in which the trait follows an autosomal dominant pattern and causes shortened fingers affected by a malformation. Many of the cancer genes or oncogenes also exhibit low to moderate penetrance.

The degree to which a penetrant gene or the genotype expresses the respective phenotype in an individual is known as expressivity. Hence, a phenotype manifestation not only depends on the frequency at which a gene expresses the phenotype but also depends on the degree to which it gets expressed. Expressivity also depends on both the factors such as the genotype and the environment. Osteogenesis imperfecta is a condition involving variation in expressivity. The disease gets characterized by the blueness of the sclerae, bone fragility, and hearing loss. It exhibits 100% penetrance. One more condition involves both incomplete penetrance and variable expressivity. Neurofibromatosis is a condition in which fibrous tumor-like outgrowths manifests on an individual’s skin. This autosomal dominant disease shows 50-80% penetrance and also the variable expressivity. The pigmented areas on the skin are known as café-au-lait spots since they resemble the color of the coffee.

Environmental effects:

Various environmental factors affect the expression of the genotype including the age of onset, sex, temperature, and exposure to chemicals. Many genes show their levels of expression at a particular age. Thus, not all the genes get involved in expression all the time. Pattern baldness in males is a common example of the age of onset. Certain gene expressions exclusively depend on the sex of the individual. For example, pattern baldness is more common in males than in females. Such genes are known as sex-linked genes. Their expression depends on the different complements present on the sex chromosomes, mainly the X and the Y chromosomes. On the other hand, sex-limited trait gets expressed due to the genes present on the autosomes resembling in the expression of the genes present on the sex chromosomes. There is one more concept associated with the sex-influenced trait. These traits also get related to autosomes. However, the levels of expression in both the sexes differ. One example of sex-influenced traits is pattern baldness. The autosomal gene becomes dominant in males and recessive in females. The gene expression depends on sex hormones. The male sex hormone known as testosterone plays an important role in the expression of the genes.

Various biochemical reactions require enzymes for catalyzing the steps and taking the process forward. Certain genes (not all the genes) express or encode for temperature-sensitive enzymes. The fur color in Himalayan rabbits is an example of temperature-dependent gene expression. Certain genotypes lead to the appearance of dark fur in the ears, nose, and paws since the local temperature is low there. Certain chemicals also act as environmental factors affecting the gene expression. For example, phenylketonuria is an autosomal recessive disorder characterized by excessive piling up of the amino acid known as phenylalanine. Changes in the chemical composition of the environment also influence the expression of the genes. For example, the administration of new drugs or exposure to bacteria or viruses produces phenocopies. A phenocopy is a non-hereditary phenotypic modification caused due to environmental changes.

Nature versus nurture:

Examples of nature versus nurture are many. Tall parents mostly have tall children. The same concept is applicable to the short parents and their children. Achondroplasia is a kind of dwarfism affecting the development of the leg bones affecting the height and other factors in an individual. A single genotype shows the ability to express a range of phenotypes depending on the exposure to a range of environmental conditions. It is known as the norm of reaction. Thus, many human behavioral traits depend on gene-environment interactions.

References:
[1] Genetics, 9th Edition, Verma P.S. & Agarwal V.K.

[2] Advanced Genetics, Gurbachan S. Miglani
[3] Thompson & Thompson Genetics in Medicine,  Robert L. Nussbaum
[4] Emery's Elements of Medical Genetics, Peter D Turnpenny, Sian Ellard

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