The environmental factors influence on realization of genotype to phenotype

All organisms have an adaptation to environmental factors which act during centuries such as gravitation, magnetic field, sun radiation and so on and which act locally such as food deficiency, freezing, overheating, noise and etc. That’s why human has a high level of adaptation to environmental factors. It is caused by gene ability to determine not only a definite trait but also limits of its variation. It leads to less dependence of organism from environment, to increasing complicity of genetic apparatus and gene controlling system. Realization of gene in phenotype occurs in proper environmental condition, which may be illustrated in following scheme.

The expressing of gene effects has particular characteristics as long as one gene in different organisms may be expressed by different ways. It is caused by different environment condition of gene expression.

The level of phenotypic gene expression is called expressivity of gene. For example, we may observe different grades of polydactylia manifestation.

One trait controlled by one gene may be expressed in one individual and be absent in another. Such event is called penetrance of gene. The penetrance is measured by calculating percentage of individuals having mutant phenotype in population, which is homozygous on this gene. The complete penetrance (100%) means that every individual express the trait, the incomplete penetrance (30-40%) means that only part of individuals express the trait. For example, Colombo (the defect of eye) is inherited dominantly with penetrance around 50%, amniotrophyc sclerosis has the same inheritance.

The syndrome of blue sclera (thin blue external eye coat, otosclerosis, deafness and frequent bone fractures) give 100% penetrance for blue sclera, 63% for frequent bone fractures and 60 % for deafness. All three symptoms are expressed in 44% of individuals (pic 8.4).

Ріс. 8.4. The gene penetrance in “blue sclera ” syndrome: black symbols are individuals with blue sclera; oblique line - frequent fractures; I - absence of penetrance; I-IV - generations (by J. Bell et al., 1928)

The terms “penetrance” and “expressivity” is more often referred to autosome dominant traits. The autosome recessive traits are expressed only in homozygotes with complete penetrance and high expressivity The expressivity and penetrance are determined by genotype interaction and different reaction of genotype to environmental condition.

The trait formation is determined by not only existence of specific gene in genotype. In some cases, the trait may be formed only by specific combination of external factors. The phenotype changes, which are look like genotype determined, but really having induced by environmental factors are called phenocopies. Thus, child rachitis may be due to low consumption of vitamin D or due to hereditary defect. Eye cataract, which is caused by German measles during pregnancy, looks like eye cataract due to gene defect.

Phenotype is formed also with help of environment and ontogenesis. So, tendency to presbyopia is inherited. But with lens accommodation some sight correction occurs. However, in elderly accommodation fails and it leads to presbyopia. The accommodation failure may be explained by working conditions.

We may state that phenotype formation depends on many factors. This principle shows a dialectic unity of genetic and environmental factors in development.