In this article we will discuss about the role of environment in changing the form and degree of expression of some genes resulting in a corresponding change in the phenotype.
Penetrance and Expressivity:
The presence of a gene does not always bear an absolute relationship with the appearance or absence of a trait. In the ABO blood group system, the genes are expressed in an absolute way. But in many other instances the gene is expressed in a variable manner, i.e. the visible phenotype shows varying intensities.
The terms penetrance and expressivity are used to describe variable gene expression. Penetrance is the proportion of individuals that show an expected phenotype. When a gene is completely penetrant it is always expressed; when incompletely penetrant, the gene is expressed in some individuals, not in others, the proportions depending upon the degree of penetrance.
For example in the recessive traits which Mendel studied, the phenotype was expressed fully when the gene was in homozygous condition; this is due to 100 per cent penetrance. Suppose instead that in a hypothetical cross, only 60 per cent of individuals show the expected trait when all 100 are carrying the gene; we say that in this case penetrance is 60 per cent.
Expressivity is the degree to which a gene is expressed in the same or in different individuals. Thus the gene for lobe eye in Drosophila may show a complete range of phenotypic expression in different individuals. Some flies may have a normal sized eye, in others the eye is smaller, in still others the eye is absent.
The earliest studies related to the effect of temperature on genetic constitution were done on the Himalayan breed of rabbits and Siamese cats. Coat colour in rabbits is controlled by multiple alleles of a gene.
When one of the recessive alleles ch is present in the homozygous condition (chch ), the Himalayan coat colour results. Such a rabbit is a mosaic with white fur all over the body except the nose, paws, ears and tail which are black (Fig. 5.1).
The black extremities are the portions which have lower temperature (less than 34°C) than the rest of the body. If the extremities are exposed to higher temperature artificially, the new hair which starts growing is white.
Similarly, if some portion of the body bearing albino fur is artificially kept at a lower temperature, the new hair formed is black. These observations explain the temperature sensitive behaviour of the allele (ch) which controls Himalayan trait in the homozygous state.
The allele codes for an enzyme used in pigment formation which is temperature sensitive and is inactivated by temperatures above 34°C resulting in albino phenotype; if temperature is lower the same alleles promote synthesis of pigment and the phenotype is black.
When rabbits of this genotype are grown at cold temperatures, they are completely black. The Siamese cat shows the same pigmentation pattern as the Himalayan rabbit due to the presence of similar type of temperature-sensitive allele.
In Drosophila temperature changes the penetrance of the gene known as tetraptera which controls wing development. At 25°C the gene has 35 per cent penetrance so that the corresponding number of flies develop wings whereas 65 per cent do not. At 17°C penetrance is much reduced so that only one per cent of flies show the winged phenotype.
The recessive gene vg/vg which produces vestigeal wings in Drosophila is also influenced by temperature. At 32°F the wings are feebly developed and extend very little from the body (Fig. 3.1). At 40°F the wings are better developed and have some venation. At 88°F wings are well developed with conspicuous venation.
Some temperature-sensitive mutations are exhibited in bacteriophages. In general the temperature at which normal phenotypes are produced is referred to as permissive temperature: that which produces mutant phenotypes is called restrictive temperature. Some lethal mutations in viruses and in Drosophila are temperature sensitive. Among plants, colour of flower in primrose changes from red to white when temperature is raised above 86°F.
There is a gene in maize plants which controls anthocyanin pigment formation. When ears of plants carrying the homozygous gene are exposed to sunlight by removing the green leafy coverings on the young cobs, the kernels become bright red in color (“sunred”).
If however, the blue violet rays of the light spectrum are prevented from reaching ears of maize plants (by wrapping red cellophane paper around them so that only red rays penetrate the cells) the sunred phenotype is not visible.
In this case sunlight interferes with one or more chemical reactions leading to pigment formation. The reddish freckles on the sensitive skin of white skinned human races are also caused by sunlight in a similar way.
In human beings a skin cancer known as xeroderma pigmentosum is caused by a homozygous recessive gene. The skin becomes extremely sensitive to sunlight so that even a minor exposure to faint light gives rise to pigmented spots on the facial skin. The spots can become cancerous and if they spread to other parts of the body, death results. If an individual homozygous for the recessive gene is not exposed to light, the gene is not able to express itself.
Environment and Sex Determination:
The marine worm Bonellia demonstrates the effect of environment on sex. In this sexually dimorphic organism the female is very large, about 10 cm in length; the male is 3 mm long and lives inside the cloaca of the female.
If the free swimming larvae that have arisen from fertilised eggs remain in the sea bed away from the females, they develop into female worms. But if females are available, the larva settles on the female proboscis, draws nourishment from it, and develops into a male.
Of the many experiments performed with Bonellia, one is most interesting and relevant here. If Bonellia are raised in the laboratory in a tank containing artificial sea water, the free-swimming larvae settle down at the bottom of the tank and develop into females. But if the artificial sea water is agitated by some mechanical device, the larvae develop into males.
Depending upon the extent to which the environment influences the genotype, the changes in the phenotype may be subtle or dramatic. Sometimes the phenotype becomes altered by the environment in such a way that the new phenotype resembles another phenotype produced by known genes. The induced phenotype is not inherited and is called a phenocopy.
In many instances phenocopies result from application of specific treatments like radiation, chemicals poisons, temperature shocks etc. The Himalayan rabbit described develops a coat that is all black if the rabbit is made to live in a cold environment.
The Himalayan rabbit is a phenocopy of the genetically black rabbit. If both rabbits live together at moderately high temperature, the Himalayan rabbit has a phenotype very different from the genetically black rabbit.
One of the most striking examples of phenocopies could be observed in what were known as thalidomide babies in the early 1960’s. A number of deformed children were born in West Germany and Great Britain to mothers who had taken the tranquilizing drug thalidomide in their sixth week of pregnancy.
The abnormal children showed deformities in limbs; some had one, two or three limbs, others had no limbs at all. The abnormalities showed a great resemblance to another phenotype known as phocomelia caused by a recessive gene.
Diabetes mellitus is a heritable human trait associated with reduced amounts of the hormone insulin that is secreted by the pancreas. In the presence of insulin glucose is absorbed by the cell membranes. When the hormone is not produced in sufficient quantity, the unabsorbed glucose passes into the blood and urine. The exact mode of inheritance of diabetes is not properly understood.
There are different types of diabetes arising from different causes; it therefore seems likely that there are several gene pairs controlling the trait. On the other hand the study of a pair of genetically identical twins, one of whom had diabetes the other not, indicates that the condition is due to a recessive gene with low penetrance.
If proper doses of insulin are administered to a diabetic person, he reverts to the normal phenotype. In other words, control of diabetes produces a phenocopy of the normal individual. There are many other examples in human beings where, by giving drugs, the mutant genotype produces a phenocopy of the normal phenotype.
In haemophiliac patients, a specific protein required for blood clotting is either defective or deficient. If however, an anti-haemophiliac factor isolated from humans is injected into a patient, a phenocopy of the normal individual results. Similarly, if thyroxine is administered to a child whose thyroid gland does not secrete this substance in adequate quantities, the normal phenotype is produced.
The creeper trait in chickens is observed sometimes in domestic fowl when the newly hatched chickens have the legs drawn up under the body. The affected chicken is not able to walk but creeps along the ground.
The creeper trait (Fig. 3.2) is expressed by the heterozygous condition of a dominant gene which is lethal when homozygous. Creeper chickens can also be produced if incubating eggs of normal fowls are treated with boric acid or insulin. Such induced creepers are phenocopies of the genetically controlled heterozygous creeper chickens.
Due to a recessive gene, maize plants become dwarfed, because they are deficient in the plant growth hormone known as gibberellic acid. But if the hormone is supplied to the dwarf plants they grow to normal height producing phenocopies of normal plants.
Environmental Effects and Twin Studies:
In human beings it is not possible to perform controlled breeding experiments. Twin studies are perhaps the best way of determining as to whether the observed differences between individuals are due to heredity.
Twins are of two types—monozygotic or identical twins that arise from a single fertilised egg and have identical genotypes; dizygotic or fraternal twins which arise from two fertilised eggs and are therefore no more genetically alike than siblings (brothers and sisters).
The correct identification of twin types is difficult and unreliable unless done by a physician. For assessing the role of environment in heredity, the percentage of concordance (both twins showing identical phenotype) and discordance (different phenotypes) for a given trait must be determined for twins of both types.
In general if concordance percentage for a trait is high in the case of monozygotic twins, and much less in dizygotic twins, one can conclude that heredity has played a role. If the concordance rate is similar in monozygotic and dizygotic twins, it suggests that the environment is determining the phenotype. From studies of a large number of twins it has been found that measles (caused by infection with Rubelia virus in early pregnancy) is largely controlled by the environment.
On the other hand conditions like diabetes, schizophrenia, Rickets and tuberculosis appear to be controlled by the genotype. Another useful aspect of twin studies is to determine the effects of different environments on identical genotypes by analysing those rare cases of monozygotic twins that have been separated from birth and reared apart. However in absence of adequate data it is not possible to conclude much on this aspect as yet.
A number of studies have been done to determine how much of human intelligence and I.Q. are controlled by the genotype and how much by the environment. Both clarifications and complications have been revealed. The differences in intelligence among different racial groups have been extensively studied by Arthur Jensen in 1969. This work is highly controversial and has been much debated.
Nevertheless, it is generally agreed that intelligence is under the control of several gene pairs interacting with the environment. From twin studies it has been further estimated that about one-half to three-fourths of human intelligence is determined genetically; the remainder is controlled by the environment.
It is fairly well established that mosquitoes develop resistance to DDT and other insecticides used for eradicating malaria. The resistance develops due to change in the genotype in response to the environment, and is inherited.
Similar resistance is reported also in insects which carry the causal agent for some other diseases like dengue fever, yellow fever, filariasis and river blindness. A number of pests which are harmful to major crops such as rice, maize, cotton, wheat and potato are also known to have become resistant to a wide range of insecticides.