In the old days there was a superstitious notion that the character and fate of a person are determined by the location of the planets at the hour of his birth. By the end of the last century, when a person became more aware of the role of heredity in shaping body and mind, there was a tendency to give hereditary influences the same role that astrologers assigned to stars. In writings such as Zola and Ibsen, the moment of conception, when a combination of genes is formed given the same fatal meaning as the moment of birth among astrologers.
Modern medicine, psychology and pedagogy, on the contrary, emphasize the great role of the environment in the development of the individual. Many seek to reconcile these two points of view, dividing the signs into hereditary and non-hereditary. It is assumed that the latter are formed under the influence of external agents; the former are regarded as inevitable rock or inalienable property. For example, for many practitioners, the disease is either hereditary or non-hereditary, in the first case nothing can be done about it, and in the second there is hope for successful prevention and treatment.
Similarly, some people believe that mental abilities are hereditary. Others believe that this is not the case: all people are able to achieve the same level of mental development if they are provided with equally favorable learning conditions. The former believe that children who inherit the genes of good mental abilities will become highly intelligent individuals even without much training, while children who are not lucky with genes will not be able to benefit from even the best education.
In relation to crime, there are the same two systems of thought. Some consider the propensity to crime as hereditary rock, from which there is no salvation. There is a book called Crime as fate. The opposite point of view was expressed by one former employee of the British police, who writes in his biography: We must get rid of looking at the criminal as a hereditary type; he is you, or me, or any person living next to you.
Is crime inherited? Is hereditary cancer? Are high mental abilities the result of learning? – Similar questions for genetics do not make sense. These questions express the idea of contrasting heredity and environment. The idea of contrasting heredity and environment, nature and upbringing is incorrect (Galton, 1874). In fact, every trait, normal or abnormal, is the result of the development process and is determined by both genes and the environment. Although the relative participation of heredity and environment in individual traits varies greatly, only a few traits are completely determined by heredity, and the traits determined only by the environment are even smaller. A gunshot wound is caused by an external agent, but the wounded person could be involved in a quarrel because of its aggressive nature, which at least partially could be inherited. Moreover, the impact of the wound on the general condition of the patient and the speed of recovery depend on his constitution, partly hereditary. At the other end of the scale, nature-upbringing contains a number of signs that are entirely determined by the genes of the blood group, eye color, and diseases like the cholera of Genington.
For some of these diseases in the future, it is possible that they will find treatment with external influences; this is not possible for others, because the connection between the gene and the effect of its action is too direct for a person to intervene. Between these extremes lies the vast majority of signs, which are determined by the interaction of genes and the environment.
The first environment for a mammal is the mother’s womb. We have already met with some examples of the interaction of the uterine environment and the genes of the embryo. A child with a positive Rh factor develops normally in an Rh-positive mother, but its development can be greatly impaired if the mother is Rh-negative. A genetically female calf can develop into sterile freemartin, since the action of its sex genes is suppressed by the hormones of the male twin embryo. Another example is shown in Fig. 44. When a large heavy truck and a small Scottish pony are crossed, the size of the foal depends on how the parents are selected. A Scottish mare, covered with a large stallion, produces a smaller foal than a large mare, covered with a Scottish stallion. Genetically, both foals have the same set of genes that determine size (sex-linked genes do not play a role in this case), but only in the womb of a large mare can these genes manifest themselves completely.
Fig. 44. The influence of different conditions of embryonic development on the same genotype. Reverse crossings of ponies and heavy hauliers
Immediately after the birth of the animal, the influence of the new environment on both himself and the effect of his genes begins. Whether a cow will be a high-yielding one depends as much on care and feeding as on its genes. Some people stick to a diet in order not to gain too much weight; others can eat sweets without losing their slim figure. The same interaction of heredity and environment is observed in relation to more specific features. In rabbits that are homozygous for one of the recessive genes, yellow fat is formed from the substances contained in fresh grass. From a consumer point of view, yellow fat is a manifestation of an undesirable gene. A rabbit breeder, in which animals have a gene for yellow fat, can completely mask its action by eliminating greens from food.
The manifestation of many genes depends on temperature. The Himalayan type of coloring of rabbits is determined by genes that ensure the formation of pigment only at low temperatures. Therefore, only the most prominent parts of the body are painted. If you pull out the fur on the back and on the side and hold the animals in the cold, the newly grown wool will be black and dark spots will form on the skin (Fig. 45). The characteristic color of a Siamese cat is determined by a gene with a similar temperature dependence; Siamese kittens growing in the cold, darker kittens growing in a warm room. A special curly breed of chickens carries a dominant gene that causes feather curliness. In homozygous curly birds, feathers are abnormal and fragile, and often the birds are completely naked. If such a naked bird is wrapped in a woolen blanket, its plumage develops over its entire body.
Fig. 45. The effect of temperature on the manifestation of the Himalayan gene in rabbits. A black spot appears when new hair grows out at a low temperature on the spot of the plucked out
The life of green plants depends on the light. They use nutrients from the air with the help of green chlorophyll, and this process goes only in the light. Moreover, in most plants, chlorophyll cannot form in the dark. For the formation of chlorophyll requires a lot of genes. When one of them is lacking or it is defective, the seedling grows up in an albino, without chlorophyll, and dies of starvation (see Chap. 4, Fig. 6). The seedling will not turn green without light, even if it has all the genes necessary for the formation of chlorophyll. Thus, the same phenotype – a dying white seedling – may be due either to a defective genotype (Fig. 46, c), or to deficiencies in the environment (see Fig. 46, a).
Fig. 46. The same sign (the absence of chlorophyll) is manifested as a result of the action of the medium (a) or lack of a gene (c). Only 5 pairs of chlorophyll formation genes are shown; in fact, there are many more of them: ● – normal chlorophyll formation genes; ○ – mutant gene that prevents the formation of chlorophyll
Some plants only bloom when the day is long enough, they bloom well on long summer days in countries located far from the equator. Other plants for flowering, in contrast, require light short days, as in equatorial countries. The differences between these two groups of plants are determined genetically, and the phenotype of a flowering plant is determined by the interaction of the corresponding genes and the length of the day (Fig. 47).
Fig. 47. The interaction of the environment and the genotype in the formation of flowers in tobacco. N. Silvestris (top row) only blooms during the long light days of the temperate zone; N. Tabacum (bottom row) only blooms with short light days in tropical countries
Most genes, therefore, do not give a stereotypical effect, but only determine the nature of the body’s reaction to its environment. This is also true for genes that cause the appearance of deformities or human diseases. We saw in Chapter 12 that many of these genes are characterized by incomplete penetrance. This means that they are often not found in those individuals who genetically carry the potency for the development of the disease. In these cases, environmental conditions, usually not yet studied by doctors, prevent the manifestation of a harmful gene. In turn, genotypic differences often determine the response to environmental conditions that are the causes of the disease.
It is clear that the source of infectious diseases is the environment. There can be no infectious diseases without penetration of microbes into the body. But the strength of the infection, expressed in the lung or acute course of the disease or in the absence of it in general, depends on many circumstances; the general state of health during an epidemic or immunity due to a previous, possibly unnoticed, weak infection by the same microbe plays a role. Often, in addition, genetic differences in susceptibility play a role. As early as 1898, doctors in Maine (USA) noticed that infantile paralysis (polio, at the time the disease just described and poorly understood) tends to infect children from the same family. In 1943, a middle school teacher from the Mac Dowwell region of the Zap. Virginia has collected information on all people who have had polio in the last 50 years. He found that they were all related. The geneticist, who understood these data, came to the conclusion, which was later confirmed by mass research of families in the state of Indiana. Apparently, there is a recessive gene that is not sex-linked, allowing the virus to enter the nerve cell, which causes paralysis. The gene has an incomplete manifestation, and about one-quarter of people who are homozygous for it suffer a mild disease or remain healthy. The same is true of persons who are heterozygous for this gene or do not carry it at all. Thus, the virus, genotype and external factors (for example, exercise at an early stage of infection) interact in the process of infestation with childhood paralysis. A similar picture is observed in other infectious diseases, in particular, in tuberculosis. We will come back to this in the next chapter.
Cancer is such a common disease that the discovery of several cases of cancer in one family cannot serve as evidence of its inheritance, especially since relatives are likely to be exposed to the same factors that are believed to cause cancer or are carcinogenic, such as work in uranium mines or excessive smoking. In fact, there are no sufficient grounds for recognizing a genetic propensity for cancer in general. On the other hand, it seems as though there is a genetic element in the tendency to develop a certain type of cancer. In mice, genetically pure lines were bred, in which a large number of animals developed tumors of a specific type, for example, a chest or lung tumor.
Since even close relatives are very different in genotype in a person, the risk of a relative of a cancer patient to get the same type of cancer is low. A relative’s cancer disease should not over-bother them, but such people should be more attentive to the symptoms that may be the beginning of the type of cancer that prevails in the family.
Everything that is correct with regard to the properties of the body is also true for the features of the mind and emotions. The level of mental development, special abilities, personal qualities – all this is the result of the interaction of genetic factors and environmental factors. Assessing the relative role of the genotype and the environment in determining such traits is a difficult task. In the next chapter, we will look at one of the methods for resolving it.
Now back to the beginning of this chapter. The discussion has taken us far from the notion that our genes – like astrologers’ stars – determine our constitution, our personality, and therefore influence a lot in our destiny. We should, perhaps, look at our genes, as the player looks at the cards he has got, from which he must extract everything he can.
Such a comparison is actually not only in this sense. We will see in Chapter 19 that both the cards are shuffled before the distribution, and the genes of the parents are mixed up before they go to the children. And even more, just as the value of any card can increase or decrease with other cards of a given player, so the value of an individual gene can depend on other genes of a given individual. If you have two X chromosomes, a gene that increases the uric acid content in the blood will rarely harm you. If you have one X and one Y chromosome, the gene can cause you to have severe bouts of gout. The aesthetically pleasing combination of the blue eyes gene and the black hair gene is useless for a person who is homozygous for albinism, as the latter inhibits the formation of any hair and eye color. Sometimes an unusual combination of genes can cause extreme abnormality. Some varieties of small platypus fish have a glaring spot. If this fish is crossed with its distant cousin, the horseshoe crab, the hybrids often develop nasty black tumors (Fig. 48). In this case, the genes in a normal environment are harmless, and perhaps even useful, when combined with other people’s genes become dangerous. I hasten to add that such crosses between different genera of fish are in no way comparable to those of different races in humans.
Fig. 48. The influence of genes on the manifestation of others. The horseshoe crab (left), paired with a spotty platy-spetsilia (right). Hybrid (below) developed black tumors
We start life with genes in our hands, as a player starts a game with cards in his hands. Sometimes the distribution can be so bad that it is difficult to expect even moderate success. Even more rarely, it is so excellent that no effort is required to achieve success; more often, success depends on the skill and experience of the player. It is fair to say that the limits of his success are determined by the cards that he and other players have; but how close he is to the limit depends on his skill. A good player with bad cards can achieve greater success than a bad player with good ones. In the life game of genes, a person takes only a limited part; a large role belongs to circumstances that are not yet under our control. However, progress in this direction is fast. Smooth hair due to permanent curling can be made wavy, dark hair can be dyed in a light color. More importantly, the speech defects caused by the hereditary cleft lip can be eliminated by surgery, and the gene responsible for susceptibility to childhood paralysis can be neutralized with the help of a polio vaccine. The more we know about the pathways of action of genes and the nature of their manifestation in various environmental conditions, the better we will learn to promote the manifestation of good genes and weaken the action of bad ones.