Health > Mental Health > Heredity
We all know that children resemble their parents, grandparents, brothers and sisters to some extent, but just how far is it possible to predict the way they will turn out? Some of the answers are provided by the study of heredity.
Every time we say ‘it runs in the family’, or ‘she has her mother’s eye’s, we are talking about heredity, or, in scientific language, genetics-the study of genes.
Genes are probably best described as biochemical codes. They are tiny, too small to be seen even under an electron microscope, but scientists know they are carried on the chromosomes, which are the tiny, thread-like structures, that can be seen under a microscope within all human cells at some time.
Everyone has 46 chromosomes, arranged in 23 pairs, one member of each pair coming from the sperm of the father, the other from the mother’s egg. Together, these structures make a complete chemical blueprint of an entire person.
SIMPLE FORMS OF HEREDITY
This pairing of chromosomes is just most significant to the way heredity works because each pair contains similar genes, and the simplest forms of heredity can be traced to the operation of single pairs of genes.
Genes acting in this way can occur in two different forms, one the dominant, the other the recessive. Dominant genes are distinguished by the tendency to make their characteristics evident in the physical make-up of a person, even if they are only present in a single does. A pair of similar recessive genes-one inherited from each parent-must be present if they are to make themselves obvious.
Geneticists have identified various dominant and recessive genes. For example, the gene for curly hair is dominant, so if a child inherits it form say the father, and also a gene for straight hair (which is recessive) from the mother, the curly hair gene will dominate, and the child will have curly hair.
In practical terms, this does not mean that you can predict whether your child will inherit your curly hair. The actual ‘passing on’ of genes is a random happening, the curly-haired dominant gene having slightly better than even chances of being transferred. But the principle does help us in a negative way-making it clear that we cannot count on a child having straight hair (if that is preferred) if one partner is curly and the other straight-haired.
This form if heredity, known as single factor inheritance, is relatively simple, and tell us some important, and quite reassuring things about how our children will turn out in terms of their general health and make-up. This is because, fortunately, the majority of ‘normal’ characteristics are governed by dominant genes. So- as one might expect-most babies born to normal mothers are themselves normal and healthy.
Just a few heredity diseases are inherited on dominant genes, but recessive genes cause many more, including albinism (lack of pigment, or colouring) and some types of deafness.
Another important point to understand is that recessive genes, of their nature, can be ‘carried’ by a human all through life without characteristics they convey actually showing up. Also, there is nothing to stop them being passed on to the next generation and, if the circumstances are right, showing up there. It is important to realize that this is putting the theory at its simplest. There are complications even in single factor inheritance.
OTHER TYPES OF HEREDITY
There are several other ways in which genes can work, varying in complexity. Perhaps one the most interesting is the polygene system, which governs such characteristics as skin colour, height and probably intelligence. Polygenes can be thought of as groups of genes working together. The rules of dominance apply to each gene in the group, and the effect of the genes is cumulative, that is, they build up to produce an overall effect.
So although it is impossible to lay down a general rule, it is fair to say that tallness or shortness tends to run in families because the polygene for height contains more ‘tall’ or ‘short’ genes in some families than others. And, if a taller-than-average man has children by a shorter-than-average woman, the geneticist would expect them to be closer to the average height than either parent. In a similar way, plygenes governing skin colour produce a whole range of complexions among the so-called ‘white’ races of the world, from dark through to pale.
Children born to parents of completely different skin colours- say ‘white’ and ‘black’-will tend to be intermediate between their parents, although because the ‘dark’ genes are dominant over the ‘pale’, such a child would tend to be darker than the exact halfway shade.
So there is no simple pattern of either ‘black’ or ‘white’ skin being inherited, and the old wive’s tale about throwbacks with ‘black’ skin colouring being born to two white parent because there was a coloured ancestor somewhere far back in the family tree are without foundation.
Intelligence is the most argued-over aspect of the whole subject. The only certain fact-and it does not take one far-is that intelligence is both inherited and affected by environment, that is, the atmosphere and conditions in which a child is raised, and lives during adult life.
So it cannot be precisely explained why, for example, musical talent tends to run in families to such an extent. No one knows how much of it is inherited, and how much occurs as a result of being brought up in an environment where music is part of family life. The same applies to acting ability, sporting skill, literacy ability and many other talents.
Environment may also act as the trigger for an inherited physical characteristic. Two people, may, for example, be born with a tendency to go brown easily (which in fact involves the ability to make the pigment melanin in the skin). But if one of them stays indoors for most of the time, their skin is unlikely to go brown, while the other, doing an outdoor job, will quickly develop a healthy tan. The question of heredity is further complicated by the fact that genes show their strength not only in terms of dominance over other genes, but by the degree to which they penetrate, which geneticists call penetrance.
Penetrance may be weak or strong. For example, the defect of fingers known as camptodactyly is produced by a dominant gene, and can thus show up by being inherited in the single-factor method. However, the degree to which a person suffers from it will vary from severe stiffness of several fingers (full penetrance) to stiffness of just one finger (partial penetrance).
Geneticists suspect that longevity, the ability to live a long time-is determined by genes, possibly polygenes.
But they also recognize that however long your genes have programmed you to live, their effect can be counteracted by maltreating the body in any of the usual ways-like smoking or overeating.
These various principles of heredity are only tiny corners of a super-complex jigsaw. Most geneticists are concerned with trying to unravel the way disease sand abnormalities are inherited. Although possible, it is unlikely that they will be able to start playing ‘God’ by tampering with genes.