The sum of allwe inherit is our genotype or ‘blueprint’. The sum of the effects of the (plus the influence of the environment) govern the development of our individual body with its own particular characteristics. This is known as the phenotype.
An example of a characteristic controlled by only one pair of genes is eye colour. Brown eyes and blue eyes are the most commonly found. We inherit one gene of each gene pair from each of our parents. What we inherit depends on the whole genetic constitution (genotype) of our parents, not only on that part of the genetic information which happened to express itself in their bodies (phenotype) giving them their apparent characteristics. If both parents have brown eyes, this does not mean that both parts of each of their gene pairs are necessarily for brown eyes. The two parts of that pair are called alleles. As it is common to use letters to explain the functioning of such alleles, letters B and b will be used for the alleles governing eye colour. B stands for brown, b for blue. Someone with brown eyes may either have two identical alleles (BB), or two different alleles (Bb). It seems strange that this last combination still produces brown eyes. The reason is that an allele for brown (B) is dominant over an allele for blue (b). Only when both alleles are b (the gene is then bb), will the eye colour be blue. It seems as though the allele for blue eyes is weaker.
Such an allele is called recessive, in contrast to the dominant allele for brown eyes.
Predicting the eye colour of your children is still very difficult with this theoretical background. Although you know the eye colour of you and your partner, the gene pairs underlying this are not normally apparent. Only when both the father and the mother have blue eyes (they both have the gene pair bb) is it certain that offspring will also have blue eyes. This means that they receive the allele b from both parents. Even if both parents have brown eyes it is still possible that one or more of their offspring may possess blue eyes. This occurs if both parents have the gene type Bb – one half of their gene pairs have the allele B and the other half the allele b. In this case there is a 25 per cent chance that any offspring will have the combination bb, producing blue eyes. It is of course, more likely that offspring will have brown eyes, which is the result of either of the combinations BB or Bb. In the case of BB the genetic constitution is said to be homozygous (alleles the same). The genetic constitution of Bb is heterozygous (alleles not the same).
In humanthere are very few characteristics that are controlled by one pair of genes; eye colour is one of the few exceptions. Height, intelligence, skin colour and many other aspects of our appearance and behaviour are influenced by any number of genes. One must also remember that environmental forces also play their part. For example, a child may inherit all the genes to give him a tall stature but he may be the victim of malnutrition at a critical stage in his development; the chances are that he will remain short in adult life. The Japanese have always been regarded as people of small stature, but in recent years, with a more varied diet, they have grown taller. Genetically, therefore, we can say that the Japanese have always carried genes for tall stature. Most characteristics or traits are controlled by a combination of genes. These are called polygenic, and the degree of variation seen tends to follow the normal statistical distribution. Thus there are as few giants as there are dwarfs and most of us are somewhere in the middle the hump of the distribution curve, at average height.
For every characteristic there are, then, at least two controlling alleles and for some there are several. One system controlled by three alleles is that of human blood types. The three different alleles are A, B and 0. Everyone posseses two of these alleles. The resulting four possible blood groups are A (genotype AA or AO), B (genotype BB or BO), AB (genotype AB) and O (genotype O).
Blood-grouping is useful in deciding which donor blood matches with the blood of a patient. It can also be useful in excluding the possibility that someone is the father of a child. For example, a mother who has blood group O and who has a child of the same blood
group, could not have conceived the child by a man who has bloodgroup AB. This method however cannot be used the other way round. It is not possible to prove by blood-grouping that someone is the father. In the given example it is only possible to make a statement such as: the father must have blood group O, A, or B and because this is true of a large majority of the population, this does not prove much.