The Mechanism of Heredity and the Role of Genes:
Man’s biological heredity begins with the union of the sperm ejected by the male with the egg- cell in the mother’s womb. This is the contribution of each parent to the formation of the first cell. This first cell called the ‘germinal cell’ divides repeatedly into thousands and millions of cells.
The first cell contains 23 pairs of chromosomes which correspond to the human species. Each parent has contributed one half to this cell. Every human cell has the same number of chromosomes except the reproductive cells.
The reproductive cells which proceed from each parent for the procreation of the new being have only one half of this number, that is, 23 unpaired chromosomes. If there is a single special chromosome called ‘Y’ the offspring is going to be male; and if all chromosomes are ‘X’, the offspring is bound to be a female.
If, in the mother’s womb, two egg-cells are fertilised by two sperms contributed by the father, twins will be born. These may be of the same sex or of different sex. ‘Identical’ or ‘equal’ twins are born when only one egg-cell is fertilised by the sperm which at the time of its first division breaks into two independent cells, which multiply as normally as the others. These identical twins are very much alike and they necessarily belong to the same sex.
The Role of Genes:
The chromosomes that are contributed by each parent contain thousands of ultramicroscopic particles called “genes”. These genes are the carriers of heredity. They determine the physiological characteristics of the new being.
Not all inherited characteristics appear at birth; many like baldness or deafness manifest themselves later in life. Intensive researches carried on by Gregory Mendel, Thomas Hunt Morgan, Hugo de Vries and others, have further confirmed the fact that the genes largely decide the heredity of the offspring.
It was previously believed that every physiological trait has a definite gene corresponding to it. Now it has been proved that some inherited qualities are due to the combination of a few genes.
Experiments have shown that when two characteristics, one from each parent, combine, the offspring is a hybrid uniting in itself the two characteristics. Of these two characteristics, one is ‘dominant’ and the other is ‘recessive’ and the former always prevails over the latter.
Thus, as Mendel showed in experiments on rats, mice, and most fur-bearing animals, black is the dominant character over white; smooth hair over curly and short over long hair. In the case of man, brown eyes are dominant over blue eyes.
It may also happen that sometimes, some white complexioned parents may get a child with black complexion, and ‘black’ parents may get ‘white’ child. Because some genes which were recessive in the previous generation of those parents become dominant at that time and express qualities peculiar to themselves.
Still it can lot be said that every characteristic with which we are born is strictly inherited. Many of them are ‘exogenous’ or due to environmental influences. Every characteristic in order to be ‘ endogenous’ or inherited must have its basis in the cell genes and be capable of being transmitted through them. According to H.S. Jennings, the following somatic characteristics may be strictly inherited.
“Sex, colour of eyes and hair, complexion, form of the features, form the distribution of hair,; finger prints and palms and sole patterns, structure and form of the hands and feet, form of the body (stoutness, slenderness, stature, and the like), chemical composition of the blood, the blood types, the glandular types to which individuals belong, the senses and their efficiency, efficiency of the brain, vigor and weakness of constitution, susceptibility and immunity of various diseases, and is many other physical and physiological peculiarities”.
Recent investigations have revealed that even the genes undergo changes. “The changes occurring in the genes which may be transmitted by generation are called ‘mutations”.” These must be differentiated from ‘ variations’. A variation refers to a change in the degree or intensity of a certain characteristic or trait.
For example, in a group of short men some may be shorter than others; of two pairs of black eyes one may be more intense than the other. But a change from tallness to shortness, from blue to brown eyes, or from white to black colour, as may occur in the offspring, is the effect off a “mutation”. Variations are temporary and they indicate changes in the individual. But mutations are permanent, and found in the species.
It is only in terms of mutations the evolution of species can be understood. Hence the changes in skin colour, bodily features, or hair forms of a group have become perpetuated because of mutations in the genes.
It may be reiterated that heredity is the living link or bridge between two generations. What I actually go over the bridge are thousands of living microscopic particles packed away in the single cell which each of us received from each of our parents. These particles are the genes.