A cat produces a cat, and a rat gives birth to a rat. The neem tree does not bear mangoes and the mango tree does not bear guavas. The children of human beings resemble their parents. This is a universal truth that the offspring of all the living beings bear close resemblance to their parents. Do you know why it is so? The answer to all these questions lies in genetics — the science of heredity. It deals with all those factors, which are responsible for the close resemblance of the offspring to their parents. With the help of genetics, we also come to know what traits in the young ones would be different from those of the parents. Heredity is another name of the transmission of characteristics from parents to the offspring. These are inherited from the parents through very minute particles, called ‘genes’. The term gene was coined in 1909 by the Danish geneticist, Wilhelm Johannsen (1857- 1927). Gene a part of the cell. There are some 20,000 genes in the nucleus of every cell. Out of these, some ten thousand of them are inherited from the mother and the other ten thousand from the father. All these genes are contained in 46 chromosomes in the nucleus having a diameter of 0.01 millimetre. At the time of conception, 23 chromosomes come from the father and 23 from the mother. Thus, there are 23 pairs of chromosomes in every cell. The gene is the unit of heredity. In every plant and animal, some genes come from the male and some from the female. Gregor Mendel (1822-1884), a Christian priest and biologist from Austria, is called the father of genetics. He conducted many experiments on the pea-plants and formulated certain rules of heredity. These are known as Mendel’s Laws. He showed through experiments that the dominating genes of parents dominate the traits of the children also. For example, if the eyes of one parent are brown, and those of the other blue, then the eyes of the child would be brown since the brown colour has dominance over the blue. If the eyes of both the parents are blue then the eyes of the children will also be blue. Similarly, children of tall parents tend to be tall and those of short parents, short. In this way, genes are responsible for the close resemblance of offspring to their parents.