All that which we inherit from our parents, grandparents, great grandparents and so on as a free gift is called heredity. It is of two types: (i) Biological heredity and (ii) Social herediry.Biological heredity is that which a person inherits from his forefathers in the forms of chromosomes and genes.Social heredity means all that one generation receives from the preceding generation in the form of social traditions customs and skills.Person belonging to Rajput race is generally found muscular with strong and wide bones. It is an example of biological heredity. And the son of a carpenter is also likely to be an expert carpenter. It is an example of social heredity.

We will talk in this part only about biological heredity which is determined by genes. These genes are located in chromosomes. A person inherits chromosomes from his forefathers. With the transfer of these chromosomes genes are also transferred which are actually the carrier of heredity. Sex of an individual is also determined by genes. In order to know how traits are fixed by genes, we must first of all know the principles of heredity.

Principles of Heredity

(a) Principle of resemblance : This principle states that like begets like. A cat can only give birth to a kitten, a human can only give birth to a baby and so on. According to this principle children generally resemble their parents or relatives.

(b) Principle of Variability : Many children do not resemble their parents. There are many instances where intelligent parents have dull children or ugly parents have beautiful children. This variation is universal in nature. That is why we see every individual unique in certain respect. This is called principle of variability of inheritance.

Mechanism of Heredity

When human sperm and egg unite, they are fertilized into zygote. Each zygote has 23 pairs of chromosomes. Every egg in the body contains the same number of chromosomes. The number of chromosomes in a given species are same but different species have different number of chromosomes.

For example, all humans contain 46 chromosomes. Half of them come from mother and half of them come from father. Within the chromosomes there are thousands of minute particles called genes. They are actually the carriers of hereditary traits from one generation to another.

Chromosomes are made up of long organic molecules whose main substance is deoxyribo nucleic acid (DNA). It is found in the chromosomes as two long strands periodically connected by chemical bonds.

Our heredity information is not found in the two long strands of DNA but in their order of appearance between the two strands of DNA. One pair of chromosomes is related to sex (gender) also. Females have two x chromosomes and males have an x and a y chromosome. If sperm of x chromosomes unite with the egg of y chromosomes, then the child will be a girl. If on the other hand, the sperm of y unite with the egg of x chromosomes, then the offspring will be a male child.

The story of hereditary influence will not be complete without knowing the influence of genes on our traits. It is because traits are determined exclusively by genes.

Genes are of two types (1) dominant and (ii) recessive. We inherit the same qualities of our parents because of dominant genes. If same characteristics are not inherited it means that recessive gene is active there. If marriages between blood relations take place, the recessive genes are developed which can cause many deformities in the body, i.e.. offspring may become physically or mentally handicapped.