All human cells have 46 chromosomes each, except for sex cells (sperms and ova) which has only 23 chromosomes. Chromosomes are the bodies in the cell nucleus which contains genes. These genes are the carriers of all hereditary characteristics from parents to offspring.

Upon the uniting of a sperm and an ovum, a single celled organism called a “zygote” is formed. This single celled organism consists of 23 pairs of chromosomes, in other words, 46 single chromosomes, of which 23 are inherited from father and the remaining 23 are inherited from the mother. All genetic information is thus carried on to the next generation and the offspring possesses equal halves of mother’s and the father’s genetic information. As the zygote divides itself through the process of mitosis, it develops physical human features (depending on the genetic information) and grows into a human baby, now called an “embryo”.

We shall now see how specific characteristics are passed on to offspring. Let us consider the gender, first. An ovum has XX pair and a sperm has XY pair. At any given time, only a single chromosome is passed on from a parent. Ultimately, the offspring too, gets two chromosomes (associated to a characteristic) but one from mother and the other from father.

In this way, a large number of characteristics are inherited from parents such as the colour of eyes, hair, and skin, abnormalities such as colour blindness, albinism, haemophilia, hypertension, diabetes, some cancers and heart diseases. More serious conditions such as Down’s syndrome and fragile X syndrome are also considered genetic disorders associated with heredity.

How genes manifest their acquired characteristics is also very interesting. Since chromosomes are in pairs, for each genetic characteristic, there are two possible genes, contributed by each parent. Among these are dominant and recessive genes. Let ‘Bb’ shows eye colour brown and ‘bb’ to shows eye colour blue. Let ‘B’ be dominant over ‘b’. Now let us see the probability of eye colours in the offspring. The particular sets of genes inherited from parents are called the genotype. It is determined at conception and cannot be change except due to environment acting upon it. Thereby, the genotype interacting with the environment produces the ‘phenotype’ which are observable and measurable expressions of an individual’s physical and behavioural characteristics. Example; height is highly heritable but it is also heavily influenced by nutrition.