Heredity is the process of transmission of characters from one generation to the next. It studies how those characters are transmitted from parents to their offspring, what are the basic laws applicable to such processes etc. The field of genetics studies heredity in detail.
Gregor John Mendel, an Austrian monk, was responsible for formulating the basic laws of heredity on the basis of his studies on the Garden Pea. For this reason, he is remembered as the Father of Heredity.
In the present century, much more has been learnt about the mechanism of heredity. Much of the credit for this research goes to Morgan Hunt, an American biologist who has added to our genetic knowledge tremendously by way of his experiments on the butterfly Drosophila.
Each chromosome has many individual hereditary units called genes. A gene is a segment of Deoxyribonucleic Acid (DNA), which is the actual carrier of all genetic information. The DNA molecule looks like a twisted ladder or a double-stranded helix as shown alongside. The gene, a segment of the DNA molecule gives coded instruction to the cell, directing it to perform a particular function (usually to make a particular protein).
Although all cells in the body contain the same genes, the special nature of each cell reults from the fact that only 5 to 10 percent of the genes are active in any given cell. While developing from a fertilized egg, each cell "switches on" some genes and "switches off" all others. When “nerve genes” are active, for example, a cell becomes a neuron because the genes direct the cell to make chemicals necessary to perform neural functions.
Genes, like chromosomes, occur in pairs. One gene of each pair comes from the sperm chromosome and the other from the egg chromosome. Thus, a child receives only half of his genes from each parent’s total genes. The total number of genes in each human chromosome is around 100- perhaps higher. Since the number of genes is so high, it is extremely unlikely that two human beings would have the same heredity, even if they were siblings. The only exception is the identical twins, who have exactly the same genes.
An important trait of many genes is dominance or recessiveness. When both members of a gene pair are dominant, the individual manifests the form of the trait specified by those genes. When one gene is dominant and the other recessive, the dominant gene again decides the form of the trait expressed. Only if the genes contributed by both parents are recessive is the trait specified by them expressed. The genes determining eye colour, for example, act in a pattern of dominance and recessiveness; blue is recessive and brown is dominant. Thus, a blue-eyed child may have two blue-eyed parents, or one blue-eyed parent and one brown-eyed parent (who carries a recessive gene for blue eyes) or two brown-eyed parents (each of whom carries a recessive gene for blue eyes). A brown-eyed child, in contrast, never has two blue-eyed parents.
Some characteristics carried by recessive genes are baldness, hemophilia, and a susceptibility to poison ivy. However, not all gene pairs follow the dominant-recessive pattern and most human traits are determined by many genes acting together.
Some human characteristics are determined by a single gene pair. For instance, Phenylketunoria (PKU) and Huntington’s Disease. In case of PKU, the person is not able to digest an essential amino acid which gets logged in the nervous system.
The X chromosome may carry either dominant or recessive genes, the Y chromosome carries a few genes dominant for some male sexual characteristics, but otherwise carries only recessive genes. Thus, most recessive characteristics carried by a man’s X chromosome (received from his mother) are expressed since they are not blocked by dominant genes. For example, colour-blindness is a recessive sex-linked characteristic. A man will be colour-blind if he inherits a colour-blind gene on the X chromosome received from the mother. Females are less often colour-blind, because a colour-blind female has to have both a colour-blind father and a mother who is colour-blind or who carries a recessive gene for colour-blindness. A number of genetically determined disorders are linked to abnormalities of the 23rd pair of chromosomes, or by recessive genes carried by this pair. They are called sex-linked disorders.
On rare occasions, a female may be born with only one chromosome instead of the usual XX. Such a condition is referred to as Turner’s Syndrome.
Sometimes, when the 23rd chromosome fails to divide properly, the developing organism get an extra X or Y chromosome. An individual with XXY condition is said to be suffering from the Klinefelter’s syndrome. Such a person is physically a male, but with marked feminine traits. A person born with XYY composition will be known as a supermale, a person with exaggerated male features.