Genes are responsible for heredity. They are present on chromosomes at fix points.

The nucleus of every cell (except RBC of mammals) contains a fixed number of chromosomes. In all the cells of eukaryotes, chromosomes show the following characteristics:

  1. They are present in pairs, one from the father and the other from the mother.

  2. They can be seen only during cell division. In a non-dividing cell, they appear in the nucleus as a jumbled up network termed chromatin.

  3. The paired chromosomes are present in a fixed number. A fixed set of chromosomes is termed the "diploid" (paired) number and designated as 2n.

  4. Each chromosome is made of one molecule of the chemical called DNA or deoxyribonucleic acid and some proteins.

  5. Before cell division, the DNA molecule of a chromosome replicates (duplicates) to give two molecules of DNA which are called "chromatids". The two chromatids of a chromosome remain attached at a point called centromere and separate to form two chromosomes during cell division.

In the bacteria, only one chromosome (that is only one molecule of DNA) is present and since there is no well formed nucleus, the single chromosome lies in the cytoplasm in the region termed nucleoid.

Human Chromosomes

Every cell of a human being contains 46 chromosomes. In other words, the diploid number in humans is 46. This can be expressed as 2n = 46. Since gametes contain only half the number of chromosomes or the haploid number, a sperm and an ovum or an egg has only 23 chromosomes.

Every species has a fixed number of chromosomes. Each chromosome possesses two identical chromatids joined by the centromere. The chromatids become independent chromosomes when they acquire a centromere at anaphase of cell division and ultimately move to different cells.

A pair of similar chromosomes (one received from the father and one from the mother) containing the same genes are termed homologous chromosomes.

Out of the 23 pairs of chromosomes, 22 pairs are termed autosomes. The 23rd pair (X and X in females and X and Y in males) are called sex chromosomes. X chromosome has several genes, some of which are necessary for survival. Y chromosome bears genes for maleness only.