Chromosomes form homologous pairs

If the chromosomes are cut out they can be arranged into matching pairs according to their size and certain other features. These are called homologouspairs. Apart from the sex chromosomes, both chromosomes in a pair normally contain the same genes (for example, for eye or hair colour). However, these may be different forms of the gene (for example, one chromosome carries the form for green eyes, the other for brown eyes).

Human cells each have 46 chromosomes (23 pairs). Other species have different numbers, for example, chimpanzee cells each have 48 (24 pairs) and cabbage plant cells each have 18 (9pairs).

One chromosome in each pair comes from the individual’s mother and the other from the father.

- Cells that have the normal two sets of chromosomes are called diploid.

- Cells that give rise to gametes (eggs and sperm) have only one chromosome of each pair, so they have half the normal number of chromosomes. Such cells are calledhaploid.

- In humans, n =23, so normal diploid cells have 46 chromosomes and the haploid gametes have 23 chromosomes.