Chromosomes Come in Pairs

Human Cells have a total of 46 chromosomes. But these 46 chromosomes come in pairs: each human somatic cell has 23 pairs of chromosomes. Many other species also have their chromosomes situated in pairs.

Forty-six just happens to be the number of chromosomes in human cells, hence human cells have 23 pairs of chromosomes. Another way of saying this is to say that human cells have two sets of chromosomes, and each set consists of 23 different chromosomes.

One set of those chromosomes came from your dad in a sperm cell. One set of those chromosomes came from your mum in an ovum. When the sperm and ovum joined in fertilisation, the resulting cell had two sets of 23 chromosomes each, for a total of 46 chromosomes. From that single cell, every other cell in the body was derived, so every other cell in the body has 46 chromosomes.

Think about the set of 23 chromosomes that came from DAD. Let' call them 1A, 2A, 3A,. all the way to 23A. Now think about the set of 23 chromosomes that came from Mom. Let's call them 1B, 2B, 3B, etc.

Chromosomes 1A and 1B are very similar (not exactly alike, but very similar). They form one of the 23 pairs of chromosomes in your cells, and we refer to them as being homologous. Chromosomes 1A and 1B form a homologous pair. So do chromosomes 2A and 2B, 3A and 3B, 4A and 4B, etc.

One last point: cells that have two sets of chromosomes (all chromosomes have a homologous partner) are said to be diploid. So human cells are diploid, and the cells of any other organism that has two sets f homologous chromosomes are also diploid.