Chromosomes: What are they and how many do humans have?

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Chromosomes are structures in the nucleus of cells in the body that contain genetic material (genes). Genes are made of a chemical called DNA, which is short for ‘deoxyribonucleic acid’. The DNA molecule is a double helix: that is, two long, thin strands twisted around each other like a spiral staircase.

Our body is made up of millions of cells. Most cells contain a complete set of genes. So we have thousands of genes. Genes contain information that control how we grow and how our bodies work. They are also responsible for many of our characteristics such as eye color, blood type and height.

The total number of chromosomes in an organism, such as an animal or plant, is important and differs for different species. Humans, like many other species, are called ‘diploid’. This is because our chromosomes exist in matching pairs – with one chromosome of each pair being inherited from each biological parent.

Humans have 23 pairs of chromosomes. Of the 23 pairs, 22 are known as autosomes. The 23rd pair is made up of the sex chromosomes, called the ‘X’ and ‘Y’ chromosome. Females have a pair of X chromosomes; males have an X and Y chromosome. The female inherits an X chromosome from her mother and an X chromosome from her father. The male inherits an X chromosome from his mother and a Y chromosome from his father.

These 46 chromosomes carry the genetic information that’s passed from parent to child through heredity. It is the very detail of this genetic material – in the DNA – that makes most people (other than identical siblings) totally unique.