DNA, genes and chromosomes (2)

Alleles

There are pairs of chromosomes in the nucleus of a body cell. The chromosomes in a pair carry the same genes in the same places. But there are different versions of the same gene.

Different versions of the same gene are called alleles. For example, the gene for eye colour has an allele for blue eye colour and an allele for brown eye colour. For any gene, a person may have the same two alleles or two different ones.

Dominant or recessive alleles

Alleles may be either dominant or recessive:

• A dominant allele is always expressed, even if the individual only has one copy of it. For example, the allele for brown eyes is dominant. You only need one copy of it to have brown eyes. Two copies will still give you brown eyes.
• A recessive allele is only expressed if the individual has two copies of it and does not have the dominant allele of that gene. For example, the allele for blue eyes is recessive. You need two copies of this allele to have blue eyes.

Mitosis

In mammals, body cells are diploid. The chromosomes need to be copied exactly so that new cells can be produced for:

• growth
• repair to damaged tissue
• replacement of worn-out cells

The type of cell division involved is called mitosis. The diagram shows how it works.

Identical cells

Mitosis produces two genetically identical cells in which the number of chromosomes is the same as in the original cell.

Meiosis

Meiosis is the type of cell division that produces gametes. A human body cell contains 46 chromosomes arranged in 23 pairs. Human gametes are haploid – so their nucleus only contains a single set of 23 unpaired chromosomes.

The diagram shows the stages of meiosis.

Reduction division

Meiosis produces four genetically different haploid cells. Unlike mitosis, meiosis is a reduction division – the chromosome number is halved from diploid (46 chromosomes in 23 pairs in humans) to haploid (23 chromosomes in humans).

	Mitosis	Meiosis
Number of cells produced	2	4
Chromosome number	Diploid	Haploid
Genetically identical	Yes	No
Used for	Growth, repair, asexual reproduction	Production of gametes for sexual reproduction