Basic Genetics

The basic tissue of living things is made up of cells.

All living things start life as a single fertilised cell called a zygote. The single-celled zygote splits into two and begins to multiply to make the building blocks of plants or animals.


The cells of plants and animals look different but their construction and operation are very similar.

Individual threads in the nucleus of the cells are called chromosomes.

When cells divide, a process called mitosis, both resulting cells have an exact copy of the number and type of chromosomes in their nucleus. The single cell divides into two, then four, then eight and so on. All cells of an organism contain the same number of chromosomes. In humans this is 46. The chromosomes occur in pairs.

There is another form of cell division called meiosis which only occurs in the reproductive organs where the sex cells, gametes, are formed. In the human body the female egg and male sperm contain 23 chromosomes which fuse together to form a zygote during reproduction which then contains the full amount of 46 chromosomes in 23 matching pairs.

The chromosomes in each pair are called homologues. One member of each homologue is from the father and the other from the mother. There are two types of chromosome; the sex chromosomes and the autosomes.

In humans as well as most animals males and females have two sex determining chromosomes. In the female they are both X chromosomes but in the male one is an X chromosome and the other a smaller Y chromosome.