Chromosomes are made from DNA. Genes are short sections of DNA. Genetically identical cells are produced by a type of cell division called mitosis. In sexual reproduction, a male gamete fuses with a female gamete to produce a new cell. This is called fertilisation. Gametes are produced by a type of cell division called meiosis. They contain a single set of chromosomes, whereas body cells contain two sets of chromosomes.

DNA molecules are large and complex. They carry the genetic code that determines the characteristics of a living thing.

A gene is a short section of DNA. Each gene codes for a specific protein by specifying the order in which amino acids must be joined together.

The cell’s nucleus contains chromosomes made from long DNA molecules.

Mitosis a type of cell division that leads to growth or repair. When a cell divides by mitosis:
*Two new cells form
*Each cell is identical to the other one, and the cell they were formed from

Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs controls the inheritance of gender - whether offspring are male or female:

*In males, the two sex chromosomes are different. They are XY.
*In females, the two sex chromosomes are the same. They are **.

Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs.

Gametes contain one set of genetic information, while body cells contain two sets of genetic information.

Fertilisation is the joining or fusion of a male gamete and a female gamete. The new cell that is formed divides over and over again by mitosis. This creates the many cells that eventually form a new individual.

Gametes are formed from cells in the reproductive organs by a type of cell division called meiosis.

                                  Male      Female

Reproduction Organ Testes    Ovaries

Gametes                   Sperm    Eggs

The cells that are formed by meiosis have half as many chromosome as the cell that formed them. Human body cells contain 23 pairs of chromosomes, while human gametes contain 23 single chromosomes. The main features of meiosis are:

*The chromosomes are copied

*The cell divides twicem forming four gametes

Alleles are different forms of a gene. They can be dominant or recessive. Genetic diagrams help us to understand the possible outcomes when parents produce offspring. Huntington’s disease is a disorder of the nervous system that is caused by a dominant allele. Cystic fibrosis is a disorder of the cell membranes caused by a recessive allele.

Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms.
Different forms of the same gene are called alleles. The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour.
Alleles are dominant or recessive:

*The characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair
*The characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair.