B1
Genes are found in our chromosomes,which parents pass on to offspring in their sex cells in reproduction. Different versions of the same gene are called alleles, and these can determine features like eye colour, and the inheritance of disorders such as cystic fibrosis.

DNA the nucleus controls the activities of a cell. The instructions for how an organism develops are found in the nuclei of its cells

Chromosomes
Chromosomes are structures found in the nucleus of most cells. They consist of long strands of a substance called deoxyribonucleic acid, or DNA for short. A section of DNA that has the genetic code for making a particular protein is called a gene. The proteins can either be: structural proteins such as the ones found in muscles and hair enzymes, such as proteases and other digestive enzymes

Variation
Individuals differ in all sorts of ways, even when they are offspring of the same parents. These differences are called variation. Most characteristics, such as height, are determined by several genes working together. They are also influenced by environmental factors. These include: Climate, diet, physical accidents, culture&lifestyle

Identical twins
Identical twins are genetically the same. They are a good example of the interaction between inheritance and the environment. For example, an identical twin who takes regular exercise will have better muscle tone than one who does not exercise. All of the differences that you see between identical twins, for example, in personality, tastes and aptitude, are due to differences in their experiences or environment.

When an egg and sperm cell come together, the now fertilised egg contains 23 pairs of chromosomes. Sex chromosomes are responsible for certain genetic traits.

Sex cells and chromosomes
Human body cells each contain 23 pairs of chromosomes. Parents pass on their genes to