B1

A nucleus contains your genetic material and is arranged into chromosomes. The human cell nucleus contains 23 pairs of chromosomes. A chromosome is one very long molecule of DNA. A gene is a short length of a chromosome and the genes control the development of different characteristics. Genes can exist in different versions. The different versions of the same gene are called alleles.

Each gene is a code for making a certain protein. Some proteins are structural e.g.skin and hair. Others are functional e.g. enzymes are proteins that help with digestion.

An organism's genotype is all the genes it has. The characteristics of an organism are called it's phenotype. Some characteristics e.g. dimples are controlled only by genes. Other characteristics e.g. scars are controlled by environmental factors and some are controlled by both.

Sperm and egg cells have half the normal amount of DNA. The sex cells are different from ordinary cells bcause they have only 23 chromosomes. When the sperm fertilises the egg (sexual reproduction) the 23 chromosomes in the sperm combine with the 23 chromosomes in the egg. The fertilised egg has 23 pairs of chromosomes.

Half a child's chromosomes have come from each parent so they resemble both parents but are identical to neither. When people produce sperm or egg cells their pairs of chromosomes separate and go into different cells. The two chromosomes in a pair are never identical because they have different allele.