Inheritance

• Each chromosome contains one double strand of DNA
• Most human cells contain 46 chromosomes (23 pairs), apart

  from gametes which contain 23.
• When two gamete fuses during fertilisation, the two nuclei fuse

  to form a single diploid cell called a zygote.
• Pairs of matching chromosomes are called homologous pairs.

  They carry the same genes in the same position.
• All the DNA of an organism (present in one diploid cell) is called the

  genome. The human genome is made of around 3.2 billion base pairs,

  containing around 20,000 genes
• Genes control the production of proteins in a cell which can contribute to a particular visible body feature (e.g. eye colour) or a feature which is not visible (e.g. type of blood group antigen on red blood cells)
• Some genes have more than one form, the different forms are called alleles
• Pairs of chromosomes carry the same genes, but they may carry different alleles
• Some characteristics are controlled by a single gene, but most

  characteristics are controlled by several genes
• For a dominant allele, only one copy is required for the allele to be expressed

  For a recessive allele, two copies are required for the allele to be expressed
• The genotype is the two alleles carried for a particular gene (e.g. TT, Tt or tt)

  The phenotype is the expression of the gene, influenced by the environment

  If an individual carries two copies of the same allele, they are homozygous

  dominant (TT) or homozygous recessive (tt)