B1, 2 & 3

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• Genes carry the instructions for the body to develop and function. They do this by coding for proteins
• Two groups: - Functional proteins enable the body to function, e.g. enzymes, antibodies and hormones - Structural proteins give the body structure, rigidity and strength, e.g. collagen and keratin

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• Each gene is a section of a molecule called DNA (deoxyribonucleic acid)
• DNA is made up of 4 bases, as well as phosphate groups and sugar molecules
• The order of bases determine the order of amino acids in a protein

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• Chromosomes are lengths of DNA found in the nuclei of cells.

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• A complete set of genes is reffered to as a genome

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• Identical twins have identical genotypes because they develop after a fertilized egg splits into two.

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• Phenotype are the observable physical features of a person, whereas genotype will be defined by the genetic makeup (written as two letters e.g. DD)

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• Chromosomes are arranged in pairs. Humans have 23 pairs of chromosomes. Sex cells only have 23 chromosomes.
• At fertilisation, the egg and sperm join to create a zygote, with 46 chromosomes (23 from each parent).

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• Alleles are the different forms in which the genes controlling a characteristic occur, e.g. one for freckles and one without.
• If two alleles of the same gene are identical the person is said to be homozygous, rather than heterozygous.

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• Traits are passed on from parents to their offspring through genes on chromosomes. In most cases, alleles for a trait can be dominant or recessive.

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• You can use punnet squares to show genetic crosses and find the probability of offspring having traits.

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• One pair of chromosomes determine out sex - the 23rd pair. A female has XX and a male has XY. Eggs and sperm has 23 chromosomes (not pairs).