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Unit Biology 2 Genetic variation Candidates should use their skills, knowledge and understanding of how science works: a) Sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent. b) In human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY). c) Some characteristics are controlled by a single gene. Each gene may have different forms called alleles. d) An allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele. e) An allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele. f) Chromosomes are made up of large molecules of DNA (deoxyribo nucleic acid) which has a double helix structure. Candidates are not expected to know the names of the four bases or how complementary pairs of bases enable DNA replication to take place. g) A gene is a small section of DNA. h) Each gene codes for a particular combination of amino acids which makes a specific protein. HT only25 i) Each person (apart from identical twins) has unique DNA. This can be used to identify individuals in a process known as DNA fingerprinting. GCSE Text Book Revision Guide Genetic disorders a) Some disorders are inherited. b) Polydactyly – having extra fingers or toes – is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder. c) Cystic fibrosis (a disorder of cell membranes) must be inherited from both parents. The parents may be carriers of the disorder without actually having the disorder themselves. It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder. d) Embryos can be screened for the alleles that cause these and other genetic disorders. Strengths: Weaknesses: ……………………………………………. …………………………………. ……………………………………………. …………………………………. ……………………………………………. …………………………………. To Improve I need to: …………………………………………………………………………… …………………………………………………………………………… …………………………………………………………………………… ……………………………………………………………………………