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HMB265H1 (242)

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Human Biology
Stephen Wright

Pages 40-50 (Chapter 2) Alleles are usually found to be identical in most of their sequences; only differ at one or several nucleotides; essentially, all alleles are different versions of the same gene Mutation – creation of a new allele when the nucleotide sequence changes due to a rare chemical “accident” - Alleles with mutations are usually recessive, since it takes only 1 copy of a wild-type gene to provide normal function - A, representing the wild-type allele, means one specific sequence of DNA; a is a shorthand that can represent any of the possible types of damage that can lead to non-functional recessive alleles If we wanted to show DNA segregation directly, we could sequence the alleles, and ½ the products would have the A sequence, and ½ would have the a sequence - This would also apply to any DNA sequence that differed in the inherited chromosomes Restriction fragment length polymorphism (RFLP) – another way of tracking/characterizing a segment of DNA - Although target sites are usually found at specific locations, any one chromosome can have an extra site or a site missing - A Southern blot will reveal an RFLP if such a site flanks the sequence hybridized by a probe – 2 bands in female and 1 band in male - These “heterozygous” fragments will be inherited in exactly the same way as a gene Molecular markers – RFLP and PCR “alleles” aren’t associated w/ a biological function, but can be used to track the inheritance of a chromosome segment at a specific position Null alleles – encoded proteins completely lack function Leaky mutations – some wild-type function seems to “leak” into the mutant phenotype Silent mutations – have no effect on phenotype Haplosufficient – one copy of a gene provides enough gene product to carry out the normal functions of the cell Haploinsufficient – a null mutant allele will be dominant, b/c in a heterozygote, the single wild-type allele can’t provide enough for normal function Mutations can be dominant or recessive, so we also need to consider the question of dominance in the analysis of inherited single mutant alleles - Standard procedure – cross with wild type Sterile mutants – in diploid organisms, a sterile recessive mutant can be propagated as a heterozygote , which can then be selfed to produce the expected 25 mutants for study - A sterile dominant mutant is a dead end and can’t be sexually propagated; however, plants and fungi can propagate them asexually Forward genetics – starts with random single-gene mutants and ends with detailed cell/biochemical analysis of them, including genomic a
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