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HMB265H1 (54)

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Human Biology
Stephen Wright

Pages 54-60 Pedigree analysis – a scrutiny of mating records Propositus – a member of a family who first comes to the attention of a geneticist; usually, their phenotype is exceptional in some way - Investigator traces the history of the phenotype through the history of the family, then draws a family tree (pedigree) To see single-gene inheritance, the patterns in the pedigree have to be interpreted by Mendel’s law of equal segregation - Typical 3:1 and 1:1 ratios are not seen in humans, since we usually have few children - 3:1 and 1:1 ratios might be seen when many similar pedigrees are combined Autosomal recessive disorders are inherited as recessive alleles, so the unaffected phenotype must be the dominant allele; 2 ways to show autosomal recessive inheritance - The disorder generally appears in the progeny of unaffected parents - The affected progeny include both males and females; this will be simple Mendelian inheritance of a gene on an autosome, rather than a gene on a sex chromosome - Heterozygotes are more common than homozygous recessive individuals –both parents must have the recessive allele to be a recessive homozygote; however, only one parent must have it in order to be a heterozygote - When relatives mate, there is a higher risk of recessive disorders in offspring (two cousins can carry the same rare recessive allele from a common ancestor) - Two unrelated people would have to inherit the recessive allele from both their families in order for them both to be heterozygous - This hypothesis doesn’t support the hypothesis of X-linked recessive inheritance, b/c an affected daughter must have a heterozygous mother (possible) and a hemizygous father (impossible, b/c the father would have expressed the phenotype of the disorder) - Mendelian rules are at work but not necessarily observed in single families – the sample size of humans is usually very small Autosomal dominant disorders – normal allele is recessiv
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