INTERPRETING HUMAN PEDIGREES - INTERPRETING HUMAN PEDIGREES...

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INTERPRETING HUMAN PEDIGREES. Certainly true that Mendel’s laws apply to humans. Established within 3 years of rediscovery; Farabee’s pedigree of brachydactly. Fact that many illnesses due to blocks in metabolic pathways also established very early. Archibald Garrod; 1909 – “A study in chemical individuality” - disease alkaptonuria; blackening of the urine in air – now known to be absence of enzyme homogentisic oxidase in tyrosinase degradation pathway. Family – recessive pattern of inheritance; gave the one gene one enzyme hypothesis, central to development of biochemical genetics. Now – thousands of genes, DNA sequenced. OMIM – online Mendelian Inheritance in Man; 10 000 inherited diseases. Human biochemical genetics perhaps better understood than any other eukaryote – but this is really chemistry! For genetics need sex; and for humans need pedigrees . In fact, human pedigrees studies much more problematic than might seem; and in OMIM formal pattern of inheritance only established for a small proportion of the diseases listed, with many disagreements about whether a particular disease is dominant, recessive or sex-linked. In principle, can see some straightforward patterns of inheritance: assume that you know the basics. Some very clear pedigrees . Attention drawn by one individual; the propositus – work out from there to try to find relatives. Formulate a hypothesis as more and more data comes in: e.g. autosomal dominant, autosomal recessive, sex- linked. Obviously important to know in terms of explaining risk to parents. Many good ex’s. Autosomal Dominant Canadian deafness, Huntington’s disease, achondroplasia, brachydactyly, etc.. Every affected individual has at least one affected parent. Because most Aa matings are with aa, 50% chance of transmitting the trait. Two affected individuals may have unaffected children. Sometimes, true dominance; e.g. very rarely get two people with Aa x Aa; have AA child (can establish with molecular probe). For true dominance, AA phenotype should be same as Aa. And, for Huntington’s disease, it is. However – more often, AA much more severely affected. E.g. AA for achondroplasia – severe, crippling dies in utero. Autosomal Recessive Albinism, cystic fibrosis, thalassemia. Characteristic patterns: Most children the offspring of unaffected parents All children of two affected parents are affected Recurrence risk to later children of a family that has had one affected child is ¼ X-linked Dominant Rather rare; but a few examples Hypophosphatemia – causes a type of rickets. Affected males have all daughters affected and no sons. Herterozygous female (affected) will pass trait to half daughters and half sons. On average, twice as many females affected as males (with the herterzygous mother that displays the disorder).
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