Silent alleles - The Bombay phenotype in humans is caused...

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Silent alleles In a multiple allele system, it is sometimes not obvious that a silent allele exists. This can give confusing results. Consider for example: A/A x A/B (phenotype A crossed to phenotype AB) | | V A/A : A/B 1 : 1 and compare with A/O x A/B (phenotype A crossed to phenotype AB) | | V A/A : A/O : A/B : B/O 1 : 1 : 1 : 1 It would be important not to lump together these two different sorts of crosses but when there are only small numbers of offspring (which is the case in most human matings) some offspring classes may not be represented in a family and it may not be obvious which type of mating you are examining. Epistasis This occurs where the action of one gene masks the effects of another making it impossible to tell the genotype at the second gene. The cause might be that both genes produce enzymes which act in the same biochemical pathway. If the product of gene1 is not present because the individual is homozygous for a mutation, then it will not be possible to tell what the genotype is at gene2.
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Unformatted text preview: The Bombay phenotype in humans is caused by an absence of the H antigen so that the ABO phenotype will be O no matter what the ABO genotype. A soap opera example of this is given on page 89 (figure 5.4) in Lewis. Pleiotropy Mutations in one gene may have many possible effects. Problems in tracing the passage of a mutant allele through a pedigree can arise when different members of a family express a different subset of the symptoms. In the case of Tuberous sclerosis , an autosomal dominant condition affecting about 1 in 6000 people in the UK, symptoms can include any subset of: o small depigmented patches on the skin o a disfiguring facial rash o kidney cysts o heart tumours o mild to severe mental subnormality o autism among others. Lewis gives the interesting example of Tourette syndrome which may cause strange behavioural problems. Pleiotropy can occur whenever a gene product is required in more than one tissue or organ....
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Silent alleles - The Bombay phenotype in humans is caused...

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