If a polypeptide acts as an enzyme or is part of an enzyme,a mutation in its gene sometimes results in a deficiency of that enzyme,with a consequent blockage in the biochemical pathway where that enzyme is required Diseases that result from such blockages are called inborn errors of metabolism Two alleles at one locus can exhibit more than one type of gene action, depending on the trait being considered Genetic heterogeneity of disease occurs because mutations in different genes can give rise to exactly the same clinical entity,and different mutations within the same gene can produce different clinical entities Recessive disorders tend to result from enzyme deficiencies,whereas dominant or co-dominant disorders are more likely to be caused by defects in non-enzymatic polypeptides。
Sometimes an environmental cause produces the same phenotype as a single-
gene disorder;such an occurrence is called a phenocopy With SNP chips comprising tens or even hundreds of thousands of DNA markers now available,in many situations it is possible to map and identify the causal mutation of a single-locus disorder with as few as 10-20 affected animals and the same number of normals(controls)
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