A lod score method is provided for mapping genes relative to the centromere using family data from autosomal trisomies. Such gene-centromere mapping can be performed whenever two or more members of a meiotic tetrad can be recovered. The critical mapping parameter is not the recombination value θ or the map distance w, but the probability of nonreduction y. In a disomic gamete arising by nondisjunction in a heterozygous host, the probability of heterozygosity (nonreduction) is 1 - y 2 for a meiosis I error and y for a meiosis II error. Under various assumptions regarding chiasma interference, y can be related to θ and w. We provide specific methods for estimating y and θ from trisomy data using maximum likelihood, so that recombination may be studied on chromosomes that underwent nondisjunction.
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