Q.
When two genetic loci produce identical phenotypes in cis and trans position, they are considered to be
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AIPMTAIPMT 1995Principles of Inheritance and Variation
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Solution:
E.B. Lewis in 1951 reported from a cross of apricot eyed and white eyed flies in Drosophila, he obtained F, having intermediate eye colour. In F2, he had expected segregation only for apricot and white, but he recovered very low frequency of wild type. Since those alleles behaved as non-alleles, Lewis preferred to call them pseudoalleles and the phenomenon as pseudoallelism. In pseudoallelism in cis position both the mutant alleles are on one chromosome. So the other chromosome will be normal and will be able to produce the end result. But in trans position the sequence of steps involved in synthesis will be interrupted due to mutations on either of the two homologous chromosomes thus leading to a mutant phenotype.