Geneticists sometimes use the following test for the nullness of an allele in a diploid organism: If the abnormal phenotype seen in a homozygote for the allele is identical to that seen in a heterozygote (where one chromosome carries the allele in question and the homologous chromosome is known to be completely deleted for the gene) then the allele is null. What is the underlying rationale for this test?

There will be null activity for a gene within in the deletion, this means if a genotype has similar phenotype. This suggests that mutant allele and the allele with deletion has the same level of activity.

Explanation:

The limitation of such interpretation is that the phenotypes have the enzymatic activity according to a threshold level. This means that the mutant phenotype is visible till the enzymatic activity is lower than a threshold. If the level rises above the threshold the wild type phenotype is observed.

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