Question

Given an individual who is heterozygous for a recessive disease (i.e., the wildtype allele is dominant), explain how a deletion could result in the expression of the recessive phenotype. Be sure to use the formal vocabulary for full points.

Answer 1

Let 'A' be the dominant allele responsible for the normal phenotype and let 'a' be the recessive allele responsible for the disease.

Then following types of genotypes and corresponding phenotypes are possible :

AA = normal homozygous individual

Aa = normal heterozygous individual

aa = affected homozygous individual

Now, in the given question, the genotype of the man is = Aa

Since the dominant allele 'A' is present, the man will exhibit normal phenotype, 'a' allele will not show its affect.

But anyhow, if 'A' allele is deleted. Then, 'a' allele is left in the chromosome. Now. There is no corresponding allele to 'a' is left in the chromosome. So, the allele 'a' will show its effect. This will be done via transcription and translation of the DNA.

This condition when only one allele is present in a diploid organism is called as hemizygous and the organism is called as hemizygote.