Question

1. If a disorder is caused by a single gene, can one conclude that only one phenotype will manifest? Why or why not? Support your answer.

Answer 1

If a disorder if caused by a single gene the phenotype manifested by that gene will depend upon the number and types of alleles of that gene. Let's say a hypothetical disorder is caused by gene A which has two alleles (A ( dominant -wild type) and a (recessive-disease causing)) causes hemoglobin to lose the oxygen carrying function. An individual will AA genotype( homozygous dominant ) will have functional hemoglobin and healthy. A homozygous recessive individual (genotype aa) will have faulty hemoglobin and will have the disorder. A heterozygous carrier (genotype Aa) will have 50% functional hemoglobina and 50% faulty hemoglobin. For this reason this individual will not be as healthy as the homozygous dominant individual or as sick as the homozygous recessive individual. So here there are three phenotypes, healthy, intermediately healthy and diseased. So it cannot be concluded that a single gene disorder will result only in one phenotype. The only situation in which this condition can occur is when that single gene has only one allele and that allele is disease causing.