MCAT® ExamBiological and Biochemical Foundations of Living SystemsMedium
A geneticist is studying a population of fruit flies where a specific gene (Gene X) is known to affect wing development. They observe that flies homozygous for the recessive allele (xx) have severely underdeveloped wings, while heterozygous flies (Xx) have slightly reduced wings, and homozygous dominant flies (XX) have normal wings. This pattern of inheritance is an example of:
- AIncomplete dominance
- BEpistasis
- CCodominance
- DComplete dominance
Show answer & explanationAnswer & explanation
Correct answer: A. Incomplete dominance
In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes. Here, 'slightly reduced' wings (Xx) are intermediate between 'normal' (XX) and 'severely underdeveloped' (xx) wings.
Why the other options are wrong
- B. Epistasis involves one gene masking or modifying the expression of another gene, which is not described here; only one gene (Gene X) is discussed.
- C. Codominance would mean both alleles are expressed simultaneously and distinctly in the heterozygote (e.g., both normal and underdeveloped wing traits are visible, not an intermediate blend).
- D. Complete dominance would mean heterozygous (Xx) flies have the same phenotype as homozygous dominant (XX) flies (normal wings).
Incomplete Dominance
A form of intermediate inheritance in which one allele for a specific trait is not completely expressed over the other allele, resulting in a third, intermediate phenotype in the heterozygote.
- Heterozygous phenotype is a blend of homozygous phenotypes.
- Neither allele is fully dominant.
- Example: Red + White = Pink flowers.
Memory trick: Dominance is a spectrum: Complete, Incomplete, Co-express.