GED Science TestLife ScienceEasy
A geneticist is studying a trait in fruit flies where red eyes (R) are dominant to white eyes (r). If a heterozygous red-eyed male (Rr) is crossed with a white-eyed female (rr), what percentage of their offspring are expected to have white eyes?
- A50%
- B25%
- C0%
- D75%
Show answer & explanationAnswer & explanation
Correct answer: A. 50%
This is a monohybrid cross between Rr and rr. Using a Punnett square, the possible genotypes are Rr, Rr, rr, and rr. This means 50% of the offspring will have the heterozygous (Rr) genotype and thus red eyes, and 50% will have the homozygous recessive (rr) genotype and thus white eyes.
Why the other options are wrong
- B. 25% would occur in a cross between two heterozygous individuals (Rr x Rr).
- C. 0% would occur if both parents were homozygous dominant or if one parent was homozygous dominant and the other heterozygous.
- D. 75% would occur in a cross between two heterozygous individuals (Rr x Rr) for the dominant phenotype.
Monohybrid Cross
A genetic cross between two individuals that are heterozygous for the same gene, or between individuals with different genotypes for a single trait.
- Involves one trait.
- Predicts offspring genotypes and phenotypes.
- Punnett squares are used to visualize outcomes.
Memory trick: Mendel's Monohybrid Magic: One trait, two parents, predictable ratios.