GED Science TestLife ScienceEasy
A geneticist is studying a population of fruit flies. They observe that a certain gene has two alleles: one for red eyes (R) and one for white eyes (r). If a male fruit fly with red eyes (heterozygous Rr) is crossed with a female fruit fly with white eyes (rr), what percentage of their offspring are expected to have white eyes?
- A100%
- B50%
- C0%
- D25%
Show answer & explanationAnswer & explanation
Correct answer: B. 50%
When a heterozygous red-eyed male (Rr) is crossed with a white-eyed female (rr), a Punnett square shows that 50% of the offspring will be Rr (red eyes) and 50% will be rr (white eyes).
Why the other options are wrong
- A. This would only happen if both parents were homozygous recessive (rr x rr) or if the white-eyed allele was dominant and at least one parent was homozygous dominant (RR x rr) or heterozygous (Rr x rr) and we were looking for the dominant trait.
- C. This would only happen if the white-eyed allele was recessive and both parents were homozygous dominant or if one parent was homozygous dominant and the other was heterozygous, and we were looking for the dominant trait.
- D. This percentage is typically seen when crossing two heterozygous individuals (Rr x Rr) for a recessive trait.
Monohybrid Cross (Rr x rr)
A genetic cross between a heterozygous individual (Rr) and a homozygous recessive individual (rr) for a single trait.
- Results in a 1:1 genotypic ratio (Rr:rr).
- Results in a 1:1 phenotypic ratio (dominant trait:recessive trait).
- Used to determine the genotype of an individual showing the dominant phenotype.
Memory trick: Punnett's squares Predict Progeny Probabilities.