GED Science TestLife ScienceMedium
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). Red eyes are dominant to white eyes. 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%
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). Therefore, 50% of the offspring are expected to have white eyes.
Why the other options are wrong
- B. This percentage is typical for a dihybrid cross or specific monohybrid crosses, but not Rr x rr for a recessive trait.
- C. This would only occur if both parents contributed at least one dominant allele.
- D. This would occur if the white-eyed trait was dominant, or in other specific genetic crosses.
Monohybrid Cross (Rr x rr)
A genetic cross between a heterozygous individual and a homozygous recessive individual for a single trait.
- One parent is heterozygous (e.g., Rr)
- Other parent is homozygous recessive (e.g., rr)
- Offspring phenotypic ratio is typically 1:1 dominant to recessive
Memory trick: Punnett's Puzzler: Hetero-Recessive is half-and-half