GED Science TestLife ScienceEasy
A geneticist is studying a population of pea plants. She performs a monohybrid cross between two heterozygous tall plants (Tt x Tt), where tall (T) is dominant to short (t). What is the expected phenotypic ratio of tall to short offspring?
- A2:1
- B3:1
- C4:0
- D1:1
Show answer & explanationAnswer & explanation
Correct answer: B. 3:1
In a monohybrid cross between two heterozygotes (Tt x Tt), the Punnett square yields genotypes TT, Tt, Tt, and tt. Since 'T' (tall) is dominant, the genotypes TT, Tt, and Tt all express the tall phenotype, while only tt expresses the short phenotype. This results in a 3:1 phenotypic ratio of tall to short.
Why the other options are wrong
- A. A 2:1 ratio is not a standard phenotypic ratio for a simple monohybrid cross.
- C. A 4:0 ratio would occur if all offspring expressed the dominant trait, which is not the case when both parents are heterozygous.
- D. A 1:1 ratio is expected from a test cross (e.g., Tt x tt).
Monohybrid Cross (Heterozygous)
A monohybrid cross involves two parents that are heterozygous for a single trait (e.g., Tt x Tt), where one allele is dominant over the other. The resulting phenotypic ratio is typically 3:1 for dominant to recessive traits.
- Parents are both heterozygous (e.g., Tt).
- Dominant allele masks the recessive allele's expression.
- Punnett square is used to predict offspring genotypes and phenotypes.
Memory trick: Three tall, one short, that's the heterozygous lot!