26. In a specific scenario, red flowers are dominant over white flowers. A plant heterozygous for red flowers and a plant with white flowers are crossed. What is the expected proportion of phenotypes in the next generation?
Answer: A
The expected proportion of phenotypes in the next generation is 2 red, 2 white.
In this scenario, the cross involves a heterozygous red flower plant (Rr) and a homozygous white flower plant (rr). The offspring from this cross will exhibit a phenotype ratio of 2 red flowers to 2 white flowers, or simply 1:1.
A) 2 red, 2 white
This option is correct because when a heterozygous red flower (Rr) is crossed with a homozygous white flower (rr), the potential offspring genotypes are Rr and rr, leading to a phenotype ratio of 1:1. Therefore, half of the offspring are expected to have red flowers, and half will have white flowers.
B) 3 red, 1 white
This option is incorrect as it suggests a 3:1 ratio, which typically arises from a cross between two heterozygous plants (Rr x Rr). Since one parent in this scenario is homozygous recessive (rr), the resulting ratio cannot yield this outcome.
C) 4 red, 0 white
This option is incorrect because it implies that all offspring would result in red flowers, which would only happen if both parents were either homozygous dominant or if the heterozygous parent was crossed with another heterozygous plant. The presence of the homozygous white parent guarantees some white offspring.
D) 1 red, 3 white
This option is incorrect because it suggests a phenotype ratio that does not fit the genetic cross presented. A 1:3 ratio would only occur in a scenario where both parents are heterozygous; however, in this case, only one parent is homozygous recessive.
Conclusion
The correct answer of 2 red and 2 white is validated by the principles of Mendelian genetics, specifically regarding dominant and recessive traits. The other options fail to reflect the expected outcomes based on the genetic makeup of the parent plants involved in the cross.