Monohybrid Cross Yielding 1:1 Phenotypic Ratio

Steps:

• The offspring ratio of 25 red : 24 yellow approximates 1:1, indicating a test cross in Mendelian genetics.
• Assume red fruit (R) is dominant over yellow (r); this ratio occurs when one parent is heterozygous (Rr) and the other homozygous recessive (rr).
• Explanation 1 fails: both homozygous parents (RR x rr or rr x rr) produce all dominant or all recessive offspring, not 1:1.
• Explanations 2 and 3 fit: one parent must be homozygous recessive (rr, carrying only recessive alleles), and the other heterozygous (Rr).

Why D is correct:

• D (2 and 3) matches the test cross definition, where heterozygous x homozygous recessive yields 50% dominant and 50% recessive phenotypes per Punnett square.

Why the others are wrong:

• A: Ignores 1:1 ratio; homozygous parents yield uniform offspring.
• B: Omits need for recessive parent to produce yellow offspring.
• C: Includes incorrect explanation 1, which contradicts the observed ratio.

Final answer: D