7. In a plant in which fuzzy leaves (F) are dominant over smooth leaves (f), which of the following crosses will reproduce only offspring with smooth leaves?
Answer: B
ff by ff
The cross between two smooth-leaved plants (ff) will produce only offspring with smooth leaves, as both parents contribute only the recessive allele for smooth leaves.
A) FF by ff
This cross will produce offspring with the genotypes Ff (fuzzy leaves) and ff (smooth leaves). Since F is dominant over f, at least half of the offspring will have fuzzy leaves, making this option incorrect for producing only smooth leaves.
B) ff by ff
This cross involves two plants that are homozygous for the recessive trait (smooth leaves). All offspring will inherit the recessive alleles (f) from both parents, resulting in 100% smooth-leaved plants.
C) Ff by Ff
The cross between two heterozygous (Ff) plants will produce offspring with genotypes of FF, Ff, and ff. This will result in a mix of 25% fuzzy leaves (FF), 50% fuzzy leaves (Ff), and 25% smooth leaves (ff), so this option does not guarantee only smooth leaves.
D) FF by FF
This cross will produce only offspring with the genotype FF, all exhibiting fuzzy leaves. Since there are no recessive alleles involved, this option cannot yield smooth-leaved offspring.
Conclusion
The only cross that ensures all offspring will have smooth leaves is the cross of ff by ff, as it only produces recessive genotypes. All other options involve at least one dominant allele that can produce fuzzy leaves, thereby failing to meet the criteria specified in the question.