If two true-breeding lines differ for a single trait and are crossed, what is the expected phenotype of the F1 generation?

• A. All dominant
• B. 50/50
• C. All recessive
• D. Mixed

Answer: (A)

When two true-breeding lines differ for a single trait, each parent is homozygous for a different allele. Crossing them produces offspring that are all heterozygous, carrying one copy of each allele. If the trait follows complete dominance, the dominant allele masks the recessive in the heterozygote, so every F1 individual expresses the dominant phenotype. For example, a tall-homozygous line crossed with a short-homozygous line yields all tall offspring in the F1 because each plant has one tall allele and one short allele, and tall appears in the phenotype. Only in cases of incomplete dominance or codominance would the F1 show a mix of phenotypes.