A dihybrid cross demonstrates independent assortment by showing

• A. If genes are linked, they assort independently.
• B. Only one gene is involved.
• C. When genes are on different chromosomes or far apart, they assort independently, producing a 9:3:3:1 phenotype ratio in the F2.
• D. A dihybrid cross shows a 1:1:1:1 ratio.

Answer: (C)

Mendel's law of independent assortment is being tested here. In a dihybrid cross, when the two gene loci are on different chromosomes or are far apart on the same chromosome, they sort independently during meiosis. That means the gametes from a heterozygous parent can be AB, Ab, aB, or ab with roughly equal frequency. When such gametes combine, you get four phenotypic classes in a 9:3:3:1 ratio in the offspring: both dominant traits, each single dominant trait, and both recessive traits.

If genes are linked, they tend to be inherited together and you don’t see that 9:3:3:1 pattern. Only one gene involved would not demonstrate a dihybrid cross at all, and a 1:1:1:1 ratio would come from a different cross situation, not from independent assortment of two unlinked genes. So the statement that when genes are on different chromosomes or far apart they assort independently, producing a 9:3:3:1 ratio, best captures what a dihybrid cross shows.