In tomatoes, tall is dominant to short, and smooth fruits are dominant to hairy fruits. A plant homozygous for both dominant traits is crossed with a plant homozygous for both recessive traits. The F₁ progeny are tested and crossed with the following results: 78 tall, smooth fruits; 82 dwarf, hairy fruits; 22 tall, hairy fruits; and 18 dwarf, smooth fruits. These data indicate that the genes are A) on different chromosomes. B) linked, but do not cross over. C) linked and show 10 percent recombination. D) linked and show 20 percent recombination. E) linked and show 40 percent recombination.

Question

Quizplus · Accepted Answer

The phenotype ratios in the offspring indicate that the two genes are linked and show 20 percent recombination. This is because the observed double recombinant class (tall, hairy and dwarf, smooth) adds up to 40 individuals out of a total of 200 (20 percent). If the genes were unlinked or not showing recombination, the two double recombinants would not exist, and the other two phenotypic classes would have equal numbers. If the genes were linked and showing only 10 percent recombination, the frequency of the double recombinants would be too low compared to the observed data.

In Tomatoes, Tall Is Dominant to Short, and Smooth Fruits