Question 4
A heterozygous tall plant is crossed with a dwarf plant. What fraction of the offspring is dwarf?
The right answer is the one marked above.
One half. The heterozygote gives T or t in equal numbers; the dwarf plant gives only t. So half the offspring are Tt and tall, and half are tt and dwarf.
This is the test cross. Because the dwarf parent contributes nothing dominant, the offspring reveal directly what the unknown parent was carrying.