The F1 of a cross between a tall and a dwarf plant was tall. The F1 was advanced to F2. How many of 120F2 plants will be dwarf?

Answer Details

When a tall plant and a dwarf plant are crossed, the resulting F1 generation is always tall because the tall trait is dominant over the dwarf trait. When the F1 generation is allowed to self-pollinate, the resulting F2 generation will have a 3:1 ratio of tall to dwarf plants. This is because each F1 plant carries one dominant tall allele and one recessive dwarf allele. Using a Punnett square to represent the possible genotypes of the F2 generation, we can see that out of 4 possible combinations, only 1 combination (Tt x Tt) will result in a dwarf plant (genotype tt). This gives us a probability of 1/4 or 25% chance of a dwarf offspring. If we have 120 F2 plants, we can expect 25% or 0.25 of them to be dwarf. To find out how many that is, we can multiply 120 by 0.25 to get 30 dwarf plants. Therefore, the answer is 30, and this is because of the 3:1 ratio and the laws of probability.