When two tall plants were crossed, they produced some offspring that were dwarf. Assuming that plant height is controlled by a single gene and the allele for tall is dominant, what is the genotype of the parents?

When two tall plants are crossed and some offspring are dwarf, it suggests that both parents are heterozygous for the plant height trait. This means that each parent contributes one allele for tallness (T) and one allele for dwarfness (t). Therefore, the genotype of each parent would be Tt.

Let's break down the genetics:

- T = Dominant allele for tallness - t = Recessive allele for dwarfness

A plant needs only one copy of the dominant allele (T) to be tall. Dwarf plants are only possible when they inherit two recessive alleles (tt).

When two heterozygous tall plants (Tt) are crossed, the possible genotypes of the offspring can be predicted by drawing a Punnett square:

T t +---+---+ T | TT| Tt| +---+---+ t | Tt| tt| +---+---+

The Punnett square represents the different combinations of alleles that can occur in the offspring. According to this square, the possible genotypes of the offspring are:

TT: Homozygous dominant, which will result in a tall plant. - Tt: Heterozygous, which will also result in a tall plant, as the dominant allele is present. - tt: Homozygous recessive, which will result in a dwarf plant.

Based on this, the ratio of phenotypes (physical expressions) would be 3 tall plants to 1 dwarf plant. The fact that dwarf plants appeared among the offspring confirms that both parents carried the recessive allele (t).