Question

In certain plants, tall is dominant to short. A member of your class accidentally did not record the genotype of the parent plants prior to mating them. Following the cross, you notice that the phenotypic ratio for tall to short plants is 1:1. Identify the cross that could have produced these results. Justify your reasoning

Answer 1

Answer:

Heterozygous tall x homozygous short individuals

Explanation:

The cross that could produce tall plant to short plant in the ratio 1:1 would be between a heterozygous tall and homozygous short individuals.

Assuming the allele for tallness is represented by T and that for shortness is represented by t. TT and Tt individuals will be tall while tt individuals will be short. A cross between a heterozygous tall individual (Tt) and homozygous short individual (tt) will be such that:

Tt   x   tt

offspring: Tt (tall), Tt (tall), tt (short), and tt (short). (see the attached image for the Punnet's square analysis)

The ratio of tall to short offspring = 1:1

Hence, the cross is between heterozygous tall and homozygous short individuals.

Answer 2

Answer:

Heterozygous tall (Tt) and homozygous short (tt) parents

Explanation:

This question involves a single gene coding for height in a certain plant. The allele for tallness (T) is dominant over the allele for shortness (t). This means that allele 'T' will always mask the phenotypic expression of allele 't' in a heterozygous state.

According to the question, the genotypes of the parent plants that were crossed were not recorded but tall and short offsprings in a ratio 1:1 were observed. Based on this phenotypic ratio, one of the parents has to be heterozygous for the trait and the other homozygous for the recessive trait in order to produce offsprings with equal ratio of tall and short traits.

This means that the heterozygous tall parent will have a genotype: Tt, and be phenotypically tall since Tallness is dominant to shortness. The other parent which is homozygous recessive will have a genotype: tt.

In a nutshell, a cross between parents with genotypes; Tt and tt will produce four possible offsprings with two distinct genotypes i.e. Tt and tt.

Tt offsprings (2) will be phenotypically tall

tt offsprings (2) will be phenotypically short

Hence, the phenotypic ratio will be 2:2 equivalent to 1:1