Question

5. In nasturtiums, flowers may be single (one layer of petals), double (two layers of petals) or superdouble (many layers of petals). Superdouble plants are female-sterile. 1. true-breeding single X true-breeding double → Fı single → F2 78 single : 27 double 2. true-breeding double X superdouble → 112 superdouble : 108 double 3. true-breeding single X superdouble → 8 superdouble : 7 single 4. true-breeding double X superdouble progeny from cross 3 = 18 superdouble : 19 single 5. true-breeding double X single progeny from cross 3 → 14 double : 16 single a. Why can't you get true-breeding superdouble plants? (1 pt.) b. From this series of crosses, determine the dominance relationships between alleles controlling the trait. (4 pts.)

Answer 1

Ans.a) True breeding superdouble plants are not obtained in the cross, because we get only one gamete, i,e. the male one from the plant as the females are sterile.

Ans.b) 1. In the cross between true breeding single and true breeding double, the F₁ generation produces all single plants. It shows that the trait producing single flower is dominant over the trait producing double flower. So in the F₂ generation we get more (78) single flower than double.

2. In the cross between true breeding double and superdouble, we get 112 superdouble and 108 double. Thus the trait producing superdouble flower is dominant over the trait producing double flower. The superdouble female is sterile but still we get more superdouble plants which means the hybrid produced in the cross, the superdouble trait is dominant.

3. In the  cross between true breeding single and superdouble we get 8 superdouble and 7 single. Here the superdouble trait is dominant over single trait.

4. In the cross between true breeding double and superdouble progeny from coss 3, we get 18 superdouble and 19 single. Here it is proved that the trait producing double flower is the recessive trait. Superdouble with only one functional gamete is the most dominant trait.

5. In the cross between true breeding double and single progeny from coss 3, we get 14 double and 16 single which shows the dominance of the single trait over the double trait.

So the most dominant alleles controlling the traits are in the following order : Superdouble > Single > Double. Double is the recessive trait here.