Monohybrid cross PROBLEM SOLVING
1. In tomato plants, fruit colour may be red or yellow. The table below gives details of
crosses between certain tomato plants and the offspring produced in each case.
Cross
1
2
3
Parents
Plant A (red) x Plant B (red)
Plant B (red) x Plant C (red)
Plant D (red) x Plant E (yellow)
Number and phenotype(s) of offspring
258 red
197 red and 65 yellow
128 red and 134yellow
Using letters of your choice, identify the genotypes of the tomato plants A, B, C, D and E.
A: RR or Rr
B: Rr
C: Rr
D: Rr
E: rr
2. In fruit flies, straight wing (T) is dominant to curled wing (t). The table below shows the
results of an investigation carried out to examine the phenotypes arising from a
monohybrid cross involving the wing type gene.
Parents of F1
F1
Parents of F2
F2
Straight wing
6 true-breeding males
168 flies of both sexes
6 males from F1
81 flies of both sexes
Curled wing
6 true-breeding females
0
6 females from F1
87 flies of both sexes
a. Explain why no curled winged flies were produced in the F1 generation.
All the F1 will have the straight wing allele (T), so therefore all will be straight wing.
b. What is the expected ratio of straight to curled in the F2 generation?
3 straight:1 curled
c. What is the expected number of straight to curled in the F2 generation?
Total offspring = (81+87) = 168
3:1 of 168 = 126 Straight : 42 curled
d. Why do the actual results vary from the expected ones?
Fertilisation is a random process