Question 4
In maize species, starchy grain (GG) is dominant to sugary grain (gg). Two pure maize varieties were crossed, one grown from a starchy grain and the other grown from a sugary grain.
- (i) With the aid of a genetic diagram, determine the phenotype of F1 grains. [9 marks]
(ii) The F1 grains grew into mature plants and were allowed to self pollinate. What is the expected ratio of starchy to sugary grains on the cobs borne on the F1 plants? [9 marks] - (b) If the total of F2 grains collected is 580, how many of these would be expected to be:
- (i) starchy;
(ii) sugary? [2 marks]
Observation
A few candidates attempted this question and most of those that did had a good grasp of genetics, although some of the candidates lost marks by not stating the parental phenotype before giving the gametes and crossing.
Some candidates could not calculate the number of plants that would be sugary or starchy.
The expected answers are:
(a) (i) Phenotype of F1 grains
G = starchy allele
g = sugary allele
Parents genotype for starchy plant/homozygous starchy -GG
Parents genotype for sugary plant/homozygous sugary - gg
F2 crossing must be derived from F1 to score; Punett square should be accepted
(b) 