| Date | May 2009 | Marks available | 2 | Reference code | 09M.2.SL.TZ1.1 |
| Level | Standard level | Paper | Paper 2 | Time zone | Time zone 1 |
| Command term | Evaluate | Question number | 1 | Adapted from | N/A |
Question
Genetic engineering allows genes for resistance to pest organisms to be inserted into various crop plants. Bacteria such as Bacillus thuringiensis (Bt) produce proteins that are highly toxic to specific pests.
Stem borers are insects that cause damage to maize crops. In Kenya, a study was carried out to see which types of Bt genes and their protein products would be most efficient against three species of stem borer. The stem borers were allowed to feed on nine types of maize (A–I), modified with Bt genes. The graph below shows the leaf areas damaged by the stem borers after feeding on maize leaves for five days.
Before the use of genetically modified maize as a food source, risk assessment must be carried out. A 90-day study was carried out in which 3 groups of 12 adult female rats were fed either:
• seeds from a Bt maize variety
• seeds from the original non-Bt maize variety
• commercially prepared rat food.
All the diets had similar nutritional qualities.
State what would be used as the control in this experiment.
Outline the effects of the three species of stem borer on Bt maize type A.
Evaluate the efficiency of the types of Bt maize studied, in controlling the three species of stem borers.
Describe the change in mean mass for the female rats during the 90-day experiment.
Evaluate the use of Bt maize as a food source compared to the other diets tested.
Markscheme
maize not modified/transformed with Bt (genes) / maize that did not have Bt gene added / not genetically modified / untreated maize
there was a decrease in damage by all three types of stem borers compared to control;
there was almost no change in damage by Eldana compared to control;
there was almost no damage/little effect (to Bt maize type A) by Sesamia (and Eldana);
Busseola caused the most damage (to Bt maize type A);
very efficient at controlling Sesamia;
type B is the most effective against the three stem borers collectively;
no type of Bt maize controlled Busseola well / vice versa i.e. Busseola not well controlled by any types of Bt maize;
all types of Bt maize decreased Sesamia damage (significantly) / Bt maize type E not damaged by Sesamia / vice versa;
Bt maize types C/H/I had more damage caused by Busseola (than was caused in the control) / vice versa;
all types of Bt maize decreased Eldana damage (to some extent) / type B was not damaged by Eldana / vice versa;
Eldana damage low in control / less effect;
cannot determine efficiency since data is about leaf damage and stem borers may feed (preferentially) on other structures/stems/roots;
mass increases in all three groups;
increase is more rapid in beginning and tapers off later in the study;
mass seems to be levelling off in rats fed Bt and non-Bt maize / rate of increase in mass is slowing down;
rats fed rat food always have higher mass/greater mass increase than those fed either type of maize;
all three foods result in the same pattern of growth/mass gain / highest rate of growth at start of study / tapering off later in the study;
Bt maize causes same amount of growth as non-Bt maize / appears to be as good a food source as non-Bt maize / there is no significant difference between Bt and non-Bt maize (in terms of mass gain);
corn (both types) appears to cause less growth/mass gain than rat food / vice versa;
genetic modification does not affect growth/mass gain;
no evidence to support risk of Bt maize to growth/mass gain;
study does not investigate other possible risks of Bt maize to rats;
sample size is small / only 12 rats (in each group) so this may not be enough to give trends;
only female rats tested, no males;
Examiners report
The best answers identified the control as maize that did not have Bt gene added, rather than just untreated maize.
Weaker candidates misread the question (perhaps a language issue) and wrote how maize type A damaged the three species of stem borers instead of how the stem borers affected maize type A. This caused a variety of ambiguous answers which caused difficulty in awarding marks
Few candidates analyzed the data in its entirety. They didn't consider that no type of Bt maize controlled Busseola well or that all types of BT maize decreased Sesamia damage. An astute evaluation made by some candidates who recognized that type B maize was most efficient in controlling all three species if they were considered together as a group.
Though the question used the term “female rats,” there three lines on the graph because there were three different groups of female rats depending on which diet they were fed. Candidates needed to qualify their answers by naming the group(s) of female rats which correlated to the change they were describing.
Most candidates answered that Bt maize causes the same amount of growth as nonBt maize and that both types appeared to have caused less growth or mass gain than the rat food. No candidates mentioned the small sample size being too small to give trends of that only female rats were tested.
