Question 21M.2.SL.TZ2.5c
| Date | May 2021 | Marks available | [Maximum mark: 3] | Reference code | 21M.2.SL.TZ2.5c |
| Level | SL | Paper | 2 | Time zone | TZ2 |
| Command term | Calculate | Question number | c | Adapted from | N/A |
Congenital lactase deficiency is a type of lactose intolerance that occurs in infants. It is inherited in an autosomal recessive pattern. Calculate the chance of congenital lactose intolerance in a child whose parents are both carriers for the disorder, showing fully how you reached your answer.
[3]
a. gametes of both parents shown as a capital and small letter (e.g. L and l) ✔
b. possible F1 genotypes ✔
c. 25 % lactose intolerant, 50 % carriers, 25 % lactose tolerant
OR
75 % tolerant and 25 % intolerant
OR
child has 25 %/1:4/ chances of inheritance of intolerance ✔
(attempted by about 40 % of the cohort)
The question was put in the context of the composition of pastry cream, rather than just diving in with questions. In part (a), the structure of starch was quite well known. The better answers included descriptions of polymers and monomers, amylose and amylopectin, and glucose. Part (b) was challenging to many, requiring a knowledge of the digestive system, enzyme action and the blood supply to the liver. It was possible to gain all of the marks by omitting the references to the blood stream. This was an example of where a clear 'essay plan' would have helped. Weaker candidates wrote at length about carbohydrate and fat digestion which was not required. Surprisingly the position of the pancreas was often confused, with pancreatic enzymes appearing in the stomach. The fundamental idea of the digestion of large molecules (proteins) to small soluble ones (amino acids) which can be absorbed into the blood stream at the villi was missed by many. Part (c) was a straightforward genetics cross, put into the context of lactose intolerance. In general, it was well answered. Although the question clearly stated that it was an autosomal recessive condition, many weaker students were determined to make it sex linked.
