Lactose is the main sugar in milk and is hydrolysed by the enzyme lactase.
Lactase is essential to newborn mammals as milk is their only source of food.
Most mammals stop producing lactase when they start feeding on other food
sources. Humans are an exception to this because some continue to produce
lactase as adults. The ability to continue producing lactase is known as lactase
persistence (LP) and is controlled by a dominant allele. A number of hypotheses
based on different selection pressures have been put forward to explain LP in
(a) One hypothesis for LP in humans suggests that the selective pressure was
related to some human populations farming cattle as a source of milk.
Describe how farming cattle as a source of milk could have led to an
Reject mutation caused by drinking milk.
2. Milk provides named nutrient;
Accept any correct named nutrient e.g. glucose,
Ignore 'sugar' 'lactose' as named nutrient
3. Individuals with LP more likely to survive and reproduce
Individuals with advantageous allele more likely to survive and
Accept 'individuals who produce lactase' for 'LP
Accept 'pass on allele/LP/characteristic' for
4. Directional selection;
5. Frequency of allele increases (in the offspring/next generation);
Accept description of increasing frequency of allele
e.g. 'higher proportion', 'more common' but ignore
increase in number of allele