| Date | November 2013 | Marks available | 1 | Reference code | 13N.3.HL.TZ0.2 |
| Level | Higher level | Paper | Paper 3 | Time zone | TZ0 |
| Command term | State | Question number | 2 | Adapted from | N/A |
Question
Cystic fibrosis (CF) is caused by a mutation of a human gene which codes for a chloride channel. The frequency of the CF allele is much higher in Europe than expected for an allele that causes a harmful condition. It has been suggested that individuals who are heterozygous for this allele may be protected against an infectious disease such as cholera or typhoid. This could cause both the CF allele and the normal allele of the chloride channel gene to persist in the population.
State the name given to the situation where two alleles of a gene persist indefinitely in a population.
CF is a recessive condition that affects approximately 1 in 2500 births in Australia. Calculate the percentage of heterozygous individuals in the population. Show your calculation.
Using CF as an example, distinguish between allele frequency and gene pool.
Markscheme
balanced polymorphism
\(2pq = (2)(0.98)(0.02) = 0.039\);
3.9%;
Award [1] for working and [1] for correct answer.
gene pool is all of the genetic information/genes/alleles present (in an interbreeding population);
allele frequency is the proportion of one allele of one/CF gene in a population;
Examiners report
The majority could name balanced polymorphism properly. Most knew that they had to apply the Hardy-Weinberg equation to find the percentage of heterozygotes in the population, but many had difficulties in determining the value of p and/or 2pq. Many found it hard to distinguish between allele frequency and gene pool using appropriate terminology. The conditions of application for the Hardy-Weinberg law were well known by the majority of candidates.
The majority could name balanced polymorphism properly. Most knew that they had to apply the Hardy-Weinberg equation to find the percentage of heterozygotes in the population, but many had difficulties in determining the value of p and/or 2pq. Many found it hard to distinguish between allele frequency and gene pool using appropriate terminology. The conditions of application for the Hardy-Weinberg law were well known by the majority of candidates.
The majority could name balanced polymorphism properly. Most knew that they had to apply the Hardy-Weinberg equation to find the percentage of heterozygotes in the population, but many had difficulties in determining the value of p and/or 2pq. Many found it hard to distinguish between allele frequency and gene pool using appropriate terminology. The conditions of application for the Hardy-Weinberg law were well known by the majority of candidates.
