Cystic fibrosis is an autosomal recessive genetic disease. People with cystic fibrosis have a homozygous recessive genotype.

1. (a)Explain the meaning of the terms homozygous and recessive. homozygous recessive[2]
2. (b)(i)In 2020: • there were 10800 people with cystic fibrosis in the UK • the UK population was estimated to be 67 100000 people. A proportion of people in the UK population are heterozygous for the gene that causes cystic fibrosis and do not have symptoms of the disease. Use the Hardy–Weinberg principle to calculate the number of people in the UK population who are expected to be heterozygous for the gene that causes cystic fibrosis. The two equations for the Hardy-Weinberg principle are provided. equation 1 p + q = 1 equation 2 p² + 2pq + q2 = 1 p = frequency of the dominant allele q = frequency of the recessive allele p² = frequency of the homozygous dominant genotype 2pq = frequency of the heterozygous genotype q² = frequency of the homozygous recessive genotype The first stage of the calculation has been completed for you. q2 = 10800 / 67 100 000 q = p = 2pq = number of people in the UK expected to be heterozygous for the gene =[3]
3. (b)(ii)The Hardy-Weinberg principle provides a useful estimate of the number of people in the UK who are heterozygous for cystic fibrosis. However, the estimate is lower than the actual number. This underestimation occurs because not all the conditions of the Hardy-Weinberg principle apply. In the UK in 2020, the mean life expectancy of: • people with cystic fibrosis was approximately 50 years • all people was approximately 80 years. Explain how this information accounts for the underestimation of the number of people in the UK that are heterozygous for cystic fibrosis.[3]
4. (c)(i)A screening programme for cystic fibrosis was introduced in 2007 for all children born in the UK. Children are tested within seven days of their birth. Children identified from the screening programme as being at high risk of having cystic fibrosis can have a genetic test to confirm whether they have the disease. Table 3.1 shows the median predicted life expectancy for people born in the UK who have cystic fibrosis. Predictions are shown for people born in 2008, 2012, 2016 and 2020. Table 3.1 year of birth: 2008, 2012, 2016, 2020. median predicted life expectancy /years: 38.8, 43.5, 47.0, 50.6. Describe the trend shown in Table 3.1 and outline how early screening for cystic fibrosis may have contributed to this trend.[3]
5. (c)(ii)In many countries, a genetic test for cystic fibrosis is available to adults who do not have cystic fibrosis but have a family member who either has cystic fibrosis or is heterozygous for the gene that causes cystic fibrosis. These adults include partners, parents, offspring, brothers and sisters of the family member. The aim is to find out if any of these adults are heterozygous for the gene that causes cystic fibrosis. Discuss the ethical and social considerations of making a genetic test for cystic fibrosis available to these adults.[3]