Explain what is meant by the terms homozygous and recessive.
In 2020, there were $10800$ people with cystic fibrosis in the UK and the UK population was estimated to be $67\,100\,000$ people. A fraction of the UK population are heterozygous for the gene that causes cystic fibrosis and show no symptoms of the disease. Use the Hardy-Weinberg principle to calculate how many people in the UK population are expected to be heterozygous for the gene that causes cystic fibrosis. The equations are $p + q = 1$ and $p^2 + 2pq + q^2 = 1$, where $p$ is the frequency of the dominant allele and $q$ is the frequency of the recessive allele. The first stage of the calculation has been completed: $q^2 = \frac{10800}{67\,100\,000}$. Calculate $q$, $p$, $2pq$, and hence the number of people in the UK expected to be heterozygous for the gene.
The Hardy-Weinberg principle gives a helpful estimate of how many people in the UK are heterozygous for cystic fibrosis. However, the estimate is lower than the actual number because not all the conditions of the Hardy-Weinberg principle are satisfied. In the UK in 2020, the mean life expectancy of people with cystic fibrosis was approximately $50$ years and of 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.
Calculate $p$. $p =$
A screening programme for cystic fibrosis was introduced in 2007 for all children born in the UK. Children are screened within seven days after birth. Children identified by 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 predicted median life expectancy for people born in the UK who have cystic fibrosis. Predictions are shown for people born in 2008, 2012, 2016 and 2020. Describe the trend shown in Table 3.1 and outline how early screening for cystic fibrosis may have contributed to this trend.
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 discover whether 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.