Two healthy parents
323 words over 29 numbered lines, then 5 parts worth 12 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice unless a source is named.
Two healthy parents
When Amira and Tom’s first baby was five days old, a midwife took a
few drops of blood from her heel. The test is offered for every
newborn baby and checks for several rare conditions. Two weeks later,
they were told that their daughter had cystic fibrosis. Neither parent
had the condition, and neither knew of anyone in their families who
had.
Cystic fibrosis is an inherited disorder that affects the lungs and
the digestive system. It is caused by a recessive allele of a single
gene. The gene codes for a protein that sits in the cell membranes of
cells lining the airways and the pancreas, where it moves chloride
ions out of the cells. Water follows by osmosis, keeping the mucus on
the surface of the cells thin and runny.
The most common cystic fibrosis allele has lost three bases from the
DNA of the gene. As a result, the protein is missing one amino acid.
It folds into the wrong shape and is broken down before it reaches the
cell membrane. Without it, the mucus becomes thick and sticky. It
blocks the small airways, where bacteria can grow, and it blocks the
tubes that carry enzymes from the pancreas to the small intestine.
A person with one normal allele and one cystic fibrosis allele does
not have the condition, but is a carrier. In many populations of
northern European origin, about one person in 25 is a carrier. Amira
and Tom were both carriers.
Newborn screening means that babies with cystic fibrosis can be
treated from the first weeks of life. Figure 1 shows illustrative
screening results from three regions. Treatment includes daily
physiotherapy to clear mucus from the lungs, and capsules of enzymes
taken with every meal. New medicines can now help the faulty protein
to fold correctly and reach the cell membrane. Most people with cystic
fibrosis now live well into adulthood, which was rare fifty years ago.
Figure 1 Newborn screening results from three regions in one year
| Region | Babies screened | Babies with cystic fibrosis |
|---|---|---|
| A | 48,000 | 16 |
| B | 62,500 | 25 |
| C | 30,000 | 6 |
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term allele.
(1)Explain how two parents who do not have cystic fibrosis can have a child with the condition.
(3)Amira and Tom are both carriers. Use a genetic diagram to show the probability that their next child will have cystic fibrosis. Use F for the normal allele and f for the cystic fibrosis allele.
(3)Explain how the loss of three bases from the gene causes the symptoms of cystic fibrosis.
(3)Use Figure 1 to calculate the number of babies with cystic fibrosis per 10,000 babies screened in Region B.
(2)Total for this question: 12 marks
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