B1 Lesson 9 – Genetic Disorders
Cystic fibrosis is called a single gene disorder . It occurs if a particular gene is defective.
1 Suggest how a gene can be defective.
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How common is this? Do some research using books or on the Internet.
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Single gene disorders are inherited. The gene for cystic fibrosis is ‘recessive’, which means only a person with two defective genes (one on each chromosome) has the condition. Look at the diagram on page 101 of your Student Book.
With a defective gene on one chromosome and a normal gene on the other chromosome, the person will not have cystic fibrosis. But they will be a carrier .
2 Explain why a carrier will not have the condition.
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We can represent the forms of the gene for cystic fibrosis using letters. The normal gene is C . The defective gene, causing cystic fibrosis, is c .
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A person without cystic fibrosis will have two C genes. We say their genotype (the genes they have for this characteristic) is CC .
A person with cystic fibrosis will have two c genes. Their genotype is cc .
A carrier has one normal and one defective gene. Their genotype is Cc .
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B1 Lesson 9 – Genetic Disorders
Reproductive cells − sperms in the male, eggs in the female – are called gametes . These are different from all other body cells: they have one of each chromosome, not a pair. When gametes are produced by an individual, half of the gametes produced will have one chromosome from each of the chromosome pairs, and the other half of the gametes will have the other chromosome of the pair.
If a ‘normal’ person and a carrier have children, we can predict the chances of them having a child with cystic fibrosis. It’s helpful to produce a diagram called a Punnett square to work this out.
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B1 Lesson 9 – Genetic Disorders
From the Punnett square on the previous sheet, we can see there is no possibility that any of the children of the ‘normal’ person and the carrier will have cystic fibrosis. We can predict that half would be carriers. But they may not, because which sperm fertilises which egg is random.
3 You are a genetic counsellor, advising couples on whether having children would put those children at risk of having a genetic disease. You are seeing a couple who have cystic fibrosis running in both their families, but neither of them has the disease themselves. They want to know what the chances are of having a child with cystic fibrosis. What would your advice be to them? Use a diagram to help.
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4 Another couple visits you. They know that they are both carriers of the condition. They already have one child who is normal. They are concerned that if they have another, it’s more likely that the next will have cystic fibrosis. Explain why this is not the case.
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