Inherited disorders

1. An inherited disorder is a condition caused by a faulty allele passed from parents to offspring.
2. Some disorders are caused by a recessive allele, so a person needs two copies to have the disorder.
3. Others are caused by a dominant allele, so a person needs only one copy to have the disorder.

Cystic fibrosis and polydactyly

1. Cystic fibrosis is a disorder of the cell membranes that causes thick, sticky mucus in the lungs and digestive system.
2. It is caused by a recessive allele, so a child only has it if they inherit the faulty allele from both parents.
3. A person with just one faulty recessive allele is a carrier: they do not have the disorder but can pass the allele on.
4. Polydactyly, having extra fingers or toes, is caused by a dominant allele.
5. Because the allele is dominant, a child can inherit polydactyly from just one affected parent.

Working out the risk

1. Write the parents' genotypes using allele symbols, such as $F$ for the normal allele and $f$ for the cystic fibrosis allele.
2. Work out the gametes and combine them in a Punnett square.
3. Count the offspring genotypes to find the chance of an affected child.
4. For two carriers ($Ff\times Ff$), the offspring are $FF$, $Ff$, $Ff$ and $ff$, so $\frac{1}{4}$ ($25\%$) are affected and $\frac{1}{2}$ are carriers.
5. For a dominant disorder, a single faulty allele is enough for the child to show it.

Embryo screening

1. In embryo screening, embryos made by IVF are tested for faulty alleles before one is placed in the womb.
2. Embryos carrying an allele for a serious disorder can be identified and not used.
3. Arguments for: it can stop children being born with serious genetic disorders and may reduce suffering and the cost of long-term treatment.
4. Arguments against: it is expensive, and some people believe it is unethical to test and reject embryos.
5. There are also fears that screening could later be used to choose other features, leading to so-called designer babies, or that people carrying alleles could face discrimination.
6. To evaluate the issue, you weigh these economic, social and ethical points and then reach a clear judgement.

Gene therapy

1. Scientists are also researching gene therapy as a way to treat some inherited disorders.
2. The idea is to insert a working copy of the faulty gene into the affected person's cells.
3. If the cells then make the correct protein, the symptoms of the disorder could be reduced.
4. Gene therapy is still experimental and is not yet a reliable, routine cure.