Reproduction » Inherited Disorders and Embryo Screening

Disorders that run in families

Some disorders are inherited. That means they are passed from parents to their children. These disorders are caused by the inheritance of certain alleles. A child gets the faulty allele from one or both parents, in the same way that they get any other allele.

You already know that an allele can be dominant or recessive. That is the key to this lesson, because the type of allele decides how likely a child is to have the disorder. The spec names two examples that you need to know.

Key terms:

• Inherited disorder: a disorder caused by the inheritance of certain alleles from the parents.
• Polydactyly: having extra fingers or toes.
• Cystic fibrosis: an inherited disorder of cell membranes, caused by a recessive allele.
• Carrier: a person who has one copy of a recessive allele for a disorder but does not have the disorder.
• Embryo screening: testing embryos for faulty alleles before one is placed in the mother's uterus.

Two inherited disorders

This is why the two disorders behave so differently in families.

Polydactyly: a dominant allele

Let D stand for the dominant allele that causes polydactyly and d for the normal recessive allele. A person with the genotype DD or Dd has polydactyly. Only a person with dd does not.

So if one parent has polydactyly and carries a D allele, each child can inherit that allele. If neither parent has the disorder, neither of them carries a D allele, so their children will not have it either.

Cystic fibrosis: a recessive allele

Let f stand for the recessive allele that causes cystic fibrosis and F for the normal dominant allele. Only a person with the genotype ff has cystic fibrosis. A person with FF is unaffected. A person with Ff is also unaffected, because the dominant allele F is the one expressed. That person is a carrier.

A carrier does not have the disorder but can pass the faulty allele on. This means two healthy parents can have a child with cystic fibrosis. It happens when both parents are carriers, and the child inherits an f allele from each of them.

Embryo screening

People who know that a serious inherited disorder runs in their family may want to avoid having a child with it. Embryo screening can help. The embryos are made by IVF, which you met in Hormones and Infertility Treatment. When the embryos are tiny balls of cells, a cell or two is taken from each one and tested to see if it will have the disorder.

Only embryos that will not have the disorder are then put into the mother's uterus. The other embryos are not used.

Screening can prevent a child being born with a serious disorder and can prevent suffering. But it also raises difficult questions. You need to be able to make informed judgements about the economic, social and ethical issues, using the information you are given.

Gene therapy: the outline

Another idea is gene therapy. Here, scientists try to treat a disorder by putting a working copy of the allele into a person's cells. This is different from screening, because the person who has the disorder is treated, rather than embryos being chosen.

Like screening, gene therapy may reduce suffering. It also raises ethical issues, such as how safe it is and whether it should be used at all. The way scientists change genes is covered in Genetic Engineering.

Exam-style question

Cystic fibrosis is caused by a recessive allele. A man and woman do not have cystic fibrosis, but their daughter does.

(a) Explain how this can happen. [2 marks]

(b) Describe what embryo screening is. [2 marks]

(c) Give one ethical issue linked to embryo screening. [1 mark]