GCSE · Biology · AQA · Spec 8461

Inherited disorders

Some disorders are inherited, not caught. Doctors can test IVF embryos and choose ones that don't carry a disorder like cystic fibrosis. Should they? Weigh it up yourself first.

Weigh it up first

Should embryos be screened for inherited disorders?

The claim

Embryo screening should be used so that a child does not inherit a disorder such as cystic fibrosis.

Place each piece of evidence to load the balance. Mark the strong ones — they count double.

  1. Screening means a pregnancy can start with an embryo found not to carry the disorder.

    Evidence 1: does it support or challenge the claim?

  2. Some people are reassured that screening can spare a child a serious inherited disease.

    Evidence 2: does it support or challenge the claim?

  3. In the UK, a condition can be screened for only after a regulator has judged that it meets legal tests of seriousness and risk of being passed on.

    Evidence 3: does it support or challenge the claim?

  4. Choosing between embryos means the ones not chosen are destroyed — and every embryo may have a right to life.

    Evidence 4: does it support or challenge the claim?

  5. Some people would rather not intervene, and accept whatever happens naturally.

    Evidence 5: does it support or challenge the claim?

  6. The tests are not perfectly accurate, and removing cells can occasionally damage an embryo so that it cannot be used.

    Evidence 6: does it support or challenge the claim?

  7. NHS funding is available only in some cases — depending on a person's situation and where they live, how severe the disorder is, and the chance a child would inherit it.

    Evidence 7: does it support or challenge the claim?

  8. Plenty of people who have a genetic condition live happy, fulfilling lives — and it falls to society to remove the physical and social obstacles they meet.

    Evidence 8: does it support or challenge the claim?

Two inherited disorders, one crucial difference

PolydactylyvsCystic fibrosis

Start with the row about one copy — it's the one people get wrong.

Focus

A person with ONE copy of the allele

Polydactyly

Has polydactyly

Cystic fibrosis

Does not have cystic fibrosis

The insight

Here's the trap: carrying one copy of a disorder allele does not automatically mean having the disorder. It depends on whether that allele is dominant or recessive.

Caused by

Polydactyly

A dominant allele

Cystic fibrosis

A recessive allele

Why one copy does (or doesn't) matter

Polydactyly

A dominant allele is always expressed, even if only one copy is present

Cystic fibrosis

A recessive allele is only expressed if two copies are present — so no dominant allele is there

What the disorder is

Polydactyly

Having extra fingers or toes

Cystic fibrosis

A disorder of cell membranes

How embryo screening works

Step through it in order — each stage depends on the one before.

  1. IVFEmbryo screening takes place during IVF. The embryos are tested in a laboratory, so a couple has to go through IVF to use screening — even if they have no difficulty conceiving.
  2. A few cells are taken from each embryoNot just from one or two embryos: a few cells are taken from every embryo, so each one can be checked.

Sort the issues

Economic, social or ethical?

Which kind of issue does each point raise?

Still to sort

Economic (0)

About money: who pays, and who can get it funded.

Where the line is: If the point is about funding or who pays, it's economic.

Social (0)

About people's lives in society, and how society treats them.

Where the line is: Social points are about living with a condition in society, not about what happens to an embryo.

Ethical (0)

About right and wrong: here, what may be owed to an embryo.

Where the line is: If the point says something is right or wrong in itself, it's ethical.

6 of 6 still to sort.

Place each point, then read why it belongs there.

Now write your judgement

Write it, then check it

Sam and Priya's child could inherit cystic fibrosis. They have no difficulty conceiving. Evaluate whether Sam and Priya should use embryo screening. [5 marks]

0 words · your answer stays on this page and is not sent anywhere.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One copy of a dominant allele is enough for polydactyly; cystic fibrosis needs two recessive copies. Embryo screening can avoid a disorder — and whether to use it is a judgement you reach by weighing economic, social and ethical issues.

What you need to know

  • Some disorders are inherited — they are caused by inheriting certain alleles.
  • Polydactyly (extra fingers or toes) is caused by a dominant allele; cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.
  • A dominant allele is always expressed, even with one copy; a recessive allele is only expressed when two copies are present.
  • Embryo screening happens during IVF: cells from each embryo are tested, and only embryos found not to carry the disorder are placed in the womb.
  • You should be able to make an informed judgement about the economic, social and ethical issues of embryo screening.

The big picture

Some disorders are inherited: they are caused by inheriting certain alleles. Polydactyly (extra fingers or toes) is caused by a dominant allele, which is always expressed even as a single copy. Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele, which is only expressed when two copies are present. Embryo screening takes place during IVF: a few cells are taken from each embryo and tested for particular alleles, and only embryos found not to carry the disorder are placed in the womb. Whether to use it raises economic, social and ethical issues, and an informed judgement weighs them.

Key points

1Inherited disorders are caused by the inheritance of certain alleles.
2Polydactyly — having extra fingers or toes — is caused by a dominant allele, so one copy is enough.
3Cystic fibrosis — a disorder of cell membranes — is caused by a recessive allele, so a person needs two copies (no dominant allele present) to have it.
4Embryo screening (pre-implantation genetic testing) takes place during IVF: a few cells are taken from each embryo and tested for particular alleles.
5Embryos found not to carry the disorder can be placed in the mother's womb; those that carry it are discarded. In the UK it can be used for disorders including cystic fibrosis.
6Because embryos are tested in a laboratory, a couple must go through IVF to use screening even if they have no difficulty conceiving.
7Issues include: an ethical objection that destroying unchosen embryos is wrong if every embryo may have a right to life; the economic point that NHS funding is available only in some cases; the social point that many people with genetic conditions live fulfilling lives.
8The tests are not perfectly accurate, removing cells can occasionally damage an embryo, and in the UK a condition must meet legal tests of seriousness and risk of being passed on before it can be screened for.
9These questions often have no single correct answer; an informed judgement weighs the points and says which matters most.

Worked example

Problem

Alex has one copy of the polydactyly allele. Ben has one copy of the cystic fibrosis allele and one dominant allele for the same gene. Which of them has the disorder?

⚠ Watch out

Thinking one copy of a disorder allele always means having the disorder. That's only true for a dominant allele, like polydactyly. Cystic fibrosis needs two copies of its recessive allele. And screening doesn't 'fix' embryos — it chooses between them.

🧠

Memory hook

Dominant: one is enough. Recessive: it takes two. Polydactyly needs one; cystic fibrosis needs two.

✓

Check yourself

Without looking: why does one polydactyly allele cause polydactyly, when one cystic fibrosis allele doesn't cause cystic fibrosis? Then list the screening steps in order, and name one economic, social and ethical issue.

Flashcards

(15)
What causes an inherited disorder?
The inheritance of certain alleles.
What is polydactyly, and what type of allele causes it?
Having extra fingers or toes. It's caused by a dominant allele.
What is cystic fibrosis, and what type of allele causes it?
A disorder of cell membranes. It's caused by a recessive allele.
When is a dominant allele expressed?
Always — even if only one copy is present.
When is a recessive allele expressed?
Only when two copies are present, so no dominant allele is present.
What is embryo screening also called, and when does it take place?
Pre-implantation genetic testing. It takes place during IVF.
In embryo screening, what is actually tested — and for what?
A few cells taken from each embryo, tested for particular alleles.
After screening, which embryos go into the mother's womb?
Only embryos found not to carry the disorder. Embryos that carry it are discarded.
Why must a couple use IVF for embryo screening even if they can conceive without help?
Because the embryos are tested in a laboratory.
Which rule limits which conditions can be screened for in the UK?
A regulator must judge that the condition meets legal tests of seriousness and risk of being passed on. It can be used for disorders including cystic fibrosis.
State the ethical objection to embryo screening.
Choosing between embryos and destroying the ones not chosen is wrong, because every embryo may have a right to life.
What is the economic issue with embryo screening?
NHS funding is available only in some cases — depending on a person's situation and where they live, how severe the disorder is and the chance a child would inherit it.
Give the social point about people with genetic conditions.
Plenty live happy, fulfilling lives, and it falls to society to remove the physical and social obstacles they meet.
What are two practical limits of embryo screening?
The tests are not perfectly accurate, and removing cells can occasionally damage an embryo so it can't be used.
Why can two informed people disagree about embryo screening?
Some are reassured it can spare a child a serious disease; others would rather not intervene. What seems right to one person may not seem right to another.

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