6.1.5 Inherited Disorders · 6. Inheritance, Variation and Evolution | Aqa GCSE Combined Science Trilogy 8464: Biology

Section 1 of 9Inherited disorders

Some disorders are caused by faulty alleles that are passed from parents to children. Many are controlled by a single gene and follow the inheritance patterns of dominant and recessive alleles (6.1.6). Two examples are polydactyly and cystic fibrosis.

Section 2 of 9Polydactyly

Polydactyly is a condition in which a person is born with extra fingers or toes.

Dominant

It is caused by a dominant allele (D).

A person with the genotype DD or Dd has the condition, while only dd has a normal number of digits.

It is not usually life-threatening, and the extra digit may be removed by surgery.

If one parent is Dd and the other is dd, each child has a 50 percent chance of having the condition, as the cross gives Dd, Dd, dd, dd.

Because the allele is dominant, a child can only have polydactyly if at least one parent has it, apart from rare new mutations.

Section 3 of 9Cystic fibrosis

Cystic fibrosis is a serious inherited disorder of the cell membranes that results in the production of thick, sticky mucus in the lungs and digestive system.

It causes breathing difficulties and chest infections, and problems with digesting food.

Recessive

It is caused by a recessive allele (f), so a person must have two copies (ff) to have the disease.

Carriers

People with one copy (Ff) are carriers: they do not have the disorder but can pass the allele on.

Cross of two carriers, Ff × Ff:

	F	f
F	FF	Ff
f	Ff	ff

There is a 25 percent (1 in 4) chance of a child with cystic fibrosis, a 50 percent chance of a carrier child and a 25 percent chance of an unaffected non-carrier. Treatment includes physiotherapy to clear mucus, antibiotics, enzyme tablets and, in some cases, lung transplants. There is currently no cure, although gene therapy is being researched.

Section 4 of 9Embryo screening

Embryos produced by IVF (5.3.3) can be tested for faulty alleles before being placed in the uterus. A cell is removed from the embryo at an early stage and its DNA is analysed. Only embryos free of the disorder are implanted. Foetal tests during pregnancy, such as amniocentesis, can also check for abnormalities.

For screening	Against screening
Reduces the number of people suffering from serious disorders	Embryos that are not selected are destroyed, which some people believe is wrong
Parents who carry the allele can have healthy children	It is expensive and the process carries some risk
Reduces the cost to the health service of treatment	It may lead to screening for other features such as sex or appearance, the 'designer baby' problem
	No screening can guarantee a healthy baby, and may imply that people with the disorder are less valued

Section 5 of 9Links and exam technique

For inheritance questions, always define your letters, show the parents' genotypes and gametes and complete a Punnett square. Probability should be expressed as a fraction, percentage or ratio, as asked. Remember that a recessive disorder can appear in a family where neither parent shows it.

The topic links to genetic crosses (6.1.6), DNA and mutation (6.1.4), and the ethics of reproductive technology. Other inherited conditions, such as sickle cell disease, are also caused by recessive alleles, and having a single copy of the sickle cell allele protects against malaria.

Section 6 of 9Genetic counselling and testing

Families with a history of inherited disorders may be offered genetic counselling.

1

A counsellor draws a family tree, explains how the condition is inherited and estimates the chance that a future child will be affected.

2

Carrier testing can detect whether a person has one copy of a recessive allele, and in some populations is offered widely.

3

Prenatal tests during pregnancy include amniocentesis, in which a sample of the fluid around the foetus is taken to obtain cells for DNA analysis; this carries a small risk of miscarriage.

4

Newborn babies are routinely tested for a few disorders so treatment can begin early.

Information from a test is not always simple to interpret, and people may decide not to be tested. Some conditions, such as Huntington's disease, are caused by a dominant allele that shows up in adulthood; knowing the result in advance can cause distress. Doctors must respect confidentiality, and there are concerns that employers or insurers might misuse results.

In the exam, give both medical benefits and personal or social concerns, and use the ideas of probability and carrier status in your explanation.

Section 7 of 9Exam-style questions

Cystic fibrosis inheritance: carrier parents Punnett square

Punnett Square - Cystic Fibrosis Inheritance (Ff x Ff)

Both parents are carriers of cystic fibrosis. Calculate the probability that their child has the disorder. Use a Punnett square. [4]4 marks
Polydactyly is caused by a dominant allele. A man with polydactyly (Dd) has children with a woman with a normal number of digits. State the probability that a child has polydactyly. [2]2 marks
Evaluate the screening of embryos for cystic fibrosis. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Inherited disorder	A condition caused by a faulty allele passed from parents.
Polydactyly	Extra fingers or toes caused by a dominant allele.
Cystic fibrosis	A recessive disorder causing thick mucus in the lungs and gut.
Carrier	A person with one copy of a recessive allele who does not show the disorder.
Embryo screening	Testing embryos for faulty alleles before implantation.
Recessive allele	An allele that is expressed only when two copies are present.
Dominant allele	An allele expressed when only one copy is present.

Section 9 of 9Exam technique

1

Saying cystic fibrosis is dominant.

2

Saying carriers have the disease.

3

Saying that a disorder always skips generations.

4

Confusing percentage chance with the ratio.

5

Saying screening removes the faulty gene; it only selects embryos.