Inherited Disorders

This lesson covers the genetic basis of inherited disorders — conditions caused by faulty alleles that are passed from parents to their children. For AQA GCSE Biology, you need to know about polydactyly (a dominant disorder) and cystic fibrosis (a recessive disorder). You also need to understand genetic screening and the ethical debates surrounding it.

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What Are Inherited Disorders?

Inherited disorders (also called genetic disorders) are caused by mutations in genes that are passed from parent to offspring. They are not caused by pathogens and cannot be "caught" — they are inherited through the alleles received from parents.

Some inherited disorders are caused by dominant alleles, meaning only one copy of the faulty allele is needed to cause the condition. Others are caused by recessive alleles, meaning two copies of the faulty allele are needed.

Inheritance pattern	Number of faulty alleles needed	Example
Dominant disorder	One copy (heterozygous or homozygous)	Polydactyly
Recessive disorder	Two copies (homozygous recessive only)	Cystic fibrosis

Exam Tip: Remember: dominant disorders need only ONE faulty allele; recessive disorders need TWO. This is the opposite of what many students expect — the "worse-sounding" recessive conditions actually require two copies.

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Polydactyly — A Dominant Disorder

Polydactyly is a condition where a person is born with extra fingers or toes. It is caused by a dominant allele, so only one copy of the allele is needed to produce the condition.

Key facts:

• Caused by a dominant allele (we can call it D)
• A person with genotype DD or Dd will have polydactyly
• Only a person with genotype dd will have the normal number of digits
• If one parent has polydactyly (Dd) and the other is unaffected (dd):

	D	d
d	Dd	dd
d	Dd	dd

Result: 50% chance of polydactyly (Dd), 50% chance of normal digits (dd)

• Polydactyly can often be treated surgically by removing the extra digits
• It is relatively harmless and does not usually affect quality of life

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Cystic Fibrosis — A Recessive Disorder

Cystic fibrosis (CF) is a serious genetic condition that affects the lungs, digestive system, and other organs. It is caused by a recessive allele, so a person must inherit two copies of the faulty allele to have the condition.

Key facts:

• Caused by a recessive allele (we can call it f)
• The faulty gene codes for a protein called CFTR that controls the movement of salt and water across cell membranes
• When the CFTR protein is faulty, thick, sticky mucus builds up in the lungs and digestive system

Symptoms of cystic fibrosis:

System affected	Symptoms
Lungs	Thick mucus blocks airways, making breathing difficult; frequent chest infections
Digestive system	Mucus blocks the pancreatic duct, preventing digestive enzymes from reaching the small intestine; poor digestion and nutrient absorption
Reproductive system	Can cause infertility, particularly in males
Sweat glands	Excessive salt in sweat

Inheritance of Cystic Fibrosis

Using F for the normal allele and f for the CF allele:

Genotype	Phenotype
FF	Unaffected (does not carry the allele)
Ff	Carrier — unaffected but carries one copy of the recessive allele
ff	Has cystic fibrosis

A carrier is someone who has one copy of the recessive allele but does not show symptoms. They can pass the allele to their children.

Cross: Two carriers (Ff x Ff)

	F	f
F	FF	Ff
f	Ff	ff

Result:

• 25% (1 in 4) chance of being unaffected (FF)
• 50% (2 in 4) chance of being a carrier (Ff)
• 25% (1 in 4) chance of having cystic fibrosis (ff)

graph TD
    A[Father - Carrier Ff] --> C[Possible offspring]
    B[Mother - Carrier Ff] --> C
    C --> D[FF - Unaffected 25%]
    C --> E[Ff - Carrier 50%]
    C --> F[ff - Cystic fibrosis 25%]

Exam Tip: Cystic fibrosis is the most commonly examined recessive disorder. Always remember that two unaffected parents can have a child with CF if both parents are carriers (Ff). This is a very common exam scenario.

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Comparing Polydactyly and Cystic Fibrosis

Feature	Polydactyly	Cystic fibrosis
Allele type	Dominant	Recessive
Genotype for condition	DD or Dd	ff
Can carriers exist?	No (having the allele = having the condition)	Yes (Ff = carrier)
Severity	Usually mild; treatable by surgery	Serious; life-shortening
Frequency	Relatively common	About 1 in 2,500 births in the UK
Treatment	Surgical removal of extra digits	Physiotherapy, medication, lung transplants

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Genetic Testing and Screening

Genetic testing involves analysing a person's DNA to check for the presence of alleles linked to genetic disorders. This can be done:

• Before birth (prenatal testing) — using amniocentesis or chorionic villus sampling (CVS)
• After birth — through blood tests or cheek swabs
• During IVF — through pre-implantation genetic diagnosis (PGD), where embryos are tested before being implanted

Embryo Screening

In pre-implantation genetic diagnosis (PGD), embryos created by IVF are tested for genetic disorders. Only embryos without the disorder are implanted into the mother's uterus.

Exam Tip: Ethical questions about embryo screening often appear as 6-mark extended response questions. Practise writing balanced arguments with points for and against, and finish with a justified conclusion.

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Ethical Issues of Genetic Testing

This is a topic where AQA expects you to discuss ethical, social, and economic arguments.

Arguments FOR genetic testing and screening: