Genetic Disorders

This lesson covers genetic disorders — conditions caused by mutations in genes that are inherited — as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand cystic fibrosis (recessive), polydactyly (dominant), how to use Punnett squares for these conditions, and the implications of genetic testing.

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What Are Genetic Disorders?

A genetic disorder is a disease or condition caused by a mutation in one or more genes. Genetic disorders can be:

• Inherited — passed from parents to offspring through their alleles.
• Caused by dominant or recessive alleles.

The two genetic disorders you need to know for Edexcel GCSE Combined Science are:

Disorder	Inheritance pattern	Allele
Cystic fibrosis	Autosomal recessive	f (recessive), F (normal dominant)
Polydactyly	Autosomal dominant	D (dominant affected), d (normal recessive)

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Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disorder that affects the lungs, digestive system and other organs. It is caused by a recessive allele.

Key Facts

• Caused by a mutation in the gene coding for the CFTR protein (a chloride channel in cell membranes).
• The faulty protein means that chloride ions cannot be transported properly across cell membranes.
• This leads to thick, sticky mucus building up in the airways, digestive tract and reproductive system.

Symptoms

• Persistent coughing and lung infections (mucus blocks airways and traps bacteria)
• Difficulty breathing
• Problems with digestion (mucus blocks the pancreatic duct, so enzymes cannot reach the small intestine)
• Reduced fertility (especially in males)

Inheritance of Cystic Fibrosis

Since CF is recessive, an individual must inherit two copies of the faulty allele (ff) to be affected.

• FF = unaffected (homozygous dominant)
• Ff = carrier — unaffected but carries one faulty allele
• ff = affected — has cystic fibrosis

Punnett Square: Two Carriers (Ff x Ff)

	F	f
F	FF	Ff
f	Ff	ff

Results:

• 1 in 4 (25%) = FF (unaffected)
• 2 in 4 (50%) = Ff (carrier, unaffected)
• 1 in 4 (25%) = ff (affected with cystic fibrosis)

Exam Tip: For a recessive disorder, both parents must carry the faulty allele for an affected child to be born. Two unaffected carrier parents (Ff x Ff) have a 1 in 4 chance of having an affected child.

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Polydactyly

Polydactyly is a condition where a person is born with extra fingers or toes. It is caused by a dominant allele.

Key Facts

• Only one copy of the dominant allele is needed to cause the condition.
• The condition is relatively common and usually not life-threatening.
• The extra digits can be surgically removed if desired.

Inheritance of Polydactyly

Since polydactyly is dominant, an individual only needs one copy of the allele (D) to be affected.

• DD = affected (homozygous dominant)
• Dd = affected (heterozygous)
• dd = unaffected (normal number of digits)

Punnett Square: Affected Heterozygous x Unaffected (Dd x dd)

	D	d
d	Dd	dd
d	Dd	dd

Results:

• 2 in 4 (50%) = Dd (affected with polydactyly)
• 2 in 4 (50%) = dd (unaffected)

Exam Tip: For a dominant disorder, an affected person can be either DD or Dd. If one parent is Dd and the other is dd, there is a 50% chance that each child will be affected.

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Family Pedigrees

A family pedigree (or pedigree diagram) is a chart that shows how a genetic trait is inherited through a family over several generations.

Standard Symbols

Symbol	Meaning
Square	Male
Circle	Female
Filled shape	Affected individual
Half-filled shape	Carrier (for recessive conditions)
Horizontal line	Mating/partnership
Vertical line	Offspring

graph TD
    A["Generation I: Father (Ff) x Mother (Ff)"] --> B["Generation II"]
    B --> C["Child 1: FF (unaffected)"]
    B --> D["Child 2: Ff (carrier)"]
    B --> E["Child 3: Ff (carrier)"]
    B --> F["Child 4: ff (affected)"]

Interpreting Pedigrees

When interpreting a pedigree for a recessive disorder:

• If two unaffected parents have an affected child, both parents must be carriers.
• If an affected child has the genotype ff, each parent must have at least one f allele.

When interpreting a pedigree for a dominant disorder:

• An affected individual has at least one dominant allele (D).
• If an affected parent has an unaffected child, the affected parent must be heterozygous (Dd).

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Genetic Testing and Screening

Genetic testing involves analysing an individual's DNA to detect whether they carry alleles for a genetic disorder.

Types of Testing

Test	When used
Pre-natal testing	During pregnancy — amniocentesis or chorionic villus sampling
Carrier testing	Before pregnancy — identifies carriers of recessive conditions
Newborn screening	After birth — e.g. heel prick test for conditions like PKU
Pre-implantation genetic diagnosis (PGD)	During IVF — embryos are tested before implantation

Ethical Considerations

Argument for testing	Argument against testing
Parents can prepare for a child with a genetic condition	May lead to difficult decisions about termination
Can allow early treatment to improve quality of life	Risk of miscarriage from invasive procedures
Carriers can make informed family planning decisions	May cause anxiety and emotional distress
PGD can prevent implantation of affected embryos	Concerns about "designer babies" and eugenics

Exam Tip: Questions about genetic testing often ask for arguments for and against. Always give balanced answers with specific points, not vague statements.

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Summary