Inherited Disorders and Family Trees

Some characteristics passed from parents to offspring are not harmless differences like eye colour but inherited disorders — conditions caused by faulty alleles. In this lesson you will meet two classic single-gene disorders, polydactyly (caused by a dominant allele) and cystic fibrosis (caused by a recessive allele), and use the Punnett-square technique from the previous lesson to work out the chance that a child is affected or is a carrier. You will also learn to read family (pedigree) trees, see how the sex of a baby is determined, and weigh up the arguments around embryo screening. This is part of Topic B5 of OCR Gateway Science A and builds directly on monohybrid inheritance.

By the end of this lesson you should be able to describe how polydactyly and cystic fibrosis are inherited, use Punnett squares to find the probability of an affected or carrier child, interpret family trees, explain sex determination, and discuss embryo screening even-handedly.

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Single-Gene Inherited Disorders

An inherited disorder is a disease caused by faulty alleles passed from parents to offspring. Some are caused by a dominant allele and some by a recessive allele, and this difference changes completely how the disorder is inherited. OCR focuses on two examples.

Polydactyly — caused by a dominant allele

Polydactyly is a condition in which a person is born with extra fingers or toes. It is caused by a dominant allele (write it D). Because the allele is dominant, a person needs only one copy to have the condition. This means:

• An affected person can be homozygous dominant (DD) or, more usually, heterozygous (Dd).
• An unaffected person must be homozygous recessive (dd) — they carry no dominant allele.
• Because it is dominant, polydactyly usually appears in every generation of an affected family — an affected child almost always has an affected parent.

Cystic fibrosis — caused by a recessive allele

Cystic fibrosis is a disorder of cell membranes that causes thick, sticky mucus to build up in the lungs and digestive system, making breathing and digestion difficult. It is caused by a recessive allele (write it f). Because the allele is recessive, a person must inherit two copies — one from each parent — to have the condition. This means:

• An affected person is homozygous recessive (ff).
• A person with the genotype Ff is a carrier: they have one faulty allele but, because it is recessive, they do not have the disorder themselves — yet they can pass the faulty allele to their children.
• The disorder can skip generations: two healthy carrier parents (both Ff) can have an affected (ff) child even though neither parent has cystic fibrosis.

Exam Tip: The word carrier only applies to recessive disorders. A carrier is heterozygous (Ff) — they carry one faulty recessive allele but do not show the condition. There is no such thing as a "carrier" of polydactyly, because a dominant allele always shows.

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Working Out the Risk with Punnett Squares

The real power of the Punnett square is predicting the probability that a child inherits a disorder. The method is exactly as before.

Worked Example 1 — Cystic fibrosis (two carriers)

Both parents are carriers of cystic fibrosis (Ff). What is the probability that a child has cystic fibrosis?

Step 1 — parents' genotypes: Ff × Ff.

Step 2 — gametes: each parent passes on F or f.

Step 3 and 4 — the Punnett square:

	F	f
F	FF	Ff
f	Ff	ff

Step 5 — read off the results:

• FF (1 box) — unaffected, not a carrier.
• Ff (2 boxes) — unaffected carriers.
• ff (1 box) — has cystic fibrosis.

Answer: the probability a child has cystic fibrosis (ff) is $\frac{1}{4} = 25\%$41​=25%. The probability a child is a carrier (Ff) is $\frac{2}{4} = \frac{1}{2} = 50\%$42​=21​=50%, and the probability a child is completely unaffected and not a carrier (FF) is $\frac{1}{4} = 25\%$41​=25%.

Common error: forgetting that the Ff children are healthy. Only the ff child has the disorder; the two Ff children are carriers but do not have cystic fibrosis.

Worked Example 2 — Polydactyly (one affected heterozygous parent)

One parent is heterozygous for polydactyly (Dd) and the other is unaffected (dd). What is the probability that a child has polydactyly?

Step 1 — parents' genotypes: Dd × dd.

Step 2 — gametes: the Dd parent passes on D or d; the dd parent can only pass on d.

Step 3 and 4 — the Punnett square:

	D	d
d	Dd	dd
d	Dd	dd

Step 5 — read off the results. Dd (2 boxes) has polydactyly; dd (2 boxes) is unaffected.

Answer: the probability a child has polydactyly is $\frac{2}{4} = \frac{1}{2} = 50\%$42​=21​=50%. Each child has a 1 in 2 chance of being affected — which fits the way a dominant disorder tends to appear in every generation.

Exam Tip: Read carefully whether the disorder is dominant or recessive before you start, and choose your letters to match (D dominant for polydactyly; f recessive for cystic fibrosis). The whole calculation depends on getting this right.

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Reading Family (Pedigree) Trees

A family tree (or pedigree) is a diagram that shows how a characteristic is inherited through several generations of a family. You need to be able to read one and work out genotypes and probabilities. The usual conventions are:

• Circles represent females; squares represent males.
• A shaded symbol means the person has the characteristic (is affected); an unshaded symbol means they do not.
• A horizontal line between two people shows they are parents; the vertical lines below lead to their children.

To analyse a family tree, work through it logically:

1. Decide if the allele is dominant or recessive. If two unaffected parents have an affected child, the allele must be recessive (the parents are carriers). If every affected child has an affected parent, it is likely dominant.
2. Work out genotypes where you can. For a recessive disorder, every affected person is homozygous recessive. Unaffected parents of an affected child must both be carriers (heterozygous).
3. Use a Punnett square for any couple to predict the chance their next child is affected.

In the tree above, two unaffected grandparents (Generation 1) have an affected daughter (the shaded circle in Generation 2), which tells you the allele is recessive and that both grandparents are carriers. The affected daughter, with an unaffected partner, then has an affected son in Generation 3.

Exam Tip: The single most useful deduction in a pedigree question is this: two unaffected parents with an affected child means the allele is recessive and both parents are carriers. Spotting that one relationship unlocks most family-tree questions.

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Sex Determination