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, and to weigh up the arguments around embryo screening. This is part of Topic B5 of your OCR Gateway Combined Science course 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, and discuss embryo screening even-handedly.

This lesson develops AO2 (applying Punnett squares to probability calculations), AO3 (interpreting family-tree data and evaluating the ethics of embryo screening) and AO1 (recall of the two named inherited disorders).

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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 completely changes how the disorder is inherited. Two examples are used again and again.

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. A common misconception is to describe a carrier as "having" cystic fibrosis; they do not. There is also no such thing as a "carrier" of polydactyly, because a dominant allele always shows.

Dominant and recessive disorders side by side

The contrast between a dominant disorder like polydactyly and a recessive disorder like cystic fibrosis is worth setting out clearly, because it decides how each one behaves in a family and how you approach the genetics.

Feature	Dominant disorder (e.g. polydactyly)	Recessive disorder (e.g. cystic fibrosis)
Allele needed to be affected	Just one copy (D)	Two copies (ff)
Genotype of an affected person	DD or Dd	ff only
Genotype of an unaffected person	dd only	FF or Ff
Are there carriers?	No — a dominant allele always shows	Yes — Ff carriers are healthy
How it appears in a family	Usually in every generation	Can skip generations
Two unaffected parents having an affected child?	Not possible	Possible (two carriers)

Reading down this table gives you a quick way to recognise which kind of disorder a question is about, even before you draw a Punnett square. If two unaffected parents have an affected child, it must be recessive; if an affected child always has an affected parent, it is likely dominant.

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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.

Worked Example 3 — Polydactyly (two heterozygous parents)

Both parents have polydactyly and are heterozygous (Dd). (a) What proportion of their children would be expected to have polydactyly? (b) What proportion would be unaffected, and what genotype would the unaffected children have?

Step 1 — parents' genotypes: Dd × Dd (both affected, both carrying one recessive allele).

Step 2 — gametes: each parent passes on D or d.

Step 3 and 4 — the Punnett square:

	D	d
D	DD	Dd
d	Dd	dd

Step 5 — read off the results. The four boxes are DD, Dd, Dd, dd.

• DD and Dd (3 boxes) each carry at least one dominant D allele, so they have polydactyly.
• dd (1 box) carries no dominant allele, so this child is unaffected.

Answer: (a) $\frac{3}{4}$43​ ($75\%$75%) of the children would be expected to have polydactyly; (b) $\frac{1}{4}$41​ ($25\%$25%) would be unaffected, and every unaffected child must be homozygous recessive (dd).

Notice the contrast with a recessive disorder. For dominant polydactyly, the affected children can be either DD or Dd, but the unaffected ones can only be dd — the single genotype with no dominant allele. For recessive cystic fibrosis it is the other way round: the affected children can only be one genotype (ff), while the unaffected ones may be FF or carrier Ff. Getting clear on which "side" has the single fixed genotype is the key to reading these crosses quickly.

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.