Inherited Disorders and Sex Determination

Some disorders are passed from parents to children through their genes. These inherited disorders are caused by inheriting particular alleles — versions of a gene — rather than by infection, diet, or lifestyle.

The allele behind a disorder behaves like any other allele. A dominant allele shows its effect even when only one copy is present. A recessive allele shows its effect only when two copies are present, so a person needs to inherit the recessive allele from both parents.

An allele is a version of a gene. A dominant allele is expressed with one copy; a recessive allele is expressed only with two copies.

This single distinction — dominant or recessive — controls how a disorder moves through a family and how likely a child is to inherit it. The AQA specification names two disorders you must know: polydactyly, caused by a dominant allele, and cystic fibrosis, caused by a recessive allele. The rest of this lesson works through both, then shows how the same allele rules decide a baby's sex.

Polydactyly means having extra fingers or toes. It is caused by a dominant allele.

Because the allele is dominant, a person needs only one copy to have the condition. Using D for the dominant polydactyly allele and d for the normal recessive allele, anyone with at least one D allele has polydactyly:

Genotype	Alleles	Phenotype
DD	Two dominant	Polydactyly
Dd	One of each	Polydactyly
dd	Two recessive	Normal number of digits

The practical consequence: a person with polydactyly almost always has a parent with it, because a single dominant allele is enough to pass it on. An affected parent who is Dd has a 1 in 2 (50%) chance of passing the D allele to each child.

Dominant disorder = only one faulty allele needed. It tends to appear in every generation of an affected family.

Polydactyly is usually not life-threatening, and the extra digit is often removed surgically after birth.

Cystic fibrosis is a disorder of cell membranes. It is caused by a recessive allele, so a person must inherit two copies — one from each parent — to have the condition.

The faulty allele disrupts the proteins that move salt and water across cell membranes. This produces thick, sticky mucus in the lungs and digestive system, causing breathing problems and difficulty absorbing food.

Using F for the normal dominant allele and f for the recessive cystic fibrosis allele:

Genotype	Alleles	Phenotype
FF	Two dominant	Unaffected
Ff	One of each	Unaffected carrier
ff	Two recessive	Has cystic fibrosis

A person who is Ff is a carrier: they do not have the disorder, but they carry the faulty allele and can pass it on. Two healthy carriers can have a child with cystic fibrosis — which is exactly what the next slide works through.

A carrier has one recessive allele (Ff). They are unaffected themselves but can pass the allele to their children. Carriers explain how a recessive disorder can "skip" generations.

Two parents are both unaffected carriers of cystic fibrosis, so both have the genotype Ff. What is the chance their child has cystic fibrosis?

(Higher Tier only — constructing a genetic cross is Higher Tier. Foundation students must be able to complete and interpret a Punnett square that is partly given, and read off the ratios.)

Each parent passes one allele to the child. Cross Ff × Ff:

The four equally likely outcomes are:

Offspring genotype	Count	Phenotype
FF	1	Unaffected
Ff	2	Unaffected carrier
ff	1	Has cystic fibrosis

Reading off the square:

• 1 in 4 (25%) chance the child has cystic fibrosis (ff).
• 3 in 4 (75%) chance the child is unaffected (FF or Ff).
• The ratio of unaffected to affected is 3 : 1.

Check: 1 + 2 + 1 = 4 outcomes; the affected fraction is $\frac{1}{4}$. These are probabilities for each child, not guarantees — every child independently faces the same 1 in 4 chance.

Two technologies can address inherited disorders, and AQA asks you to weigh the issues they raise.

Embryo screening checks the genes of an embryo for faulty alleles. During IVF (fertilisation outside the body), cells are taken from each early embryo and tested; embryos free of the disorder can then be chosen for implantation. Doctors can also test cells during pregnancy.

Gene therapy aims to treat a disorder by inserting working alleles into a patient's cells — for example, delivering a healthy allele to lung cells in cystic fibrosis.

(Extra context — not required by AQA 8461.) Embryo screening before IVF implantation is formally called pre-implantation genetic diagnosis (PGD).

The specification asks you to make informed judgements about the economic, social and ethical issues of embryo screening. Present both sides:

Arguments for embryo screening	Arguments against embryo screening
Reduces suffering by avoiding serious inherited disorders	There are concerns it could lead to selecting other traits ("designer babies")
May lower the long-term cost of treating the disorder	The screening process itself is expensive
Lets parents make an informed choice	Some people feel it is wrong to destroy embryos
The disorder allele could become rarer over generations	Implies people with the disorder are less valuable, which many find offensive

There is no single "correct" answer — a good exam response gives points on both sides and reaches a reasoned judgement.

Ordinary human body cells contain 23 pairs of chromosomes — 46 in total.

22 of these pairs control general characteristics only. The remaining pair is the sex chromosomes, which carry the genes that determine whether a person is male or female:

Sex	Sex chromosomes	Description
Female	XX	The two are the same
Male	XY	The two are different

A useful memory hook: females are XX (same), males are XY (different).

Body cells have 23 pairs of chromosomes. 22 pairs control characteristics; 1 pair is the sex chromosomes — XX in females, XY in males.

When gametes (egg and sperm) form, the pair separates so each gamete carries just one sex chromosome. Every egg carries an X. A sperm carries either an X or a Y — and that is what decides the baby's sex, as the next slide shows.

Cross a mother (XX) with a father (XY) to predict the sex of their children.

(Higher Tier only — constructing the cross is Higher Tier. Foundation students complete and read off a Punnett square that is partly provided.)

The mother can only pass an X. The father passes either an X or a Y:

The four equally likely outcomes:

Offspring	Count	Sex
XX	2	Female
XY	2	Male

Reading off the square:

• 2 out of 4 offspring are XX (female) = 50%.
• 2 out of 4 offspring are XY (male) = 50%.
• The ratio of female to male is 1 : 1, or roughly 50 : 50.

The father's sperm decides the sex, because the egg always supplies an X — it is whether the fertilising sperm carries an X or a Y that sets the outcome. The 50:50 split is a probability: in any real family the actual numbers vary by chance, just as flipping a coin many times gives roughly, but not exactly, half heads.

1. Swapping which disorder is dominant and which is recessive

Polydactyly is dominant (one allele is enough); cystic fibrosis is recessive (two alleles needed). Mixing these up is the single most common error on this topic. Anchor it: extra fingers stand out, like a dominant allele showing with one copy.

2. Forgetting that carriers are unaffected

A cystic fibrosis carrier (Ff) has no symptoms. They are not "slightly affected" — the recessive allele needs two copies to show, so a single copy produces a healthy carrier who can still pass it on.

3. Treating probabilities as guarantees

A 1 in 4 chance of cystic fibrosis does not mean exactly one in every four children is affected. Each child independently faces the same 1 in 4 chance; a family of four could have none affected or more than one.

4. Saying the mother determines the baby's sex

The egg always carries an X. It is the sperm (X or Y) that decides the sex of the child.

5. Confusing the number of chromosomes and pairs

Human body cells have 46 chromosomes, arranged as 23 pairs. Of these, 22 pairs are non-sex chromosomes and 1 pair is the sex chromosomes.

6. Giving a one-sided answer on embryo screening

Judgement questions reward balance. A response that lists only benefits, or only objections, is unlikely to gain full marks — give economic, social and ethical points on both sides, then state a reasoned conclusion.