Lesson 6.05 · Spec 4.6.1.7–4.6.1.8 · Paper 2
Inherited Disorders & Sex Determination
In June 2024 Paper 2 Question 6.4, students were asked to explain how cystic fibrosis is inherited from two carrier parents. Only 15% scored full marks — the lowest success rate of any genetics question that series. The mark scheme required four specific points. Most students gave one or two. In the next 10 minutes, you'll know exactly what those four points are.
By the End of This Lesson…
You will be able to:
Explain how sex is determined by sex chromosomes (XX vs XY) and carry out a sex-determination Punnett square
Describe the two named inherited disorders — polydactyly (dominant) and cystic fibrosis (recessive) — and explain how each is inherited
Discuss the economic, social and ethical issues surrounding embryo screening, with balanced pros and cons
Spec Refs
4.6.1.7 — Inherited disorders (polydactyly, cystic fibrosis, embryo screening)
4.6.1.8 — Sex determination (XX / XY, genetic cross)
Paper 2 · Higher Tier · ⭐ Priority topic
Sex Chromosomes
Human body cells contain 23 pairs of chromosomes (46 total). 22 pairs control characteristics only — they do not determine sex. The 23rd pair carries the genes that determine sex.
♀ Female
Two X chromosomes — the same.
All eggs contain one X.
♂ Male
One X, one Y — different.
Sperm contain either X or Y (50/50).
Key Fact
The father determines the sex of the offspring — the mother always provides an X chromosome. If a Y-sperm fertilises the egg → XY → male. If an X-sperm fertilises the egg → XX → female. Probability = 50/50 each time.
Sex Determination: Punnett Square
Cross: XX (mother) × XY (father) — this is the Jun 2023 P2 Q3.5 cross. Over 75% of students got this right. Make sure you're in that group.
Parent gametesX (egg)X (egg)
X (sperm)XX FemaleXX Female
Y (sperm)XY MaleXY Male
Offspring ratio
2 XX : 2 XY → 1:1 female:male — always. The probability of any individual child being male or female is 50% (0.5).
Examiner trap — Jun 2023
25% of students wrote female gametes as X and Y — wrong. Females are XX; they only produce X gametes. Males (XY) produce X or Y. Never switch these.
The Two Named Inherited Disorders
The spec names exactly two disorders. Memorise both, and critically — memorise which is dominant and which is recessive.
Polydactyly
Dominant allele
Having extra fingers or toes
Only ONE copy of dominant allele (D) needed
Can be inherited from just one parent
Genotype: DD or Dd → affected
Cystic Fibrosis (CF)
Recessive allele
Disorder of cell membranes — thick, sticky mucus in lungs and digestive system
TWO copies of recessive allele (f) needed to show the disorder
Each parent must contribute at least one recessive allele (f)
Genotype: ff only → affected; Ff → carrier (no symptoms)
Memory tip
Dominant = Dactyly (polydactyly) · recessive = CF — needs two copies to show. If you mix these up, every Punnett is wrong.
Polydactyly: Dominant Disorder
The most commonly examined cross is a heterozygous affected parent (Dd) × unaffected parent (dd). This gives a 1:1 ratio — which students frequently confuse with the 3:1 ratio from a carrier cross. Work through it carefully.
Dd (affected) × dd (unaffected)dd
DDd PolydactylyDd Polydactyly
ddd Unaffecteddd Unaffected
Ratio
2 Dd : 2 dd → 1:1 affected:unaffected. Probability of an affected child = 0.5 (also expressed as 50%, 1/2, or "1 in 2" — all are accepted; match the form requested in the question).
Critical
Dominant disorders give 1:1 ratio (Dd × dd), NOT 3:1. The 3:1 ratio is for two heterozygous parents where the trait is recessive. Confusing these costs marks every series.
Cystic Fibrosis: Recessive Disorder
Definition
Cystic fibrosis — a disorder of cell membranes caused by a recessive allele. Causes thick, sticky mucus in the lungs and digestive system, leading to difficulty breathing and digesting food.
Because CF is recessive, a person can carry the allele without showing symptoms — they are a carrier.
GenotypeStatusHas CF?Can pass on allele?
FFHomozygous dominantNoNo
FfCarrier (heterozygous)NoYes ← key
ffHomozygous recessiveYesYes
Definition
Carrier — a person who is heterozygous for a recessive disorder. They have one copy of the recessive allele but show no symptoms. They can pass the allele to their children.
CF Punnett: Two Carriers
The most commonly examined CF cross is two carrier parents (Ff × Ff). This is also the hardest-marked CF question type — it requires four distinct points: identifying both parents as carriers, showing gametes correctly, completing the Punnett, and stating the probability of an ff offspring.
Ff (carrier) × Ff (carrier)Ff
FFF UnaffectedFf Carrier
fFf Carrierff Has CF
25%
Chance of CF (ff)
50%
Chance of carrier (Ff)
25%
Unaffected non-carrier (FF)
Genotype ratio
1 FF : 2 Ff : 1 ff → Phenotype ratio: 3 unaffected : 1 affected. But remember — 2 of those 3 unaffected children are carriers.
Expressing probability — all forms accepted
The probability of an ff child = 1 in 4. AQA mark schemes accept any equivalent form: 1/4 · 0.25 · 25% · 1 in 4 · ratio 1:3. Always match the form asked for in the question. Do not write "25 out of 100" — use the simplest form.
Embryo Screening
Couples at risk of passing on an inherited disorder can use embryo screening before pregnancy.
1
IVF
Eggs fertilised by sperm in the lab
2
Cell removal
A single cell is taken from each embryo
3
DNA testing
Cell's DNA is tested for the disorder allele
4
Implantation
Only disease-free embryos are implanted
Spec wording (memorise)
The spec requires students to make informed judgements about the economic, social and ethical issues concerning embryo screening. This means evaluation questions (Pattern 13) — always give pros AND cons plus a balanced conclusion.
Embryo Screening: Evaluate
Any evaluate question on embryo screening requires both sides. One-sided answers are capped at roughly half marks. Here is the full argument.
✓ For (Pros)
[Ethical] Reduces suffering of children born with serious genetic conditions
[Economic] Reduces long-term healthcare costs of treating the disorder over the patient's lifetime
[Social] Allows high-risk couples to have biological children without passing on the disorder
[Economic] Avoids the cost and burden of caring for a child with a serious lifelong condition
✗ Against (Cons)
[Ethical] Destruction of "non-suitable" embryos raises moral concerns about the status of embryos
[Social] Could lead to "designer babies" — selection for non-medical traits beyond disease prevention
[Social] Could increase prejudice against people living with genetic disorders
[Economic] Expensive procedure — creates inequality; not equally available to all who need it
[Ethical] Embryos cannot give consent to being tested or discarded
Examiner alert
For evaluation questions, always end with a balanced conclusion — e.g. "Overall, embryo screening may be justified when the risk of serious disease is high, provided access is equitable — but the ethical concerns about embryo destruction must be weighed carefully."
Worked Example
Polydactyly inheritance cross4 marks
A woman has polydactyly. Her genotype is heterozygous. Her partner does not have polydactyly. Use the letters D (polydactyly allele) and d (normal allele) to explain the probability that their child will have polydactyly.
1
Woman's genotype: Dd (heterozygous, has polydactyly). Her gametes: D and d. Partner's genotype: dd (unaffected). His gametes: d and d.
2
Punnett square (Dd × dd): offspring genotypes = Dd, Dd, dd, dd.
3
Offspring with Dd = polydactyly (dominant allele present). Offspring with dd = unaffected.
4
2 out of 4 offspring are Dd → probability of polydactyly = 2/4 = 1/2 = 0.5 = 50% = 1 in 2. (All forms are accepted — match the form the question asks for.)
Always fully annotate your Punnett square: label gametes on both axes, fill all four cells, and explicitly state which offspring genotypes show the condition. Missing any one of these steps costs a mark. Also annotate the probability in the form requested — ratio, fraction, decimal or "1 in N".
Examiner Alerts
Alert 1 — Jun 2024 Q6.4: allele vs gene confusion
The examiner report noted "clear confusion between the terms allele and gene" throughout Q6.4. A gene is a section of DNA that codes for a protein. An allele is a version of that gene. CF is caused by a specific allele — not by "having the gene".
Alert 2 — Jun 2024 Q6.2: reading family trees (30% success)
Only 30% of students could explain why Person 1 is heterozygous from the family tree. Strategy: if a parent shows a dominant trait but has at least one unaffected child, the parent must be heterozygous (Dd) — a homozygous dominant parent (DD) would never produce an unaffected child from an unaffected partner.
Alert 3 — All series: wrong allele letters for CF
Some students wrote CF parents as XX and XY — those are sex chromosomes, not CF allele notation. CF uses F and f (or any agreed letter). The sex chromosome notation is a completely separate system.
Common Errors
✗ Wrong
"Female gametes are X and Y"
✓ Right
"Female gametes are both X; male gametes are X or Y"
✗ Wrong
"Polydactyly is recessive"
✓ Right
"Polydactyly is dominant; cystic fibrosis is recessive"
✗ Wrong
"Dominant disorder Punnett gives 3:1 ratio"
✓ Right
"Dd × dd gives 1:1; Ff × Ff gives 3:1 (phenotype)"
✗ Wrong
"Embryo screening cures cystic fibrosis"
✓ Right
"Embryo screening prevents implantation of affected embryos — it does not treat or cure the disorder"
Recap: Three Takeaways
1
Sex determination: XX = female; XY = male. Father determines sex (provides X or Y). Mother always provides X. Sex-determination Punnett always gives a 1:1 ratio. Females produce only X gametes — never X and Y.
2
Inherited disorders: Polydactyly = dominant (Dd × dd → 1:1 affected:unaffected). Cystic fibrosis = recessive (Ff × Ff → 3:1 unaffected:affected, but 1 in 4 has CF). Carriers (Ff) show no symptoms but can pass the allele on.
3
Embryo screening: IVF → remove cell → test DNA → implant disease-free embryo. For evaluation questions give both pros and cons + a balanced conclusion. One-sided answers are capped at roughly half marks.
Checkpoint
You should now be able to:
✓ State that females are XX and males are XY, and explain how sex is determined at fertilisation
✓ Complete a sex-determination Punnett square and state the 1:1 ratio
✓ Define polydactyly (dominant, extra fingers/toes) and cystic fibrosis (recessive, cell membrane disorder)
✓ Complete Punnett squares for dominant (1:1) and recessive (3:1) disorders, including carrier crosses
✓ Define carrier and explain why carriers don't show symptoms of a recessive disorder
✓ Describe the embryo screening process and evaluate it with pros, cons, and a balanced conclusion
Next lesson
Lesson 6.06 — Variation & Evolution: genetic causes (mutations + sexual reproduction), environmental causes, continuous vs discontinuous variation, and natural selection.
Next: By the End of This Lesson…