You carry two copies of almost every gene — one from each parent. That single fact decides whether a condition appears in every generation of a family, or skips silently through people who never know they carry it.
A gene is a heritable factor that controls a specific characteristic. A chromosome is a body of condensed DNA carrying hundreds of those genes, lined up at fixed positions.
An allele is a specific form of a gene. The gene sits at the same locus on both chromosomes of a pair — but the two copies need not be identical. Each version is an allele, and each can have a different effect on the characteristic.
By convention we write the dominant allele as a capital letter and the recessive allele as the same letter in lower case — B and b.
The genotype is the combination of alleles an organism carries for a gene. The phenotype is the physical characteristic that results — what you actually observe.
Two identical alleles make you homozygous — homozygous dominant (BB) or homozygous recessive (bb). Two different alleles make you heterozygous (Bb).
A dominant allele shows its effect even with one copy. A recessive allele only shows when no dominant allele is present — so its phenotype needs two recessive copies.
This is why a heterozygote can be a carrier: Bb looks identical to BB on the outside, yet quietly carries one copy of the recessive allele to pass on.
Gregor Mendel could not see genes — he inferred them from counting peas across generations. His law of segregation states that the two alleles of a gene separate during the formation of gametes, so each gamete receives only one.
A Bb parent therefore makes two kinds of gamete in equal proportion: half carry B, half carry b. Which allele a given offspring inherits is a matter of chance.
Mendel's first cross is named the F1 generation — first filial, from the Latin filius / filia, son and daughter. Crossing the F1 with itself gives the F2.
Nature of science: Mendel built an abstract model — invisible "factors" obeying simple ratios — decades before anyone saw a chromosome. The model earned its place by predicting results, not by being observed directly.
Tap any allele to flip it between the dominant A and recessive a. The grid and the ratios update live. A Punnett grid is just an organised way of pairing every gamete from one parent with every gamete from the other.
Teal = dominant allele. Coral = recessive allele. Try a carrier × carrier cross (Aa × Aa), then make one parent homozygous recessive (aa) and watch the recessive phenotype jump.
| A | a | |
| A | AA | Aa |
| a | Aa | aa |
With both parents heterozygous, three in four offspring show the dominant phenotype and one in four shows the recessive — the classic 3 : 1 ratio Mendel counted in his peas.
Cystic fibrosis is autosomal recessive. A couple come to a genetic counsellor: neither has the condition, but both are carriers — each genotype is Ff, where f is the cystic fibrosis allele.
What is the chance that any child of theirs has cystic fibrosis?
25% — one in four. Ff × Ff gives offspring in the ratio 1 FF : 2 Ff : 1 ff. Only the ff quarter is affected; the two Ff children are unaffected carriers, just like their parents.
| F | f | |
| F | FF | Ff |
| f | Ff | ff |
This is the signature of a recessive condition: it can appear in a child whose parents both seem perfectly healthy, because a carrier shows no symptoms.
IB exam tip: A question asking you to determine this probability wants the worked Punnett grid and the fraction. A question asking you to state it only wants "25% / 1 in 4" — but the grid is what earns the marks.
Cystic fibrosis is an autosomal recessive disorder caused by a mutation in the CFTR gene on chromosome 7. Affected individuals produce unusually thick, sticky mucus that clogs the airways and the ducts of the digestive system.
A heterozygous carrier has one working CFTR allele — enough to avoid symptoms. The disorder only appears in the homozygous recessive genotype, which needs a recessive allele from both parents.
PKU (phenylketonuria) follows the same recessive logic. A mutation on chromosome 12 disables the enzyme phenylalanine hydroxylase, so phenylalanine builds up. Two recessive alleles are needed to be affected.
Because PKU is detectable and manageable, most newborns are screened within a day or two of birth — untreated, high phenylalanine can damage the developing brain, but a controlled low-phenylalanine diet prevents it.
Huntington's disease is autosomal dominant: a single copy of the allele H is enough to cause it. One parent is heterozygous and affected (Hh); the other is unaffected (hh).
What chance does each child have of inheriting Huntington's disease?
50% — one in two. Hh × hh gives 1 Hh : 1 hh. Half the children inherit the H allele, and because it is dominant, every one of them will develop the disease.
| h | h | |
| H | Hh | Hh |
| h | hh | hh |
No carriers exist for a dominant condition: if you carry the allele, you show the phenotype. That is why an affected parent appears in the line of every affected child.
IB exam tip: An explain question here wants both the mechanism (one H allele is sufficient) and the consequence (no unaffected carriers, condition appears each generation). A state question only wants the 1 in 2.
Huntington's disease is an autosomal dominant disorder caused by a mutation in the HTT gene on chromosome 4. The gene contains a repeating three-base sequence, CAG, normally present in low numbers.
When the repeat count grows past roughly forty, the huntingtin protein misfolds and causes progressive neurodegeneration — the molecular reason a single faulty allele is enough to cause disease.
Symptoms — uncontrolled spasmodic movements (chorea) and dementia — usually emerge in middle age, often after a person has already had children. This late onset is part of why the allele persists in populations.
Compare the two patterns: a recessive allele can pass invisibly through carriers for generations, while a dominant allele declares itself in everyone who carries it — so it leaves an unbroken trail down a pedigree.
A mutation is simply a change to the genetic code. New alleles — the different versions of a gene that inheritance shuffles — can only arise because mutations occur.
A mutation is not inherently good or bad. Whether a new allele spreads, persists, or disappears depends on the selective pressures acting on it. Over time, this sorting of variation by selection is what we call evolution.
So the CFTR and HTT alleles are not anomalies — they are part of the same variation that natural selection works on. Inheritance, mutation and evolution are one connected story, viewed at different timescales.
You carry two alleles per gene; gametes carry one. From those two facts, every cross and every pedigree follows.
Needs two copies to show. Travels silently through unaffected carriers, so it can appear from two healthy parents — cystic fibrosis, PKU.
State = "needs two recessive alleles". Explain = carriers (heterozygous) show no symptoms → allele passes unseen.
One copy is enough. No carriers exist, so it surfaces in every generation with an affected parent — Huntington's disease.
State = "one allele is sufficient". Explain = no unaffected carriers → unbroken trail down a pedigree.
The two alleles separate into different gametes at random. A Punnett grid pairs the gametes to give the offspring ratios.
State = "alleles separate during gamete formation". Explain = each gamete gets one allele → predictable ratios.
Attempt each one on paper first, then reveal the mark scheme. Notice how the command term tells you how much depth the marks expect.
Cystic fibrosis is an autosomal recessive condition. A couple, neither of whom has cystic fibrosis, have a child who is born with the condition. Use F for the dominant allele and f for the recessive allele.
STATE = give the specific answer, no explanation needed. Award the mark only if both parents are correct.
DETERMINE = obtain the answer by showing the working. Award 1 per point, max 3.
Accept: probability expressed as a fraction, decimal or percentage. Do not accept: 25% without an ff genotype shown in the grid.
EXPLAIN = give reasons / mechanism — both what and why. Award any 2.
Accept: "carrier" for heterozygous. Do not accept: vague "they don't have the gene" (they do carry the allele).
Huntington's disease is an autosomal dominant condition; cystic fibrosis is an autosomal recessive condition.
COMPARE AND CONTRAST = give both similarities and differences, referring to both throughout. Award 1 per valid point, max 8. (Nine points listed — award any 8.)
Accept: correct allele symbols defined by the candidate. Do not accept: claims that males and females are affected differently (these are autosomal, not sex-linked).