Key Idea: Topic D3.2 is about how features are passed from parents to offspring — and the skill running through every part of it is the same: read the genotypes, then predict the offspring. You first lock down the vocabulary (gene, allele, genotype, phenotype, dominant, recessive, homozygous, heterozygous). Then you use a Punnett grid to predict ratios for a simple monohybrid cross, before meeting the non-Mendelian twists (codominance, multiple alleles, incomplete dominance), sex determination and sex-linkage, pedigree charts, and finally karyograms, DNA profiling and where variation comes from. This is a heavyweight on Paper 1 (quick ratio / pedigree / DNA-profile MCQs) and Paper 2 (draw-a-Punnett-grid and explain-the-pattern questions).
📖 Genetics vocabulary & alleles (4.8.1)
A gene is a length of DNA coding for a feature; an allele is one version of that gene. The genotype is the pair of alleles an organism carries; the phenotype is the feature you can actually see. A dominant allele shows in the phenotype even with just one copy, so a heterozygote (Bb) looks the same as the dominant homozygote (BB). A recessive allele only shows when both alleles are recessive (bb) — which is exactly why a recessive feature can skip a generation and appear in children of unaffected carrier parents.
| Term | What it means | Quick example |
|---|---|---|
| Gene | A length of DNA coding for one feature | The gene for flower colour |
| Allele | One version of a gene | The 'purple' allele and the 'white' allele |
| Genotype | The alleles an organism carries | Bb |
| Phenotype | The feature you actually see | Purple flowers |
| Dominant | Shows in the phenotype even with one copy | B (purple) over b |
| Recessive | Only shows when both alleles are recessive | bb gives white |
| Homozygous | Two identical alleles | BB or bb |
| Heterozygous | Two different alleles (a carrier) | Bb |
In a heterozygote (Bb) the dominant allele is expressed, so the recessive allele is carried but not shown in the phenotype. It only reappears when two carriers each pass on their recessive allele, giving a bb child. That is why two unaffected parents can have an affected child — a classic exam clue that the condition is recessive.
🟩 Monohybrid crosses & Punnett grids (4.8.2)
A monohybrid cross follows one gene. To predict the offspring you: (1) write the parents' genotypes, (2) work out the gametes each can make, and (3) combine them in a Punnett grid. Reading the grid gives the genotype ratio and, once you apply dominance, the phenotype ratio. The classic result is a cross between two heterozygotes (Bb × Bb), which gives a 3 : 1 phenotype ratio — three dominant to one recessive. A carrier × carrier cross for a recessive disease therefore gives each child a 1 in 4 (25 %) chance of being affected.
A monohybrid cross between two heterozygotes (Bb × Bb). Each parent's gametes (B or b) head the rows and columns; combining them fills the four cells — BB, Bb, Bb, bb. Three cells show the dominant phenotype and one the recessive, giving the classic 3 : 1 phenotype ratio.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Drawing a Punnett grid (always the same five moves)
- Choose clear symbols — a capital for the dominant allele, the same letter in lower case for the recessive (e.g. H and h).
- Write both parent genotypes, e.g. Hh × Hh.
- Work out the gametes each parent can make (Hh → H or h) and head the rows and columns with them.
- Fill every cell by combining the row and column allele.
- Count the offspring and state the genotype ratio, then the phenotype ratio after applying dominance.
A 1 in 4 chance applies independently to each child. If carrier parents already have one affected child, the next child still has a 1 in 4 (25 %) chance — the dice have no memory. Stating '1 in 3 because one is already used up' is a classic lost mark.
🩸 Codominance, multiple alleles & incomplete dominance (4.8.3)
Not every gene follows simple dominant/recessive rules. In incomplete dominance the heterozygote is an in-between blend (red × white snapdragons give pink). In codominance both alleles are fully expressed at once (roan cattle show red and white hairs). A gene can also have multiple alleles — more than two versions in the population, although any one organism still carries only two. The ABO blood group system ties all three ideas together: three alleles (Iᴬ, Iᴮ, i), with Iᴬ and Iᴮ codominant to each other and both dominant over the recessive i.
| Pattern | How the alleles behave | Heterozygote looks like… | Classic example |
|---|---|---|---|
| Complete dominance | Dominant allele masks the recessive | The dominant phenotype only | Bb purple = BB purple |
| Incomplete dominance | Neither allele fully masks the other — they blend | A blend / intermediate | Red × white snapdragons → pink |
| Codominance | Both alleles are fully expressed together | Both phenotypes at once | Roan cattle (red + white hairs); AB blood |
| Multiple alleles | More than two alleles exist for the gene (still 2 per person) | Depends on the pair | ABO blood: alleles Iᴬ, Iᴮ, i |
| Blood group (phenotype) | Possible genotypes | Why |
|---|---|---|
| Group A | Iᴬ Iᴬ or Iᴬ i | Iᴬ is dominant over i |
| Group B | Iᴮ Iᴮ or Iᴮ i | Iᴮ is dominant over i |
| Group AB | Iᴬ Iᴮ | Iᴬ and Iᴮ are codominant — both show |
| Group O | i i | i is recessive — needs two copies |
Incomplete dominance → heterozygote is a blend (pink). A red × white cross giving pink F1, then self-crossed, gives a 1 red : 2 pink : 1 white F2. Codominance → heterozygote shows both phenotypes side by side (red AND white hairs, not pink). Blend = incomplete; both-at-once = codominant.
♀️♂️ Sex determination & sex-linkage (4.8.4)
Human sex is set by the sex chromosomes: females are XX, males are XY. Eggs always carry an X; sperm carry either X or Y, so it is the father who determines a child's sex, with a roughly 50 : 50 split. A sex-linked gene sits on the X chromosome. Because males have only one X (XY), a single recessive allele is enough to show the condition — they have no second X to mask it. That is why X-linked recessive conditions such as red-green colour blindness are far more common in males; females need two copies (one on each X) to be affected and are otherwise unaffected carriers.
| Female | Male | |
|---|---|---|
| Sex chromosomes | XX | XY |
| Gametes (eggs / sperm) | All eggs carry X | Half of sperm carry X, half carry Y |
| Who decides the baby's sex? | — | The father — whether his X or Y sperm fertilises the egg |
| Ratio expected | ≈ 50 % girls | ≈ 50 % boys |
Key Idea: Write the alleles on the X: Xᴮ (normal) and Xᵇ (colour-blind). A male is XᴮY (normal) or XᵇY (colour-blind) — one allele decides it. A female is XᴮXᴮ (normal), XᴮXᵇ (unaffected carrier) or XᵇXᵇ (colour-blind) — she needs the recessive allele on both X chromosomes. Needing two copies is rarer than needing one, so males are affected far more often.
🌳 Pedigree charts (4.8.5)
A pedigree is a family tree for a genetic condition. Squares are males, circles are females, a filled symbol means affected, and horizontal lines join mating pairs with vertical lines dropping to their children. Read a pedigree in two steps: first decide whether the condition is dominant or recessive, then whether it is autosomal or sex-linked — and finally fill in the genotypes you can be sure of. The key giveaway: two unaffected parents with an affected child means the allele must be recessive (both parents are carriers).
How to read a pedigree: squares are males, circles are females, and a filled symbol means affected. A horizontal line joins a mating pair; a vertical line drops to their children. Trace which individuals are affected to deduce the inheritance pattern and work out genotypes.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| What you see in the pedigree | Most likely conclusion |
|---|---|
| Affected children born to two unaffected parents | Recessive (the parents are carriers) |
| Every affected child has at least one affected parent | Dominant |
| Many more affected males than females (often skips a generation) | X-linked recessive |
| Affected fathers passing it to ALL daughters but NO sons | X-linked dominant |
| Roughly equal numbers of affected males and females | Autosomal (not sex-linked) |
To show a dominant allele is not X-linked, look for an affected father with an unaffected daughter. An X-linked dominant father would pass his single X (and the allele) to every daughter, so they would all be affected. One unaffected daughter breaks that rule → the allele is autosomal, not X-linked.
🧬 Karyograms, DNA profiling & variation (4.8.6)
A karyogram arranges a cell's chromosomes in numbered homologous pairs by size and banding. You read off the diploid number (humans: 46, i.e. 23 pairs) and the sex — two large matching X chromosomes means female (XX), one X plus a much smaller Y means male (XY). Comparing karyograms across species shows different chromosome numbers and shapes. A DNA profile compares the lengths of repeating DNA between individuals. Because a child inherits half of its bands from each parent, every band in a child's profile must match a band in one of its parents — the basis of paternity and forensic testing. All of this variety ultimately comes from sexual reproduction, which shuffles alleles in three ways below.
| Source of variation | What it does | When it happens |
|---|---|---|
| Crossing over | Swaps allele blocks between homologous chromosomes | Prophase I of meiosis |
| Independent assortment | Random which chromosome of each pair goes to each gamete | Metaphase I of meiosis |
| Random fertilisation | Any sperm can fuse with any egg | At fertilisation |
| Mutation | Creates brand-new alleles | Any time, at low rate |
Line up the bands. Every band in the child must appear in one of the parents — roughly half come from the mother and half from the father. For a paternity test: the candidate whose bands account for all of the child's non-maternal bands is the biological father. A man missing even one of those bands is ruled out.
✍️ Worked examples
IB-style question — chance of an affected child
Albinism is caused by a recessive allele (a). Two parents with normal pigmentation have a child with albinism. State the parents' genotypes and the probability that their next child also has albinism. [3]
Model answer:
Deduce the parents' genotypes. Both parents have normal pigmentation but produced an albino (aa) child, so each must carry a hidden recessive allele — both parents are Aa (carriers).
Set up the cross. Aa × Aa. Gametes A or a from each parent. The Punnett grid gives 1 AA : 2 Aa : 1 aa.
Read off the probability. Only aa is albino, so the chance is 1 in 4 (25 %) — and this applies to each child independently, so the next child is still 25 %. (Mark 1: both parents Aa. Mark 2: 1/4 or 25 %. Mark 3: independent of previous children / per child.)
Both parents are Aa (carriers); each child has a 1 in 4 (25 %) chance of albinism, independently of any earlier children.
IB-style question — predict the F2 of an incomplete-dominance cross
In a species of flower, red (CR CR) crossed with white (CW CW) gives all pink F1 plants. Predict the phenotype ratio when two pink F1 plants are crossed, and name the inheritance pattern. [3]
Model answer:
Identify the pattern. The heterozygote (CᴿCᵂ) is pink — an in-between blend of red and white — so this is incomplete dominance (neither allele fully masks the other).
Cross the pink F1. CᴿCᵂ × CᴿCᵂ. The Punnett grid gives genotypes 1 CᴿCᴿ : 2 CᴿCᵂ : 1 CᵂCᵂ.
Translate to phenotypes. CᴿCᴿ = red, CᴿCᵂ = pink, CᵂCᵂ = white, so the phenotype ratio is 1 red : 2 pink : 1 white. (Unlike complete dominance, the heterozygote is visibly different, so the ratio is 1:2:1, not 3:1.)
Incomplete dominance; crossing two pink plants gives a 1 red : 2 pink : 1 white phenotype ratio.
IB-style question — deduce inheritance from a pedigree
A pedigree shows a condition in which two unaffected parents have an affected daughter, and affected individuals appear in roughly equal numbers of males and females. Deduce whether the allele is dominant or recessive and whether it is autosomal or X-linked, giving a reason for each. [4]
Model answer:
Dominant or recessive? Two unaffected parents have an affected child, so the affected allele must be recessive — the parents are unaffected carriers and the allele was hidden in them.
Give the reason. A dominant allele would have shown in at least one parent; because neither parent shows it yet the child does, the allele is recessive.
Autosomal or X-linked? Affected males and females appear in roughly equal numbers, which points to autosomal rather than X-linked (X-linked recessive conditions affect many more males).
Clinch it. An affected daughter of an unaffected father is also hard to explain by X-linked recessive (her father would have to be affected), so autosomal recessive fits best. (Marks: recessive + reason; autosomal + reason.)
Autosomal recessive: unaffected parents with an affected child means recessive, and roughly equal affected males and females (plus an affected daughter of an unaffected father) means autosomal, not X-linked.
✅ Quick self-check
Tap each card to check yourself.
Why doesn't a recessive allele always show in the phenotype? In a heterozygote the dominant allele is expressed and masks the recessive one, so the recessive allele is carried but hidden; it only shows when both alleles are recessive (e.g. bb).
Two carriers cross — what is the chance their child is affected? Aa × Aa gives 1 AA : 2 Aa : 1 aa, so a 1 in 4 (25 %) chance of an affected (aa) child — independently for each child, regardless of earlier children.
What is the difference between codominance and incomplete dominance? Incomplete dominance blends the two alleles in the heterozygote (red + white → pink); codominance shows both alleles fully and separately at once (red and white hairs, or AB blood).
Why is red-green colour blindness more common in males? The allele is X-linked recessive. Males (XY) have only one X, so a single recessive allele shows; females (XX) need it on both X chromosomes, which is rarer, so they are usually carriers.
What does an affected child of two unaffected parents tell you? The condition must be recessive — both parents are unaffected carriers and each passed on the recessive allele, giving an affected (homozygous recessive) child.
How do you read a DNA profile in a paternity test? Every band in the child must match a band in one parent; about half come from each. The candidate father whose bands account for all the child's non-maternal bands is the biological father.
Exam Tips
- The master skill of the topic: write the genotypes, work out the gametes, fill a Punnett grid, then read the ratio — keep genotype ratio and phenotype ratio separate.
- Two unaffected parents with an affected child = the condition is RECESSIVE and both parents are carriers. This single clue unlocks most cross and pedigree questions.
- A carrier × carrier cross gives a 1 in 4 (25 %) affected chance for EACH child, independently — never reduce it because an earlier child was affected.
- Heterozygote tells the pattern: looks dominant = complete dominance; a blend = incomplete dominance; both phenotypes at once = codominance.
- ABO blood: three alleles (Iᴬ, Iᴮ, i); Iᴬ and Iᴮ are codominant, both dominant over i. Group O is ii; group AB is IᴬIᴮ.
- Sex is decided by the father (X or Y sperm); X-linked recessive conditions hit males far more because they have only one X to express it.
- Read a pedigree in order: dominant vs recessive first, then autosomal vs X-linked, then fill in the certain genotypes.
- On a karyogram, count the pairs for the diploid number and check the sex chromosomes — two matching X's = female, one X + a small Y = male.
- In a DNA profile, every band in the child must appear in one parent; a candidate father missing any non-maternal band is ruled out.
- Always define symbols before a Punnett grid (capital = dominant, lower case = recessive) — examiners award a mark for clear, correct allele symbols.