The big idea: A monohybrid cross follows one gene with two versions (alleles) from the parents to their offspring.
It works because of three simple facts about sexual reproduction:
Each parent carries two alleles for the gene (one on each chromosome of a pair).
Each gamete (egg or sperm) gets only one of those two alleles — they segregate during meiosis.
At fertilisation a random egg meets a random sperm, so the offspring get one allele from each parent. A Punnett grid lets you predict every possible pairing and its chance.
- Gene
- A length of DNA that codes for one characteristic (e.g. seed colour).
- Allele
- One version of a gene (e.g. B for one form, b for another). Diploid organisms carry two alleles per gene.
- Genotype
- The pair of alleles an organism has for a gene — e.g. BB, Bb or bb.
- Phenotype
- The observable characteristic the genotype produces — e.g. the actual colour you see.
- Dominant allele
- An allele (written as a CAPITAL, e.g. B) that shows in the phenotype even when only one copy is present.
- Recessive allele
- An allele (written lower-case, e.g. b) that shows in the phenotype only when BOTH alleles are recessive (bb).
- Homozygous
- Two identical alleles for a gene (BB or bb).
- Heterozygous
- Two different alleles for a gene (Bb) — often called a carrier when b is a disease allele.
- Gamete
- A sex cell (egg or sperm) that carries only ONE allele of each gene.
- Punnett grid
- A square diagram that combines the parents' gametes to predict the offspring genotypes and their ratios.
A monohybrid cross between two heterozygous parents (Bb × Bb). Each parent makes B and b gametes; combining them across the grid gives BB, Bb, Bb, bb — a 3 : 1 dominant : recessive ratio.
Interactive diagram
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Capital vs lower-case is the whole code: Use the same letter for both alleles of a gene: CAPITAL = dominant, lower-case = recessive.
So B and b are the two alleles of one gene — never mix unrelated letters. A heterozygote is Bb, and because B is dominant it looks the same as BB.
A Punnett grid is just an organised way to pair every gamete from one parent with every gamete from the other.
Follow the same five steps every time and you can predict any monohybrid cross.
The five steps
- Write the genotypes of both parents (e.g. Bb × Bb).
- List the gametes each parent can make — split the genotype into single alleles (B and b).
- Draw the grid: one parent's gametes across the top, the other's down the side.
- Fill each cell by combining the letter at the top of its column with the letter at the side of its row.
- Count and group: write the genotype ratio, then group genotypes by phenotype to get the phenotype ratio.
| Step | What you do | Why |
|---|---|---|
| 1. Genotypes | Write each parent's two alleles (e.g. Bb and Bb) | A diploid organism has two alleles for each gene |
| 2. Gametes | Split each genotype into single-allele gametes (B and b) | Meiosis puts ONE allele of each gene into each gamete (segregation) |
| 3. Grid | Put one parent's gametes on top, the other's down the side | Every column-row meeting is a possible fertilisation |
| 4. Fill cells | Combine the heading letters in each cell (BB, Bb, Bb, bb) | Random fertilisation recombines the alleles |
| 5. Ratio | Count genotypes, then group by phenotype (3 dominant : 1 recessive) | The dominant allele masks the recessive in the phenotype |
How to build it: write one parent's gametes (B, b) along the top and the other's (B, b) down the side, then fill each cell with the two letters that meet there. Four cells = the four equally likely combinations.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
The classic 3 : 1 result: For Bb × Bb the four cells are BB, Bb, Bb, bb.
Genotype ratio = 1 BB : 2 Bb : 1 bb.
Phenotype ratio = 3 dominant : 1 recessive (because BB and Bb both show the dominant phenotype, and only bb shows the recessive one).
Memorise this 3 : 1 — two heterozygous parents is the most common cross the exam sets.
Turning a ratio into a probability: Each of the four cells is equally likely, so a count out of 4 is a probability.
1 cell in 4 = a 1/4 (25%) chance. 3 cells in 4 = a 3/4 (75%) chance.
So from Bb × Bb the chance a child is homozygous recessive (bb) is 1/4 — and that is the same whether it is the first child or the fourth, because each fertilisation is independent. A family that already has an affected child still has a 1/4 chance next time.
| From the Bb × Bb grid | Count | As a probability |
|---|---|---|
| Homozygous dominant (BB) | 1 of 4 cells | 1/4 = 25% |
| Heterozygous (Bb) | 2 of 4 cells | 2/4 = 1/2 = 50% |
| Homozygous recessive (bb) | 1 of 4 cells | 1/4 = 25% |
| Dominant phenotype (BB or Bb) | 3 of 4 cells | 3/4 = 75% |
| Recessive phenotype (bb only) | 1 of 4 cells | 1/4 = 25% |
Probability does not have a memory: The grid gives the chance for each pregnancy separately. Past children do not change the next one's odds — two carriers face a 1/4 chance of an affected child every time, not 'one in four children'.
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How this is tested: On Paper 2 you are often told to draw / construct a Punnett grid for a couple — frequently using given symbols (e.g. H and h). Show the parent genotypes, the gametes, the four filled cells, and the ratio; carrier-disease versions ask how two unaffected parents can have an affected child.
On Paper 1 a 1-mark item gives two carrier parents and asks the probability a child inherits a recessive disease (cystic fibrosis, sickle cell…). The answer is read straight off the grid: 1/4 (25%).
Always use the symbols the question gives you and label the grid clearly — unlabelled cells lose marks.
IB-style question — construct a grid for two carrier parents
A recessive allele causes a metabolic disorder. Two parents are both unaffected carriers. Using the symbols D (healthy) and d (disorder), construct a Punnett grid for their children, state the genotype and phenotype ratios, and give the probability that any one child is affected. [4]
How to score all four marks
- Parent genotypes. Both parents are unaffected carriers, so each is heterozygous: Dd × Dd. (An affected person would be dd; an unaffected non-carrier would be DD.)
- Gametes. Each parent makes two kinds of gamete — D and d — because the two alleles segregate into separate gametes.
- Fill the grid. Combining D/d across the top with D/d down the side gives the four cells DD, Dd, Dd, dd. Genotype ratio = 1 DD : 2 Dd : 1 dd.
- Phenotypes and probability. DD and Dd are unaffected (D is dominant); only dd is affected. Phenotype ratio = 3 unaffected : 1 affected, so the chance any one child is affected is 1/4 (25%). (Marks: correct parent genotypes; correct gametes/grid; correct genotype ratio; correct affected probability.)
Final answer
Parents Dd × Dd → grid cells DD, Dd, Dd, dd → genotype ratio 1 DD : 2 Dd : 1 dd → phenotype ratio 3 unaffected : 1 affected → probability affected = 1/4 (25%).
Disease version: read B as the healthy allele and b as a recessive disease allele. Both parents are unaffected carriers (Bb). The single bb cell is the affected child — a 1 in 4 (25%) chance each pregnancy.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
✓ Why this scores full marks: It does the four separate things the command asks: names the parent genotypes, shows the gametes/grid, states the genotype and phenotype ratios, and gives the probability as a clear fraction.
It also uses the symbols the question supplied (D and d) — inventing your own letters or leaving cells unlabelled throws away easy marks.