Genetic Inheritance and Punnett Squares
Inheritance Basics
Inheritance is the passing of genetic information from parents to offspring. Understanding how characteristics are inherited requires knowing some key terms.
Key Terminology
| Term | Definition |
|---|---|
| Gene | A short section of DNA that codes for a specific protein/characteristic |
| Allele | A version of a gene — there can be two or more alleles for each gene |
| Dominant | An allele that is always expressed, even if only one copy is present (represented by a capital letter, e.g. B) |
| Recessive | An allele that is only expressed when two copies are present (represented by a lowercase letter, e.g. b) |
| Homozygous | Having two copies of the same allele (e.g. BB or bb) |
| Heterozygous | Having two different alleles (e.g. Bb) |
| Genotype | The combination of alleles an organism has (e.g. BB, Bb, or bb) |
| Phenotype | The physical characteristic that is expressed (e.g. brown eyes) |
Punnett Squares
A Punnett square is a diagram used to predict the possible genotypes and phenotypes of offspring from a genetic cross.
Example: Two heterozygous parents for brown (B) vs blue (b) eyes:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
Offspring genotypes: 1 BB : 2 Bb : 1 bb
Offspring phenotypes: 3 brown eyes : 1 blue eyes
Ratio: 3:1
Worked Example — Cystic Fibrosis
Cystic fibrosis is caused by a recessive allele (f). A person must be homozygous recessive (ff) to have the disease. Carriers (Ff) carry one copy but are unaffected.
Cross: Two carriers (Ff × Ff)
| F | f | |
|---|---|---|
| F | FF | Ff |
| f | Ff | ff |
- Probability of child having CF: 1 in 4 (25%)
- Probability of child being a carrier: 2 in 4 (50%)
- Probability of child being unaffected non-carrier: 1 in 4 (25%)
Genetic Disorders
Polydactyly (extra fingers or toes):
- Caused by a dominant allele (D)
- Only one copy of the allele needed (Dd or DD)
- Can be inherited from just one affected parent
Cystic fibrosis:
- Caused by a recessive allele (f)
- Must have two copies (ff) to be affected
- Both parents must carry the allele (both at least Ff)
- Causes thick, sticky mucus in the lungs and digestive system
Sex Determination
In humans, sex is determined by the 23rd pair of chromosomes — the sex chromosomes:
- Females have XX
- Males have XY
Punnett square for sex:
| X | X | |
|---|---|---|
| X | XX | XX |
| Y | XY | XY |
There is a 50% chance of each sex — a 1:1 ratio.
The Y chromosome carries the SRY gene, which triggers male development.
Sex-Linked Inheritance
Some genes are carried on the X chromosome — these are sex-linked. Because males have only one X chromosome, they are more likely to express recessive sex-linked conditions.
Example — Colour blindness:
- The allele for colour vision is on the X chromosome
- Normal vision (X^N) is dominant; colour blindness (X^n) is recessive
- A female needs two recessive alleles (X^n X^n) to be colour blind
- A male needs only one (X^n Y) — he has no second X to mask it
- This is why colour blindness is more common in males
Carrier female (X^N X^n) × normal male (X^N Y):
| X^N | Y | |
|---|---|---|
| X^N | X^N X^N | X^N Y |
| X^n | X^N X^n | X^n Y |
- Daughters: 50% normal, 50% carrier (none colour blind)
- Sons: 50% normal, 50% colour blind
Probability and Ratios
When interpreting Punnett squares:
- Results show probability, not certainty — having a 1 in 4 chance does not mean exactly 1 out of 4 children will be affected
- Express as ratios (3:1), fractions (1/4), or percentages (25%)
- Larger sample sizes give results closer to the predicted ratio
Exam Tips
- Always use a Punnett square when asked to work out genetic crosses — show your working
- Use capital letters for dominant and lowercase for recessive — and make sure they look different (e.g. B and b, not C and c which can look similar)
- State the genotype AND phenotype of offspring in your answer
- Sex-linked conditions are more common in males — explain why (only one X chromosome)