DNA, Genetics and Inheritance
DNA and the genome
The instructions for building an organism are stored in DNA. DNA is found in the nucleus of cells, arranged into structures called chromosomes. A gene is a small section of DNA that codes for a particular protein (which controls a characteristic). The genome is an organism's entire set of DNA.
- DNA is a double helix (two twisted strands).
- Humans have 23 pairs of chromosomes (46 in total).
Key genetics terms
| Term | Meaning |
|---|---|
| Gene | Section of DNA coding for a protein |
| Allele | A different version of a gene (e.g. brown vs blue eyes) |
| Dominant | An allele that shows even if only one copy is present (written as a capital, e.g. B) |
| Recessive | An allele that only shows if both copies are present (lower case, e.g. b) |
| Homozygous | Two of the same allele (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
| Genotype | The alleles an organism has (e.g. Bb) |
| Phenotype | The physical characteristic shown (e.g. brown eyes) |
Genetic crosses — Punnett squares
A Punnett square predicts the offspring of two parents. Cross a heterozygous parent (Bb) with another (Bb):
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
- Offspring genotypes: 1 BB : 2 Bb : 1 bb.
- If B (brown) is dominant: 3 brown : 1 blue — a 3:1 ratio.
- Probability of a bb (blue-eyed) child = 1 in 4 (25%).
Inherited disorders
Some conditions are caused by faulty alleles:
- Polydactyly (extra fingers/toes) – caused by a dominant allele.
- Cystic fibrosis (affects cell membranes/mucus) – caused by a recessive allele (need two copies).
Sex determination
Sex is controlled by one pair of chromosomes:
- Female = XX, Male = XY.
- A Punnett square of XX × XY gives a 50:50 chance of male or female.
Worked example
Two parents are both Bb (B = brown eyes, dominant). What is the chance of a blue-eyed child?
- The cross gives 1 BB : 2 Bb : 1 bb. Only bb is blue-eyed → 1 in 4 = 25%. ✓
Common mistakes
- Mixing up genotype (the alleles, e.g. Bb) and phenotype (the visible feature).
- Forgetting a recessive trait needs two recessive alleles to show.
- Writing dominant alleles in lower case — use a capital for dominant.
Exam tips
- Learn the key terms precisely — they're worth easy marks.
- Practise drawing Punnett squares and stating the ratio and probability.
- Remember XX = female, XY = male, and the 50:50 outcome.
Key facts to remember
- Gene = DNA section coding a protein; allele = a version of a gene; genome = all an organism's DNA.
- Dominant shows with one copy; recessive needs two; distinguish genotype vs phenotype.
- A Bb × Bb cross gives a 3:1 phenotype ratio (25% recessive); sex is XX (female) / XY (male).