The vocabulary examiners insist on
Genetics marks live in precise definitions. Learn these exactly:
- Gene: a length of DNA that codes for a protein / a unit of inheritance.
- Allele: a different/alternative version (form) of a gene.
- Genotype: the genetic makeup / the alleles present (e.g. Bb).
- Phenotype: the observable features/characteristics (e.g. brown eyes).
- Dominant: an allele expressed even if only one copy is present.
- Recessive: an allele only expressed when two copies are present (homozygous).
- Homozygous: two identical alleles (BB or bb); heterozygous: two different alleles (Bb).
The most common confusion is genotype versus phenotype. Genotype is the letters, phenotype is the appearance. When a question asks for the genotype, give the letters; when it asks for the phenotype, describe the feature. Define a chromosome as a thread of DNA made up of genes.
Drawing a monohybrid cross: the layout that scores
Follow this exact structure every time, because each line is a mark:
- Define your symbols/key: e.g. 'Let B = allele for brown eyes (dominant), b = allele for blue eyes (recessive).'
- Write the parental genotypes and phenotypes.
- Show the gametes (circle them). Each gamete carries one allele.
- Draw the Punnett square and fill in the offspring genotypes.
- State the offspring genotype and phenotype ratio.
For Bb × Bb:
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
This gives genotype 1 BB : 2 Bb : 1 bb and phenotype 3 brown : 1 blue. Always define the key: an undefined symbol can cost the first mark even if the square is perfect. Show every step so method marks survive a slip.
Ratios, probabilities and predicted outcomes
Examiners test whether you understand that a genetic ratio is a probability, not a guarantee. A 3:1 ratio means each offspring has a 3 in 4 (75%) chance of the dominant phenotype, not that exactly three of every four will show it. If asked for the chance of a particular outcome, express it as a fraction, ratio or percentage and state it clearly.
A test cross determines whether an organism showing the dominant phenotype is homozygous (BB) or heterozygous (Bb): cross it with a homozygous recessive (bb). If any offspring show the recessive phenotype, the unknown parent must be heterozygous. The exam mark is explaining the reasoning, not just the cross. When sample sizes are small, actual results may not match the predicted ratio exactly. Examiners reward the answer 'fertilisation is random / small sample size' for this. Punnett squares connect to meiosis, which produces genetically varied gametes.
Mitosis, meiosis and cell division (Extended)
Distinguish the two divisions carefully. Mitosis produces two genetically identical daughter cells with the same number of chromosomes as the parent (diploid); it is used for growth, repair and asexual reproduction. Meiosis (Extended only) produces four genetically different cells with half the chromosome number (haploid gametes), and is a reduction division.
The key marks: mitosis = identical, same chromosome number; meiosis = varied, halved chromosome number. (Extended only) Meiosis is the source of genetic variation in gametes, and fertilisation restores the full chromosome number. Students often say meiosis 'makes identical cells' (that is mitosis). Keep them separate. A diploid cell has chromosomes in pairs; a haploid gamete has one of each. This underpins why offspring vary, linking to variation and natural selection.
Co-dominance and sex linkage (Extended)
(Extended only) Co-dominance is when both alleles in a heterozygote are expressed equally: neither is recessive. The classic example is the ABO blood group, where IA and IB are co-dominant (giving group AB), and both are dominant to IO. When writing co-dominant crosses, use capital-letter alleles with superscripts rather than upper/lower case.
Sex determination: females are XX, males are XY; a cross shows a 1:1 ratio of male to female offspring. Sex linkage refers to genes carried on the X chromosome; because males have only one X, a recessive allele (e.g. for red-green colour blindness or haemophilia) is expressed in males more often. The exam skill is showing the carrier mother (XHXh) cross and explaining why sons are more likely affected. State the alleles as superscripts on the X chromosome. To practise these crosses with a specialist, book a free trial class.