Reproduction and Inheritance · 6 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 17% of your exam marks.
Genetic crosses, Punnett squares, and dominant/recessive allele questions appear on almost every paper.
Inheritance is a topic where the right words matter. Most exam questions on genetics rely on you knowing what a gene, an allele, a genotype and a phenotype are, and being able to use these terms precisely.
| Term | What it means |
|---|---|
| DNA | The chemical that carries all the genetic information of an organism. Stands for deoxyribonucleic acid |
| Chromosome | A long, tightly-coiled molecule of DNA found in the nucleus. Humans have 23 pairs in every body cell |
| Gene | A short section of DNA on a chromosome. Each gene codes for one specific protein |
| Allele | One of the different versions of the same gene. A gene might have two or more alleles |
| Genotype | The combination of alleles an individual has for a particular gene (e.g. BB, Bb, bb) |
| Phenotype | The observable characteristic that results from a particular genotype (e.g. brown eyes, blood group A) |
| Dominant | An allele whose effect is seen in the phenotype even when only one copy is present. Written as a capital letter (e.g. B) |
| Recessive | An allele whose effect is only seen in the phenotype when both copies are present. Written as a lowercase letter (e.g. b) |
The two alleles you carry for a given gene can be the same or different:
If you are heterozygous and one allele is dominant, the dominant version is the one you show in your phenotype. The recessive allele is still there in your genotype, but it is masked by the dominant one.
An example: the PTC tasting gene. Some people can taste a bitter chemical called phenylthiocarbamide (PTC, found naturally in some vegetables like sprouts); others can't taste it at all. The ability to taste PTC is controlled by one gene with two alleles:
| Genotype | Possible alleles | Phenotype |
|---|---|---|
| TT | T from each parent | Taster (homozygous dominant) |
| Tt | T from one parent, t from the other | Taster (heterozygous; T masks t) |
| tt | t from each parent | Non-taster (homozygous recessive) |
Note that two different genotypes (TT and Tt) produce the same phenotype (taster). The only way to be a non-taster is to have two copies of the recessive allele.
Define: recessive allele / state what is meant by homozygous or heterozygous
What comes up: the exam regularly asks you to state the meaning of recessive allele, homozygous, or heterozygous for one mark each.
Write (one mark each): - Recessive allele: an allele that is only expressed in the phenotype when two copies are present (i.e. in a homozygote); it is not expressed when a dominant allele is also present.
Watch out: do not define "recessive" simply as "the weaker allele" or "always hidden" — the mark scheme credits the idea of requiring two copies (homozygous) to be expressed. The word "masked" alone without explaining the two-copy requirement is not enough for the mark.