The big idea: Genetics has its own vocabulary, and every inheritance question uses it.
Get these few words right and the crosses, the Punnett grids and the pedigrees all become easy.
Start with the chain: a gene has different versions called alleles; the alleles you carry are your genotype; the feature that genotype produces is your phenotype.
- Gene
- A length of DNA that codes for one characteristic (for example, the gene for stem length in a pea plant).
- Allele
- One particular version of a gene. A gene can have two or more alleles (for example, a 'tall' allele and a 'short' allele).
- Genotype
- The alleles an organism carries for a gene — written as a pair of letters, such as Tt.
- Phenotype
- The observable characteristic an organism shows, produced by its genotype (and sometimes the environment) — such as 'tall'.
Genotype vs phenotype — never mix them up: Genotype = the letters (the instructions you carry), e.g. Tt.
Phenotype = the feature (what you actually see), e.g. tall.
A memory hook: genotype = the type of alleles; phenotype = what you see (think 'phenomenon' — something observed).
| Genotype | Phenotype | |
|---|---|---|
| What it describes | The alleles you carry (the instructions) | The feature you can observe (the result) |
| How it is written | A pair of letters, e.g. Tt | A description, e.g. 'tall' |
| Can you see it directly? | No — you must read the alleles / do a cross | Yes — you can measure or look at it |
| Example | TT, Tt or tt | Tall plant or short plant |
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An organism has two alleles for most genes — one inherited from each parent.
How those two alleles combine, and which one 'wins', is described by four more words: dominant, recessive, homozygous and heterozygous.
- Dominant allele
- An allele whose effect shows in the phenotype even if only one copy is present. Written with a CAPITAL letter (e.g. T).
- Recessive allele
- An allele whose effect shows in the phenotype only when two copies are present. Written with a small letter (e.g. t).
- Homozygous
- Having two of the SAME allele for a gene (e.g. TT or tt). 'Homo' = same.
- Heterozygous
- Having two DIFFERENT alleles for a gene (e.g. Tt). 'Hetero' = different.
Why a recessive allele can be 'hidden': Imagine a pea plant where T = tall (dominant) and t = short (recessive).
A heterozygous plant has the genotype Tt — it carries the short allele t, but you would never know by looking, because it is tall.
That is because the dominant T masks the recessive t: one copy of T is enough to make the plant tall.
The plant is only short if its genotype is tt — homozygous recessive, with no dominant allele to mask the recessive one. This is exactly the idea the exam asks you to explain.
| Genotype | Name | Phenotype | Why |
|---|---|---|---|
| TT | Homozygous dominant | Tall | Two dominant alleles — definitely tall |
| Tt | Heterozygous | Tall | The dominant T masks the recessive t |
| tt | Homozygous recessive | Short | No dominant allele present, so the recessive allele is shown |
Homozygous
- Two of the same allele
- TT (homozygous dominant) → tall
- tt (homozygous recessive) → short
- 'Homo' = same
Heterozygous
- Two different alleles
- Tt → tall (the dominant T is shown)
- Carries the recessive t, but hides it
- 'Hetero' = different
Notation rules: Same letter, two cases: the dominant and recessive alleles of one gene share a letter — capital for dominant (T), small for recessive (t).
Never mix letters (do not write Tg for one gene) and never write the recessive with a different letter from its dominant partner.
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How this is tested: On Paper 2 a 1-mark Define item asks you to define genotype (the alleles an organism carries for a gene) — a free mark if you have the word right.
A 3-mark Explain then asks why a recessive allele is not always shown in the phenotype, using a worked example like pea stem length. Score it by naming the heterozygous genotype, saying the dominant allele masks the recessive one, and noting the recessive phenotype only appears in a homozygous-recessive organism.
IB-style question — recessive alleles in the phenotype
In pea plants, the allele for tall stems (T) is dominant to the allele for short stems (t). Explain why a plant can carry the short-stem allele yet still be tall. [3]
How to score all three marks
- Name the genotype. A plant that carries the short allele but is tall is heterozygous — its genotype is Tt (one tall allele, one short allele).
- Say what dominance does. The tall allele T is dominant, so its effect is shown in the phenotype even though only one copy is present — it masks the recessive short allele.
- State when the recessive phenotype appears. The short phenotype only shows when the plant is homozygous recessive (tt) — i.e. it has no dominant allele to mask the recessive one. (Award 1 mark per distinct point, up to 3.)
Final answer
The plant is heterozygous (Tt). The dominant T is expressed even with one copy, so it masks the recessive t and the plant is tall. The short phenotype only appears in a homozygous-recessive (tt) plant with no dominant allele present.
Why this scores full marks: Three separate points: (1) the genotype is heterozygous (Tt); (2) the dominant allele is expressed/masks the recessive one; (3) the recessive phenotype needs two recessive alleles (tt).
Writing 'the tall one wins' three different ways would score one mark, not three — use the proper terms.
| Term | What it means | Quick example (stem length in pea plants) |
|---|---|---|
| Gene | A length of DNA that codes for one characteristic | The gene for stem length |
| Allele | One particular version of a gene | T = tall version; t = short version |
| Genotype | The alleles an organism carries for a gene | TT, Tt or tt |
| Phenotype | The observable characteristic produced | A tall plant or a short plant |
| Dominant | An allele whose effect shows even with one copy | T (tall) — shown by a capital letter |
| Recessive | An allele whose effect shows only with two copies | t (short) — shown by a small letter |
| Homozygous | Two of the SAME allele for a gene | TT (homozygous dominant) or tt (homozygous recessive) |
| Heterozygous | Two DIFFERENT alleles for a gene | Tt — one of each |