This page is vocabulary, and it is the page most students skim. That is a mistake, because examiners mark genetics answers on precision. Using “gene” when you meant “allele” can cost a mark on a question you actually understood.
📘 What you need to know
A gene is a length of DNA on a chromosome that codes for a particular characteristic, by coding for a specific protein.
An allele is a variation of the same gene. Different alleles of a gene sit at the same locus.
A locus is the position a gene occupies on a chromosome.
The genotype is the combination of alleles an individual inherits; the phenotype is the observable characteristic.
Homozygous means two identical alleles at a locus; heterozygous means two different alleles.
A dominant allele is expressed if inherited from just one parent; a recessive allele is only expressed if inherited from both.
In codominance both alleles are fully expressed; in incomplete dominance they blend into an intermediate phenotype.
Phenotype can be set by genotype alone, by the environment alone, or by an interaction of both.
Gene, allele and locus
These three get muddled constantly, so pin them down with a physical picture. A gene is a stretch of DNA in a fixed position. That position is the locus. The particular version of the gene sitting there is the allele.
Because you have two copies of every chromosome, you have two copies of every gene, and therefore two alleles of each gene — one inherited from your mother and one from your father. They may be identical or they may differ.
Every locus on this pair is either homozygous or heterozygous, independently of the others. An organism is not “a heterozygote” in general — only at a named locus.
The single most common lost mark in this whole topic: writing “the gene for blue eyes”. There is no gene for blue eyes. There is a gene for eye colour, and one of its alleles codes for blue. Say allele whenever you mean a version.
Genotype and phenotype
The genotype is what an organism carries: the combination of alleles it inherited. The phenotype is what you can observe — either by looking, like eye colour, or by testing, like blood group.
The two are not the same thing, and the phenotype is not always a straight readout of the genotype. Characteristics fall into three groups.
Determined by
What that means
Examples
Genotype only
The alleles inherited fix the characteristic; the environment cannot alter it
Blood group
Environment only
Nothing to do with alleles; produced by surroundings, diet, exposure or experience
Scars, accent
Interaction of both
Alleles set a range, and environmental factors determine where within it the individual falls
A dominant allele only needs to be inherited from one parent for its characteristic to appear in the phenotype. A recessive allele must be inherited from both parents to appear. If only one recessive allele is present it stays hidden and the dominant characteristic shows instead.
That gives three possible genotypes at a locus with two alleles: homozygous dominant (two dominant alleles), homozygous recessive (two recessive alleles), and heterozygous (one of each). Homo means same; hetero means different.
A precise way to say it. A recessive allele is not “weaker”. It usually codes for a non-functional protein, so one working copy of the dominant allele is enough to produce the normal phenotype. Explaining dominance in terms of protein function is a genuinely stronger answer than describing one allele as stronger than the other.
When the heterozygote breaks the rules
Complete dominance is not the only option. In some genes the heterozygote does not look like either parent, and the pattern it shows tells you which type of inheritance you are dealing with.
Codominance and incomplete dominance are easy to confuse because both give a heterozygote that looks like neither parent. The test is whether the two colours are visible separately or mixed.
Codominance
Codominant alleles have a combined effect, and both are expressed to an equal extent. Feather colour in chickens is the standard example. Using C for the colour gene and superscript letters for the alleles, a chicken with genotype CWCW is white and one with CBCB is black. The heterozygote CWCB is speckled — you can see both colours, separately, in the same bird.
Incomplete dominance
Here both alleles are only partially expressed, so the heterozygote shows a blend. In the marvel of Peru, CWCW plants have white flowers and CRCR plants have red ones. The heterozygote CWCR is pink — neither parent colour, but something in between.
Notation matters. When alleles are codominant, neither is “lower case”, so you cannot use the capital and lower case convention. Instead use one capital letter for the gene and superscripts for the alleles. Writing them any other way makes the answer ambiguous, which examiners cannot reward.
Worked examples
WORKED EXAMPLE
Distinguish between the terms gene and allele, and between genotype and phenotype. [4]
“Distinguish” wants matched contrasts, not four separate definitionsGene and allelea gene is a length of DNA coding for a characteristic; an allele is one particular version of that geneboth alleles of a gene occupy the same locusGenotype and phenotypegenotype is the combination of alleles inherited; phenotype is the observable characteristic that resultsphenotype can also be affected by the environment, genotype cannot4 marks: one for each half of each contrast
WORKED EXAMPLE
Two snapdragon plants with pink flowers are crossed. The offspring are red, pink and white. Deduce the type of inheritance and give the expected phenotype ratio. [3]
The pink parents are the cluepink appears in both parents and offspring, and is not present in the red or white formspink must be the heterozygote, showing a blend, so this is incomplete dominanceCross the two heterozygotesthe four combinations give 1 red : 2 pink : 1 whiteIncomplete dominance, ratio 1 red : 2 pink : 1 whiteunder incomplete dominance the genotype and phenotype ratios are the same, because every genotype looks different
💡 Exam tips
Never use gene and allele interchangeably. This is explicitly flagged by examiners and it is easy marks lost.
Say “heterozygous at this locus” rather than “heterozygous”, if the question involves more than one gene.
For codominance and incomplete dominance, use the capital letter plus superscript notation.
Under incomplete dominance the phenotype ratio equals the genotype ratio, because each genotype has its own appearance. Under complete dominance they differ.
Common names and scientific names are both acceptable for named examples, so “marvel of Peru” scores as well as Mirabilis jalapa.
⚠️ Common mix-ups
Confusing codominance with incomplete dominance. Codominance shows both phenotypes at once; incomplete dominance shows a blend.
Describing a recessive allele as “weak”. Say instead that it produces a non-functional protein and is masked when a dominant allele is present.
Saying an organism “has the dominant gene”. It has a dominant allele. Both individuals have the same gene.
Assuming dominant means common. Frequency in a population has nothing to do with dominance.
Forgetting the environment. Many phenotypes are not determined by genotype alone, and questions on height or skin colour want both mentioned.
Writing genotypes inconsistently. Pick your letters, define them, and use the same ones all the way through.
Up next: Inheriting Alleles — how a recessive condition like PKU passes silently between generations, and why one gene can have far more than two alleles.
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