Genetics questions are mostly short. That means the marker is checking one thing: did you use the right word? Get these terms straight and half the topic looks after itself.
📚 What you need to know
A gene is a length of DNA coding for one polypeptide; its fixed position on the chromosome is its locus.
Alleles are different versions of the same gene, sitting at the same locus on homologous chromosomes.
Genotype = the alleles you carry. Phenotype = the features that actually show.
Phenotype comes from the genotype, the environment, or both working together.
In codominance both alleles show fully and separately. In incomplete dominance they blend into an in-between phenotype.
Codominant alleles are written as a capital letter with superscripts, e.g. CR and CW — not as capital and lower case.
Gene, locus, allele
Picture one chromosome as a long shelf. Each gene has its own numbered spot on that shelf — its locus. Because chromosomes come in homologous pairs, you have two shelves side by side, and the same gene sits at the same spot on both.
The two copies do not have to be identical. Alleles differ by a small number of bases, sometimes just one. That tiny difference can change the protein enough to change the phenotype.
Homozygous and heterozygous describe one locus at a time, not the whole organism. The same person is both, at different genes.
If a question says “state the genotype”, give the letters (Bb). If it says “state the phenotype”, describe what you would see (black fur). Answering the wrong one is a guaranteed zero even when your genetics is perfect.
What decides the phenotype
Three answers, and IB expects you to know all three.
Genotype only. Blood group is fixed by your alleles. Nothing you eat or do changes it.
Environment only. A scar or an accent has nothing to do with your alleles.
Both together. Height and skin colour depend on many genes and on diet, sunlight and health.
A dominant allele needs only one copy to show. A recessive allele needs two, because a single dominant allele will mask it.
So a recessive phenotype tells you the genotype straight away — it must be homozygous recessive. A dominant phenotype leaves you with two possibilities, homozygous dominant or heterozygous, and you often need more information to decide which.
Useful shortcut. Whenever you meet a genetics puzzle, fill in the recessive individuals first. They are the only ones whose genotype you can be certain about from the phenotype alone. Everything else follows from there.
Codominance and incomplete dominance
Sometimes neither allele hides the other, and you get more than two phenotypes.
Codominance: both alleles are fully expressed, side by side. A chicken with one white-feather allele and one black-feather allele grows white feathers and black feathers — it looks speckled, not grey. Human ABO blood groups work the same way: someone with IA and IB makes both antigen A and antigen B.
Incomplete dominance: each allele is only partly expressed and the phenotypes blend. A red-flowered plant crossed with a white-flowered one gives pink flowers — not red patches on white.
The test question to ask yourself: can you still see both original phenotypes in the heterozygote? Yes = codominance. No, it is something new in between = incomplete dominance.
The ratio changes too. With normal dominance a heterozygous cross gives 3 : 1, because BB and Bb look the same. With codominance or incomplete dominance the same cross gives 1 : 2 : 1, because all three genotypes look different. Spotting a 1 : 2 : 1 phenotype ratio in data is a strong clue.
Worked examples
WORKED EXAMPLE 1
In a plant, red and white flower colour show incomplete dominance. Two pink plants are crossed. Predict the phenotype ratio in the offspring.
Step 1: work out the pink genotype
Pink is the blend, so pink must be heterozygous: CRCWStep 2: gametes
Each parent makes CR or CWStep 3: combine1 CRCR : 2 CRCW : 1 CWCWStep 4: convert to phenotypes
Every genotype looks different here
1 red : 2 pink : 1 whiteNot 3 : 1 — that only happens when the heterozygote is hidden behind a dominant allele.
WORKED EXAMPLE 2
Two black rabbits produce a litter containing one white rabbit. Deduce the genotypes of both parents and explain your reasoning.
Step 1: start with the recessive individual
White appeared from two black parents, so white must be recessive: white = bbStep 2: trace the alleles back
The white rabbit got one b from each parent
Step 3: fit that to black parents
Each parent must carry b, but each also shows black, so each carries B
Both parents are Bb (heterozygous)“Deduce” wants the reasoning as well as the answer — say where each allele came from.
💡 Exam tip
Define the letters before you use them. One line — “B = black, b = brown” — often earns a mark on its own.
Use superscripts for codominant alleles (CR, CW, IA, IB). Capital and lower case would wrongly suggest one allele hides the other.
Recessive phenotype = homozygous recessive genotype. That is the one certainty in most puzzles — start there.
If the heterozygote looks new, name it. Blended = incomplete dominance. Both features visible together = codominance.
Know one clear example of each so you can quote it if the question asks.
⚠ Common mix-up
Calling codominance “incomplete dominance”. Speckled feathers are codominance; pink flowers are incomplete dominance.
Saying an organism “is heterozygous” with no gene named. Always say which gene.
Confusing locus with allele. The locus is the place; the allele is what is sitting there.
Assuming every phenotype is genetic. Some features are set almost entirely by the environment.
Writing codominant alleles as C and c. That notation means one is recessive, which changes the biology.
Giving 3 : 1 for a codominant cross. It is 1 : 2 : 1, because there are three visible phenotypes.
Up next: Inheriting Alleles — phenotypic plasticity, recessive genetic conditions and blood groups with three alleles instead of two.
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