IGCSE Biology 0610 · Common mistakes
Common mistakes in genetics and inheritance
Updated · by the Fahmazing team
Genetic diagrams are some of the most reliable marks on the paper, as long as you avoid a few classic slips. Here they are, with the correct version for each.
Putting two alleles in a gamete
Show the gametes of a Bb parent
✗ Wrong: Gamete: Bb
✓ Right: Gametes: B and b (circle each one separately). A gamete carries only one allele of each gene.
Free lessons: Monohybrid inheritance · Meiosis
Mixing up genotype and phenotype
Give the genotype and phenotype of a heterozygous black guinea pig
✗ Wrong: Genotype: black. Phenotype: Bb.
✓ Right: Genotype: Bb. Phenotype: black fur.
Tip: Write the capital letter first (Bb, never bB) and use the same letter for both alleles. Homozygous means two identical alleles; heterozygous means two different alleles.
Free lesson: Monohybrid inheritance
Reading 3 : 1 as a schedule
Bb × Bb. The first three offspring are black. What colour is the fourth?
✗ Wrong: It must be brown, because the ratio is 3 : 1.
✓ Right: Each offspring has a 1 in 4 chance of being brown, whatever came before. The ratio is a probability, not a schedule.
Tip: 3 : 1 means 3 out of 4 and 1 out of 4, not 1 out of 3. Count phenotypes, not genotypes: RR and Rr both look round.
Free lesson: Monohybrid inheritance
Thinking dominant means common
Is a dominant allele always the most common one?
✗ Wrong: Yes, dominant alleles are the most common.
✓ Right: No. A dominant allele is expressed whenever it is present, but it can still be rare in a population.
Free lesson: Monohybrid inheritance
Codominance and blood group slips (Extended)
What blood group is a person with genotype IᴬIᵒ?
✗ Wrong: Group AB.
✓ Right: Group A. Iᴬ and Iᴮ are codominant, but Iᵒ is recessive, so only IᵒIᵒ is group O and only IᴬIᴮ is group AB.
Tip: Pink × pink flowers do not give all pink: expect 1 red : 2 pink : 1 white.
Free lesson: Codominance, blood groups and sex linkage
Sex-linkage slips (Extended)
- A female XᴮXᵇ is a carrier with normal vision, not colour-blind.
- Fathers pass their X only to daughters and their Y only to sons, so a son gets his X from his mother.
- Males are more often colour-blind because they have one X, so one recessive allele is enough.
- "The chance a son is colour-blind" means count only the sons. "The chance a child is" means count all the boxes.
Free lesson: Codominance, blood groups and sex linkage
Questions students ask
How can two black parents have a brown baby?
If black is dominant, both parents can be heterozygous (Bb). A baby that receives the recessive b allele from each parent is bb and brown.
How do I show a parent is heterozygous?
Use a test cross with a homozygous recessive individual. If even one offspring shows the recessive phenotype, the parent must be heterozygous.
Why do real results not match 3 : 1 exactly?
Fertilisation is random, so real numbers vary. Divide to find the ratio, then choose the nearest expected ratio, such as 3 : 1 or 1 : 1.