Misconception: "Dominant means more common in the population."
Reality: Dominant and recessive refer to which allele is expressed when both are present โ they say nothing about frequency in a population. A dominant allele can be very rare (for example, the allele for polydactyly is dominant but uncommon), and a recessive allele can be very common.
Misconception: "If both parents are carriers for a recessive condition, all their children will be carriers too."
Reality: When both parents are carriers (Ff x Ff), the Punnett square shows four possible outcomes: FF (1/4, unaffected, not a carrier), Ff (2/4, carrier), and ff (1/4, affected). Only half the children are expected to be carriers; a quarter will have the condition and a quarter will have two dominant alleles and not be carriers at all.
Misconception: "The Punnett square shows certainty โ exactly 1 in 4 children will be affected."
Reality: Punnett squares show probability, not certainty. A 1 in 4 chance means that each pregnancy independently has a 25% probability of producing an affected child. A couple could have four children and none be affected, or all four be affected โ the prediction is a long-run probability, not a guarantee.
This common misconceptions covers Common Misconceptions within Genetic Inheritance for GCSE Biology. Genetic inheritance patterns, alleles, and inheritance diagrams It is section 5 of 9 in this topic. Use this common misconceptions to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Genetic Inheritance
What is the term for an allele that is always expressed when present?
What is the purpose of a Punnett square in genetic inheritance?