Common Misconceptions

Misconception: "Dominant means more common in the population."

Reality: Dominant and recessive refer to which allele is expressed when both are present — 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 — 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 — the prediction is a long-run probability, not a guarantee.