There are one or two things which make a biology teacher’s (and indeed an exam marker’s) blood pressure rise.  Well in fact in my case there are many dozens of things, as some of you know, but let’s keep it to the things candidates write in genetics answers in exams.  This post is an attempt to encourage you to avoid the commonest “howler”.

The dominant allele does not have to be the more common one in a population.

Just because an allele is dominant, it does not mean it will be the most common in a population.  I often hear answers in which people think that in a population 3/4 of the population will have the dominant phenotype, 1/4 will be recessive.  This is utter nonsense of course.  The ratio of 3:1 only applies to the probabilities of offspring produced by mating two heterozygous individuals.

There is a gene in humans in which a mutation can cause polydactyly:  this rare condition results in babies born with an extra digit on each hand.  Anne Boleyn was a famous sufferer in the past.  But the allele of the gene that causes polydactyly is dominant – it is a P allele.  I would imagine everyone reading this post, (all 12 of you…..), will probably have the genotype pp.  The p allele that causes a normal hand to form is very very common in our population whereas the P allele is very very rare.

Don’t ever believe that just because an allele is common, it must be dominant.