Sciences

Dominant and Recessive Alleles: Which One Shows

A dominant allele shows with one copy; a recessive one needs both. What a carrier is, and how to read dominant or recessive off a family tree.

Dominant and recessive alleles

A dominant allele shows its effect whenever it is present; a recessive allele shows only when both of an organism's alleles are recessive.

Où les élèves le rencontrent: Grade 9 science, with Punnett squares, and again in Grade 10 when pedigree charts and inherited disorders are read.

La réponse en bref
A dominant allele produces its characteristic whenever it is present, even from a single copy. A recessive allele produces its characteristic only when both alleles are recessive. Dominant alleles are written as a capital letter and recessive alleles as the same letter in lower case, so B masks b.

Un exemple

Bb → brown eyes; bb → blue eyes

A person with Bb has one allele for brown and one for blue, and their eyes are brown. The b is still there and can still be passed on; it simply does not show while a B is present. Blue eyes require bb, two recessive alleles with no dominant one to mask them. This is why a recessive characteristic tells you the genotype exactly and a dominant one does not.

One copy is enough, or two are needed

The whole idea reduces to a counting rule. A dominant characteristic appears if at least one dominant allele is present, so both BB and Bb show it. A recessive characteristic appears only when there is no dominant allele to mask it, which means bb and nothing else.

The notation carries this. Both alleles of a gene use one letter, capital for dominant and lower case for recessive, which is why B and b are versions of a single gene while B and c would be two different genes. Using two unrelated letters for one gene is a standard way to lose marks on a Punnett square that is otherwise correct.

Be careful with what dominant means. It does not mean stronger, better or more common. Polydactyly, having extra fingers or toes, is caused by a dominant allele and is rare; plenty of recessive characteristics are widespread. Dominance describes only which allele shows when the two are together.

Carriers, and why recessive conditions skip generations

A carrier is a heterozygous individual: they hold one recessive allele, show no sign of it, and can pass it on. Nothing about them looks different, which is precisely what makes recessive conditions behave the way they do in families.

Cystic fibrosis is the standard example, caused by a recessive allele. A child develops it only by inheriting the faulty allele from both parents, so two carriers who show nothing at all can have an affected child. The Punnett square for two carriers gives a one in four chance of an affected child, a one in two chance of a carrier, and a one in four chance of a child with neither allele — and the odds reset for every pregnancy.

A dominant condition behaves quite differently. Polydactyly needs only one copy, so an affected person has an affected parent, it turns up in every generation, and it cannot be carried silently.

Reading dominant or recessive off a family tree

Pedigree questions look daunting and usually turn on one or two lines of reasoning. The most useful is this: if two unaffected parents have an affected child, the condition must be recessive, and both parents must be carriers. There is no other way for the allele to have reached the child.

Work through the chart with these checks and the genotypes fall out one at a time.

Two unaffected parents with an affected child — the condition is recessive, and both parents are heterozygous
The condition appearing in every generation, with every affected person having an affected parent — most likely dominant
Anyone showing a recessive condition — genotype known exactly, both alleles recessive
Anyone unaffected who has an affected child — must be a carrier
Anyone showing a dominant condition — at least one dominant allele, and their other allele is often undecidable from the chart alone

Questions fréquentes

What is the difference between dominant and recessive alleles?

How many copies each needs to show. A dominant allele produces its characteristic from one copy, so it appears in both the homozygous dominant and the heterozygous genotype. A recessive allele produces its characteristic only when both alleles are recessive, because a single dominant allele present alongside it will mask it entirely.

What is a carrier?

Someone heterozygous for a recessive allele: they carry one copy, do not show the characteristic, and can pass the allele to their children. Carriers are the reason recessive conditions can disappear for generations and then reappear, and the reason two healthy parents can have a child with an inherited condition neither of them has.

Can two brown-eyed parents have a blue-eyed child?

Yes, if both parents are heterozygous, Bb. Each can pass on the recessive b, and a child receiving b from both is bb and blue-eyed. Each pregnancy carries a one in four chance. If either parent is BB it cannot happen, since every gamete from that parent would carry a dominant allele.

Does dominant mean the allele is more common?

No, and this is the most persistent misunderstanding on the topic. Dominance is about which allele shows when both are present, not about how many people carry it. Polydactyly is caused by a dominant allele and is uncommon, while plenty of recessive characteristics are widespread in the population.

How do you tell from a family tree whether a condition is dominant or recessive?

Look for two unaffected parents with an affected child. That combination is only possible if the condition is recessive and both parents carry the allele. If instead every affected person has an affected parent and the condition appears in each generation, it is most likely dominant. Start there, then work out individual genotypes.

Dernière mise à jour 21 août 2026