Summary
A dominant allele shows its effect when present in one copy; a recessive allele only shows when two copies are present.
In depth
A dominant allele masks the recessive one in heterozygotes, typically because one functional copy produces enough protein. Recessive traits appear only in homozygous recessives. Dominance is a relationship between alleles, not a measure of how healthy or common an allele is.
Capital letters represent dominant alleles and lower-case recessive.
Dominance is a property of the relationship between alleles, not of the alleles themselves, and it can differ depending on the level at which the trait is measured. The recessive allele of Mendel's wrinkled pea causes loss of an enzyme that branches starch; at the molecular level it has an effect, but the round phenotype is dominant.
Dominant does not mean common or beneficial: polydactyly and Huntington's disease are dominant but rare, and many recessive alleles, such as blood group O, are widespread.
Example: in cystic fibrosis a person with one working CFTR copy is healthy, so 1 in 25 people of northern European descent are carriers but only about 1 in 2500 births is affected.
Frequently asked questions
What is Dominant and Recessive?
A dominant allele shows its effect when present in one copy; a recessive allele only shows when two copies are present.
Which area of biology covers Dominant and Recessive, and how advanced is it?
Dominant and Recessive is one of the 54 topics in Genetics. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Codominance, Penetrance and Expressivity, Homozygous and Heterozygous.
Can you give an example of Dominant and Recessive?
Cystic fibrosis is recessive; Huntington's disease is dominant.
What should I learn before Dominant and Recessive?
Dominant and Recessive is a Beginner topic, so no earlier topic is required. Other Beginner topics in Genetics that pair well with it include Heredity, Gene, Allele.
What topics are related to Dominant and Recessive?
The closest topics to Dominant and Recessive are Codominance, Penetrance and Expressivity, Homozygous and Heterozygous, Incomplete Dominance. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Dominant and Recessive?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Dominant and Recessive. To practise the whole of Genetics, use the Genetics quiz.
How is Dominant and Recessive different from Codominance?
Dominant and Recessive: A dominant allele shows its effect when present in one copy; a recessive allele only shows when two copies are present. Codominance: Both alleles are fully expressed in the heterozygote, as in the AB blood type. Read both side by side and note what each one explains.
How should I study Dominant and Recessive step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Codominance and explain the difference aloud. Finally, take the Genetics quiz and come back to this page for any question you miss.