Definition
What is a dominant allele?
A dominant allele is an allele that determines the phenotype in a heterozygote, an individual with two different alleles of a gene. One copy is enough for its trait to appear, so individuals with the genotypes AA and Aa look the same. By convention it is written with a capital letter. Mendel found that in pea plants violet flowers, yellow seeds and round seeds were dominant over their alternatives.
At the molecular level, dominance often happens because one working copy makes enough protein for normal function, or because the dominant allele makes a product that changes the cell in a way the other copy cannot offset. Dominance describes how two alleles behave together, not how common an allele is: a dominant allele can be rare in a population. It is also not always complete. In incomplete dominance the heterozygote is intermediate, and in codominance both alleles show.
In autosomal dominant conditions such as Huntington disease and Marfan syndrome, inheriting one altered copy from one parent is enough to cause the condition, so the trait usually appears in every generation of a family pedigree.
Word parts
Where does the term “dominant allele” come from?
- domin-
- to rule, to masterLatin dominari
- allel-
- of one anotherGreek allēlōn
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Terms that go with it
- Recessive alleleA recessive allele is a version of a gene whose effect appears only when an organism has two copies, because a single dominant allele masks it.
- AlleleAn allele is any of the alternative forms of a gene or DNA sequence that can occupy the same position, or locus, on a chromosome.
- HeterozygousHeterozygous describes an organism that carries two different alleles of a gene at a given locus, such as one dominant and one recessive allele.
- PhenotypeA phenotype is the set of observable traits of an organism, such as its height or blood group, produced by its genotype acting with its environment.
- Punnett squareA Punnett square is a grid used in genetics to predict the possible genotypes of offspring, and how likely each is, from the alleles of two parents.
- PedigreeA pedigree is a family tree chart that shows how a trait or genetic condition is passed down through the generations of a family.
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Written for biofideX in plain words, after checking these references:
- NHGRI Talking Glossary: Dominant
- OpenStax Biology 2e, 12.2 Characteristics and Traits
- MedlinePlus Genetics: What are the different ways a genetic condition can be inherited?
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