Dominant and Recessive Alleles
Dominant alleles show up when present; recessive only when homozygous.
About this topic
A dominant allele (capital letter) is expressed in the phenotype even when only one copy is present; a recessive allele (lowercase) is expressed only when two copies are present (homozygous recessive). Genotype is the allele combination, phenotype the observable trait. Heterozygous individuals (Aa) show the dominant trait and are carriers of the recessive allele.
Dominant doesn't mean common or better: polydactyly is dominant yet rare. Cystic fibrosis is a recessive disorder.
Dominant does not mean common or better. Huntington's disease is caused by a dominant allele yet is rare, while the recessive allele for blood group O is very common. Cystic fibrosis needs two recessive alleles, so two healthy carriers (Cc) have a 1 in 4 chance of an affected child. Homozygous means two identical alleles and heterozygous means two different ones.
Dominance happens because one working copy of a gene often makes enough protein to produce the trait, while the non-working copy contributes nothing. Pea plants, with tall and short, wrinkled and round seeds, are the classic examples. A frequent misconception is that dominant traits always become more common in a population; allele frequency depends on selection and chance, not dominance.
Key ideas
- Genotype vs phenotype
- Homozygous vs heterozygous
- Capital letters for dominant
Quick check
Use cases: where Dominant and Recessive Alleles helps
- Homework: review the key idea behind Dominant and Recessive Alleles before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Dominant and Recessive Alleles without notes.
- Visual learning: play with the interactive lab to see how Dominant and Recessive Alleles changes when you adjust its inputs.
- Teaching a friend: explain Dominant and Recessive Alleles in your own words, then compare your version with the key ideas.
Biology underpins medicine, farming, conservation and biotechnology.
Frequently asked questions
What is Dominant and Recessive Alleles?
Dominant alleles show up when present; recessive only when homozygous.
What will I learn in Dominant and Recessive Alleles?
Genotype vs phenotype. Homozygous vs heterozygous. Capital letters for dominant.
Is Dominant and Recessive Alleles part of Biology?
Yes. It is one of 150 Biology topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Dominant and Recessive Alleles lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Dominant and Recessive Alleles?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
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