# Allele

One version of a gene. An organism has two alleles for each gene, one from each parent. A dominant allele shows its effect even with one copy. A recessive allele shows only with two copies.

Biology (university) · Part 07: Genetics and inheritance · Card 1 of 8 · Last updated 2026-10-19

## Analogy

Two settings on a dial, one from each parent.

## Example

**Genotype or phenotype**

PP: purple (homozygous).
Pp: purple (heterozygous).
pp: white (homozygous).
Three genotypes, two looks.

## In practice

In pea plants, the allele for purple flowers (P) is dominant to the allele for white (p).

## Practice

### Flashcards

- **What is the difference between genotype and phenotype?** Genotype is the alleles an organism carries. Phenotype is the trait that shows.
- **What is a heterozygote?** An individual with two different alleles of a gene.

### Interview questions

**Explain Mendel's law of segregation and use a cross between two heterozygotes to show the expected ratios.**

- Alleles separate in gamete formation
- Each gamete has one allele
- Aa x Aa gives 1:2:1 genotype
- 3:1 phenotype ratio

## Questions people ask

### Does dominant mean most common?

No. Dominant describes how an allele behaves in a heterozygote, not how common it is in a population. Some dominant alleles are rare. Huntington's disease is caused by a dominant allele that few people carry, and many common traits, such as blood group O, are recessive.

### What does homozygous and heterozygous mean?

Homozygous means the two alleles of a gene are the same, such as AA or aa. Heterozygous means they are different, such as Aa. A homozygous individual breeds true for that trait. A heterozygote carries a recessive allele hidden by the dominant one, and can pass it on.

## Related concepts

- [Law of segregation](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-2)
- [Law of independent assortment](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-3)
- [Incomplete dominance and codominance](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-4)
- [Sex-linked inheritance](https://studyvisually.com/learn/science/biology-university/biology-university-part-7/biology-university-part-7-concept-5)

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