An allele is one of two or more versions of a gene found at the same locus on homologous chromosomes; different alleles produce different versions of a trait, like the R (red) and r (white) alleles of a flower-color gene.
An allele is a specific version of a gene. A gene is the instruction for a trait, and an allele is one particular spelling of that instruction. Flower color might be controlled by one gene, but that gene can exist as an R allele (red) or an r allele (white). Both sit at the same spot, called a locus, on matching (homologous) chromosomes.
Because most organisms are diploid, you carry two alleles for each gene, one from each parent. Those two alleles can be the same (homozygous) or different (heterozygous). When gametes form, Mendel's law of segregation splits that pair so each egg or sperm carries just one allele. Fertilization then recombines them, which is exactly how new allele combinations show up in offspring (EK 5.3.A.2).
Alleles are the currency of two big AP Bio ideas. In Unit 5 (Heredity), LO AP Bio 5.3.A uses alleles to explain Mendel's laws, dominant vs. recessive relationships, and the probability math behind monohybrid, dihybrid, and test crosses. In Unit 7 (Natural Selection), LO AP Bio 7.11.A zooms out: the variety of alleles in a population is its genetic diversity, and that diversity decides whether the population can survive environmental stress (EK 7.11.A.1). The same word links Big Idea 3 (Information storage and transmission) to Big Idea 1 (Evolution), which is why alleles show up across multiple units instead of just one.
Keep studying AP Biology Unit 5
Gene vs. Allele (Units 5, 7)
A gene is the slot; an allele is what fills it. Think of the gene as 'flower color' and the alleles as the specific options (red or white). One gene, many possible alleles.
Genetic Variation and Diversity (Unit 7)
More alleles in a population means more genetic variation. Per EK 7.11.A.1, that variation is the raw material natural selection acts on, so allele-poor populations are at real risk of decline or extinction.
Heterozygous Genotype (Unit 5)
Having two different alleles (like Rr) is what makes a genotype heterozygous. This is where dominance, incomplete dominance, and carrier status all come from.
Meiosis and Segregation (Units 4, 5)
Meiosis physically separates the two alleles of each gene into different gametes. That's Mendel's law of segregation in action, and it's why a single cross can produce predictable allele ratios in offspring.
Alleles show up constantly. On MCQs, you'll see them written as letters (R and r) inside cross problems, like crossing two heterozygous Rr plants and calculating that 1/4 of offspring are recessive, or working out a 1:2:1 ratio in incomplete dominance. Expect probability questions on sex-linked traits too, where you track a recessive allele through a pedigree (such as colorblindness passed from grandfather to grandson). On FRQs, alleles anchor genetics and evolution prompts: the 2022 brook trout question and the 2025 Caribbean species question both hinge on how isolation changes the allele makeup of fragmented populations. You'll need to do things with alleles, not just define them: set up Punnett squares, predict genotype and phenotype ratios, and explain how allele frequency shifts connect to natural selection and genetic diversity.
A gene is the locus that codes for a trait; an allele is a specific variant of that gene. Saying 'the gene for white flowers' is sloppy. White is the recessive allele (r) of the flower-color gene. One gene, multiple alleles, and a diploid organism carries two alleles per gene.
An allele is a version of a gene found at the same locus on homologous chromosomes, and diploid organisms carry two alleles per gene.
Mendel's law of segregation splits the two alleles of each gene into separate gametes, then fertilization recombines them to create new genotypes (EK 5.3.A.2).
Two identical alleles make a genotype homozygous; two different alleles make it heterozygous, which is where dominance relationships matter.
The number and variety of alleles in a population equals its genetic diversity, and high diversity makes a population more resilient to environmental change (EK 7.11.A.1).
An allele that's helpful in one environment can be harmful in another because selective pressures differ, which is why no allele is universally 'good.'
An allele is one of two or more versions of a gene located at the same spot (locus) on homologous chromosomes. For example, the R allele codes for red flowers and the r allele codes for white, and both are versions of the same flower-color gene.
No. A gene is the genetic instruction for a trait, and an allele is a specific variant of that gene. One gene can have multiple alleles, and a diploid organism carries exactly two alleles per gene, one from each parent.
A dominant allele (written as a capital letter, like R) shows up in the phenotype even when only one copy is present. A recessive allele (lowercase, like r) only shows up when both copies are recessive, so an Rr individual still looks red.
Alleles are the source of genetic variation in a population. Populations with more alleles are more likely to contain individuals that can survive environmental stress, while populations with few alleles are at higher risk of decline or extinction (EK 7.11.A.1).
You'll use them in Punnett squares to calculate genotype and phenotype probabilities (like the 1/4 recessive odds from an Rr x Rr cross), track them through pedigrees for sex-linked traits, and explain how allele frequencies shift in FRQs about isolated or fragmented populations.