What are Genes and Alleles?
Genes and alleles are the building blocks of heredity that explain why living things share traits with their parents yet remain unique individuals. Understanding the difference between them is the first step into genetics.
A gene is a segment of DNA that codes for a specific trait, while an allele is one of the alternative versions of that gene. Every individual carries two alleles for each gene, one from each parent.
- •A DNA segment coding for a trait
- •Located at a fixed locus on a chromosome
- •Same gene present in every individual of a species
- •Example: the gene for flower color
- •A specific version/variant of a gene
- •Can differ between individuals
- •Produces different forms of a trait (e.g., purple vs white)
- •Example: purple-flower allele vs white-flower allele
Step-by-step worked examples
Pea plants have a gene for seed color. One plant has two 'yellow' alleles, another has one 'yellow' and one 'green' allele. Identify the genotypes.
The gene is seed color. Alleles are the versions: yellow (Y) and green (y). Plant 1 genotype: YY (homozygous). Plant 2 genotype: Yy (heterozygous).
A human has the gene for eye color with alleles B (brown) and b (blue). If both alleles are B, what is the genotype and is it homo- or heterozygous?
Genotype = BB. Both alleles are identical, so it is homozygous. Since B is typically dominant, the eye color trait expressed is brown.
How many alleles does a diploid organism carry for a single gene, and where do they come from?
Diploid organisms carry two alleles per gene. One allele is inherited from the mother. The other allele is inherited from the father. Together they form the genotype for that gene.
Flashcards
Quick quiz
Q1.What best describes a gene?
Q2.An organism with two different alleles for a gene is called…
Q3.Where do an organism's two alleles for a gene come from?
Q4.Which pair below represents alleles of the same gene?
The full card deck, worked steps and AI-tutor support for “What are Genes and Alleles?” are in Notek — study by hand before your exam.
Common mistakes
Thinking a gene and an allele are the same thing. — Correct: A gene is the DNA segment; an allele is one specific version of that gene.
Assuming an organism only has one allele per gene. — Correct: Diploid organisms carry two alleles per gene, one from each parent.
Believing heterozygous means 'no trait is expressed'. — Correct: Heterozygous just means the two alleles differ — which trait shows depends on dominance.
Confusing genotype with phenotype. — Correct: Genotype is the allele combination (e.g., Bb); phenotype is the observable trait (e.g., brown eyes).
FAQ
What is the difference between a gene and an allele?
A gene is a DNA segment coding for a trait; an allele is one specific version of that gene. Genes are the 'category,' alleles are the 'options.'
What are genes and alleles in simple terms?
Genes are the instructions for traits, and alleles are the different versions of those instructions that create variation.
How many alleles does a person have for each gene?
Humans are diploid, so they carry two alleles for each gene — one from each parent.
What is an example of genes and alleles?
The pea-plant seed-color gene has two alleles: yellow and green, which combine to determine the plant's seed color.