An allele is a specific version of a DNA sequence (which can be a single base or a segment of bases) at a given genomic location. It is one of the fundamental units of heredity, determining an organism's traits.
Understanding Alleles
Alleles represent the variations that can exist for a particular gene or genetic marker. For example, while a gene might be responsible for eye color, different alleles of that gene could lead to brown, blue, or green eyes.
Key Characteristics of Alleles
- Versions of DNA Sequence: An allele is essentially a variant form of a gene, differing slightly in its DNA sequence. This variation can be as small as a change in a single DNA base pair or a larger segment of bases.
- Specific Genomic Location: Each allele is found at a precise genomic location on a chromosome, often referred to as a locus (plural: loci).
- Inheritance: For any given genomic location where such variation exists, an individual typically inherits two alleles—one from each biological parent. This pair of alleles determines the individual's genetic makeup for that specific trait.
Alleles and Inheritance
The specific combination of the two alleles an individual inherits for a particular gene forms their genotype. This genotype, along with environmental factors, ultimately influences the observable characteristic, known as the phenotype.
For instance, consider the gene responsible for human blood type. There are three common alleles: I^A, I^B, and i.
- I^A and I^B are codominant, meaning both are expressed if present.
- i is recessive.
The combination of two of these alleles determines an individual's blood type (e.g., an individual with alleles I^A I^A or I^A i would have Type A blood).
Homozygous vs. Heterozygous
When an individual inherits two alleles for a specific gene, these alleles can either be identical or different. This distinction is crucial in understanding how traits are expressed.
| Characteristic | Homozygous | Heterozygous |
|---|---|---|
| Alleles | Two identical versions of an allele (e.g., AA, aa) | Two different versions of an allele (e.g., Aa) |
| Inheritance | Inherited the same allele from both parents | Inherited different alleles from each parent |
| Expression | The trait associated with that allele is typically expressed. | The trait of the dominant allele is usually expressed, or both alleles are expressed (codominance). |
- Homozygous: If an individual inherits two identical alleles for a particular genomic location, they are considered homozygous for that allele. For example, an individual with two alleles for brown eyes (BB) would be homozygous brown-eyed.
- Heterozygous: Conversely, if an individual inherits two different alleles for a particular genomic location (e.g., one allele for brown eyes (B) and one for blue eyes (b)), they are heterozygous for that gene. In such cases, the dominant allele usually determines the trait expressed.
Understanding alleles is fundamental to the study of genetics, as it explains the diversity of traits within a population and how these traits are passed down through generations.
