Which Of The Following Is An Example Of An Allele

Which of the following is an example of an allele? This question appears frequently in introductory biology quizzes because it tests a fundamental concept in genetics: the idea that different versions of a gene occupy the same locus on homologous chromosomes. Understanding alleles is essential for grasping how traits are inherited, how genetic variation arises, and why individuals within a population can display diverse phenotypes. In the sections below, we will define alleles, explore classic examples, explain how they interact, and provide a clear answer to the sample question while reinforcing the broader significance of allelic variation in living organisms.

Introduction to Alleles

An allele is one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome. Genes encode the instructions for building proteins or functional RNA molecules, and alleles represent the specific sequence variations that can lead to different protein functions or expression levels. Because diploid organisms inherit one chromosome from each parent, they typically possess two alleles for each gene—one on each homologous chromosome. These alleles may be identical (homozygous) or different (heterozygous), and the combination determines the organism’s genotype, which in turn influences its phenotype.

The study of alleles bridges molecular biology and Mendelian genetics. Early geneticists such as Gregor Mendel inferred the existence of “factors” (now known as alleles) by observing predictable ratios of traits in pea plants. Modern molecular techniques allow us to sequence DNA and pinpoint the exact nucleotide changes that distinguish one allele from another, linking genotype to phenotype with unprecedented precision.

What Makes a Sequence an Allele?

To qualify as an allele, a DNA variant must meet three criteria:

1. Same Locus – It occupies the exact chromosomal position as other variants of the same gene.
2. Functional Relevance – It can influence the gene’s product, either by altering protein structure, changing expression levels, or affecting RNA processing.
3. Heritable – It is passed from parent to offspring through gametes.

Not every DNA difference qualifies as an allele. Variations in non‑coding regions that do not affect gene function are often termed polymorphisms or neutral mutations, but when they occur within a gene’s coding or regulatory regions and can be inherited, they are considered alleles.

Classic Examples of Alleles Below are several well‑known alleles that illustrate the concept in different organisms and traits. Each example highlights how a single gene can have multiple allelic forms, producing observable differences in phenotype.

1. Eye Color in Humans

The OCA2 gene (oculocutaneous albinism II) contains a major allele associated with brown eye color and a variant allele linked to blue eye color. The brown allele (B) is generally dominant over the blue allele (b). Individuals with genotypes BB or Bb typically have brown eyes, whereas bb individuals tend to have blue eyes. This simple dominant‑recessive relationship makes eye color a textbook example of allelic variation.

2. ABO Blood Group System

The ABO locus has three common alleles: I⁽ᴬ⁾, I⁽ᴮ⁾, and i. The I⁽ᴬ⁾ and I⁽ᴮ⁾ alleles encode enzymes that add specific sugars to the H antigen on red blood cells, while the i allele produces a non‑functional enzyme, leaving the antigen unchanged. The I⁽ᴬ⁾ and I⁽ᴮ⁾ alleles are codominant (both expressed when present together), whereas i is recessive. Possible genotypes and phenotypes include:

• I⁽ᴬ⁾I⁽ᴬ⁾ or I⁽ᴬ⁾i → Type A
• I⁽ᴮ⁾I⁽ᴮ⁾ or I⁽ᴮ⁾i → Type B
• I⁽ᴬ⁾I⁽ᴮ⁾ → Type AB (codominance)
• ii → Type O

3. Sickle Cell Trait

The HBB gene, which encodes the β‑globin subunit of hemoglobin, has a normal allele (Hbᴬ) and a mutant allele (Hbˢ) caused by a single‑nucleotide substitution (GAG → GTG) that replaces glutamic acid with valine at position six. Heterozygotes (HbᴬHbˢ) have sickle cell trait and are resistant to malaria, while homozygotes (HbˢHbˢ) develop sickle cell disease. This allele demonstrates how a single point mutation can have profound phenotypic consequences and how heterozygote advantage can maintain genetic variation in populations.

4. Flower Color in Pea Plants (Mendel’s Experiments)

Mendel studied the purple (P) and white (p) flower color alleles in Pisum sativum. The purple allele is dominant to the white allele. Crossing true‑breeding purple (PP) with white (pp) plants yielded all heterozygous (Pp) offspring with purple flowers, and self‑fertilizing the F₁ generation produced a 3:1 phenotypic ratio in the F₂ generation—cornerstone evidence for particulate inheritance.

5. Wing Shape in Drosophila

The vestigial (vg) allele in Drosophila melanogaster produces short, crumpled wings when homozygous (vg/vg), whereas the wild‑type allele (vg⁺) yields normal wings. The vestigial allele is recessive to the wild‑type form, and heterozygotes (vg⁺/vg) display normal wing morphology. This example is frequently used in laboratory genetics to teach concepts of dominance, lethality, and linkage.

How Alleles Interact: Dominance, Codominance, and Incomplete Dominance

Alleles do not always behave in a simple dominant‑recessive fashion. The nature of the interaction depends on the biochemical activity of the gene products.

• Complete Dominance – One allele masks the effect of the other (e.g., brown eye allele over blue).
• Codominance – Both alleles are fully expressed in the heterozygote (e.g., I⁽ᴬ⁾ and I⁽ᴮ⁾ in AB blood type).
• Incomplete Dominance – The heterozygote shows an intermediate phenotype (e.g., snapdragon flower color where red (R) and white (r) alleles produce pink (Rr) flowers).
• Overdominance (Heterozygote Advantage) – The heterozygote has greater fitness than either homozygote (e.g., sickle cell trait conferring malaria resistance).

Understanding these patterns helps predict inheritance outcomes and explains why certain alleles

persist in populations despite deleterious effects, or how advantageous variants can spread through a gene pool. Population genetics provides the quantitative framework for interpreting these dynamics. Under the Hardy‑Weinberg principle, allele and genotype frequencies remain constant from generation to generation in an idealized population that is infinitely large, randomly mating, and free of mutation, migration, and natural selection. Deviations from this equilibrium signal the action of evolutionary forces.

Mutation introduces new alleles into the gene pool. Although most point mutations are neutral or deleterious, rare beneficial mutations can increase in frequency if they confer a fitness advantage, as illustrated by the Hbˢ allele in malaria‑endemic regions. Gene flow—the movement of individuals or gametes between populations—can homogenize allele frequencies across subpopulations or introduce novel variants, thereby altering local adaptation trajectories. Genetic drift, especially pronounced in small populations, leads to random fluctuations in allele frequencies that can result in the loss or fixation of alleles irrespective of their selective value; this stochastic process underlies phenomena such as the founder effect and bottleneck events. Natural selection shapes allele frequencies by favoring genotypes with higher survival or reproductive success. Directional selection drives an allele toward fixation, balancing selection (including heterozygote advantage and frequency‑dependent selection) maintains polymorphism, and disruptive selection can promote the emergence of distinct phenotypic classes.

Modern molecular techniques allow us to observe these processes directly. Genome‑wide association studies (GWAS) link specific alleles to complex traits, while temporal sampling of ancient DNA reveals how allele frequencies have shifted in response to historical pressures such as domestication, climate change, or pathogen outbreaks. Integrating phenotypic data with allele frequency changes provides a powerful testbed for evolutionary hypotheses and informs applied fields ranging from conservation biology—where preserving genetic diversity is crucial for species resilience—to medicine, where understanding allele distribution guides risk assessment and personalized therapy.

In summary, alleles are the fundamental units of hereditary variation, and their interactions—whether through dominance, codominance, incomplete dominance, or more complex schemes like overdominance—determine the observable traits of organisms. Population genetics extends this view by describing how alleles behave within groups over time, revealing the balance between stochastic forces and selective pressures that shape genetic diversity. Recognizing these patterns not only deepens our comprehension of life’s complexity but also equips us to predict and manage the evolutionary consequences of human‑induced changes to the environment.