The diversity of traits in living organisms, from eye color to height, arises from variations in genetic makeup. Understanding how these traits are passed down involves exploring how genes contribute to an individual’s features.
Genetic Basics: Alleles and Genes
At the core of heredity are genes, which are segments of DNA that serve as basic units of inheritance. Each gene provides instructions, often for making proteins that carry out specific functions within the body. Genes are located at specific positions, called loci, on chromosomes. Most organisms, including humans, are diploid, meaning they possess two copies of each chromosome and, consequently, two copies of each gene.
The different forms of a gene are known as alleles. For any given gene, an individual inherits one allele from each parent. These alleles can be identical, making the individual homozygous for that gene, or they can be different, resulting in a heterozygous state. Alleles interact in various ways, with some being dominant and others recessive. A dominant allele expresses its associated trait even when only one copy is present, while a recessive allele only manifests its trait if two copies are inherited.
Multiple Alleles Explained
Multiple alleles describe a situation where a single gene has more than two possible allele forms within a population. While many different alleles might exist across a population, any individual organism, being diploid, will still only possess two of these alleles for that specific gene—one inherited from each parent.
A classic example in humans is the ABO blood group system. This trait is determined by a single gene, the ABO gene, which has three common alleles: IA, IB, and i (or IO). The IA allele leads to the production of A antigens on red blood cells, and the IB allele results in B antigens. The i allele does not produce either A or B antigens.
The inheritance pattern of these alleles demonstrates both dominance and codominance. Both IA and IB alleles are dominant over the i allele. However, IA and IB are codominant with each other, meaning that if an individual inherits both, they will express both A and B antigens, resulting in AB blood type. This combination of dominance and codominance among the three alleles leads to the four distinct blood types: A, B, AB, and O.
Polygenic Traits Explained
Polygenic traits are characteristics influenced by multiple genes, often interacting with environmental factors. The term “polygenic” means “many genes,” indicating that their expression is a cumulative effect of several genes working together. Unlike traits determined by a single gene, polygenic traits typically exhibit continuous variation within a population.
Human height is a prime example of a polygenic trait, with numerous genes contributing to an individual’s final stature. Skin color and eye color are also polygenic, influenced by the combined effects of several genes that control pigment production. For instance, eye color is determined by at least 15 genes, and skin color involves around 60 genetic loci. These genes, along with environmental influences like nutrition or sun exposure, result in the continuous variation observed.
Distinguishing Between Multiple Alleles and Polygenic Traits
The fundamental distinction between multiple alleles and polygenic traits lies in the number of genes and alleles involved in determining a characteristic. Multiple alleles refer to a single gene having more than two possible forms (alleles) within a population, though an individual still inherits only two. The ABO blood group system, for example, is controlled by one gene with three common alleles (IA, IB, i).
In contrast, polygenic traits involve multiple different genes, each typically having two or more alleles, that collectively influence a single characteristic. Their expression results from the additive or interactive effects of these many genes. Human height, for instance, is a polygenic trait because numerous genes contribute to its determination, leading to a continuous range of heights.
To summarize, multiple alleles expand the genetic variation for a trait governed by one gene by introducing more than two allele options in the population. Polygenic traits, on the other hand, achieve their broad range of variation by involving the combined action of many different genes, each contributing to the overall expression of the trait. This distinction impacts how traits are inherited and the spectrum of observable variations.