Heredity, the process where characteristics are passed from parents to offspring, forms the basis of all life. Every living organism inherits unique instructions that guide its development and functioning. Understanding these fundamental building blocks of genetic traits is a key step in grasping how life’s diversity arises and how traits are expressed across generations. This system ensures offspring share similarities with their parents while also having individual variations.
What Are Alleles?
Alleles are different versions or variations of a single gene, which is a specific segment of DNA located on a chromosome. Think of a gene as a recipe for a particular trait, like eye color. Alleles are the variations of that recipe that result in different outcomes, such as brown or blue eyes. Since most human cells contain two copies of each chromosome, individuals inherit two alleles for each gene, one from each biological parent.
Alleles are categorized as dominant or recessive, describing how their associated traits are inherited. A dominant allele will express its trait even if only one copy is present, effectively masking a recessive allele. Conversely, a recessive allele will only express its associated trait if an individual inherits two copies of it, one from each parent. For instance, the allele for brown eyes is dominant over the allele for blue eyes.
What Is a Genotype?
A genotype refers to an individual’s specific genetic makeup for a particular gene. It represents the combination of alleles inherited for a trait. For example, a person’s hair color genotype is the specific pair of alleles received from their parents for that gene.
The combination of alleles defines whether a genotype is homozygous or heterozygous. An individual is homozygous for a trait if they have two identical alleles for a specific gene, such as two dominant alleles (e.g., ‘AA’) or two recessive alleles (e.g., ‘aa’). In contrast, an individual is heterozygous for a trait if they possess two different alleles for a gene, having inherited one dominant and one recessive allele (e.g., ‘Aa’).
How Alleles Create Genotypes
An individual’s genotype for a specific trait results from inheriting two alleles, one from each parent during reproduction. Each parent passes one of their two alleles for every gene to their offspring. The combination of these two inherited alleles then establishes the offspring’s genotype for that particular trait.
For example, let ‘A’ be a dominant allele and ‘a’ a recessive allele for a gene. If an offspring inherits ‘A’ from both parents, their genotype is homozygous dominant (‘AA’). If they receive ‘a’ from both parents, their genotype is homozygous recessive (‘aa’). When an offspring inherits ‘A’ from one parent and ‘a’ from the other, their genotype is heterozygous (‘Aa’). These allele combinations determine the genetic instructions for a specific characteristic.
From Genotype to Observable Trait
The genotype determines an organism’s observable characteristics, known as its phenotype. The phenotype includes all physical, biochemical, and behavioral traits that can be seen or measured. For example, a person’s eye color, height, or blood type are all phenotypic expressions.
The interaction between inherited alleles within the genotype dictates which trait is expressed. For dominant and recessive alleles, a homozygous dominant genotype (‘AA’) results in the dominant trait. A heterozygous genotype (‘Aa’), containing one dominant and one recessive allele, also results in the dominant trait because the dominant allele masks the recessive one. Only a homozygous recessive genotype (‘aa’) will make the recessive trait observable. Thus, while the genotype provides genetic information, the phenotype is the tangible outcome of these genetic instructions.