Genotypes represent the entire collection of hereditary instructions passed from parents to offspring, acting as the fundamental genetic blueprint for every living organism. This set of instructions is encoded within the DNA, specifically contained within the chromosomes found inside the nucleus of nearly every cell. A genotype dictates the potential for all traits and characteristics an individual may develop throughout their life.
Genotype Versus Phenotype
Genotype and phenotype represent the genetic instruction set and its resulting physical expression, respectively. The genotype is the internal, unobservable code—the specific sequence of DNA an organism possesses for a particular gene or set of genes. Think of the genotype as the recipe written in a cookbook. The phenotype, on the other hand, is the observable manifestation of that recipe; it is the physical trait, biochemical property, or behavioral characteristic that results from the genotype interacting with the environment. A person’s genotype determines their potential, but environmental factors, such as diet or exposure to sunlight, can influence how the phenotype is expressed.
The Building Blocks of Genotype
The specific code that makes up a genotype is determined by variations of a gene called alleles. Since humans are diploid organisms, they inherit two alleles for every gene—one from each biological parent. These pairs of alleles combine to form the individual’s genotype for that specific trait. Genotypes are categorized into three main pairings based on the combination of these alleles. A homozygous dominant genotype occurs when an individual inherits two copies of the dominant allele, represented as ‘AA’. Conversely, a homozygous recessive genotype involves inheriting two copies of the recessive allele, represented by ‘aa’. A heterozygous genotype occurs when one dominant and one recessive allele are inherited, represented as ‘Aa’, where the dominant allele often masks the expression of the recessive one.
Common Examples of Human Genotypes
Common human traits illustrate the principles of allele interaction. One example is the ability to taste the chemical Phenylthiocarbamide (PTC), which is controlled by the gene TAS2R38. The taster allele (T) is dominant, meaning individuals with the homozygous dominant (TT) or heterozygous (Tt) genotype will taste PTC. Only individuals who inherit two copies of the recessive non-taster allele (tt) will be non-tasters.
Another example involves earlobe attachment, where the allele for free-hanging earlobes (F) is dominant over the allele for attached earlobes (f). A person with the genotypes FF or Ff will display free earlobes, while the homozygous recessive genotype ff results in attached earlobes.
The ABO blood group system offers a more complex example, involving multiple alleles and codominance. This system is governed by three alleles: $I^A$, $I^B$, and $i$. The $I^A$ and $I^B$ alleles are codominant; if a person inherits both ($I^A I^B$), they will have Type AB blood. The $i$ allele is recessive to both $I^A$ and $I^B$. A person with the genotype $I^A i$ or $I^A I^A$ will have Type A blood, while the genotype $i i$ is required to produce the Type O blood phenotype.