Heredity explains how offspring share traits with their parents and is the foundation of genetics, the scientific field dedicated to understanding biological inheritance. The process begins when reproductive cells from each parent combine, contributing unique genetic information to the new individual. This transfer of coded information dictates the potential for every measurable characteristic, from height and hair color to blood type and susceptibility to certain conditions.
Defining the Genotype
The genotype is the specific, internal genetic blueprint of an organism, representing the full collection of genes an individual carries. Stored within the DNA of nearly every cell, this code is fixed at conception and dictates the potential range of characteristics that an organism can exhibit.
The term genotype is often used more specifically to refer to the combination of genes an organism possesses for a single trait, such as eye color. The genotype is not the trait itself, but the underlying set of instructions. This internal code can only be determined through genetic testing, unlike observable traits.
The Building Blocks of Genotype
The genotype is composed of units called alleles, which are variations of a single gene. Since humans inherit one set of chromosomes from each parent, every individual possesses two alleles for each gene. These two alleles occupy corresponding positions on the pair of chromosomes, determining the genetic makeup for that particular trait.
The combination of these two alleles defines the specific genotype, which can be described as either homozygous or heterozygous. A person is considered homozygous if they possess two identical alleles for a particular gene (e.g., RR or rr). Conversely, a heterozygous genotype occurs when the two inherited alleles for a trait are different (e.g., Rr).
The relationship between these alleles is often described as dominant or recessive. A dominant allele is expressed even if only one copy is present (Rr). A recessive allele, however, will only be expressed if two copies are present (rr), meaning its instructions are masked by the presence of a dominant allele. A heterozygous individual (Rr) will display the trait associated with the dominant allele.
Genotype and the Observable Trait
The observable, physical, or biochemical characteristics that result from a genotype are known as the phenotype. The phenotype is everything that can be seen or measured, including features like height, blood pressure, or the presence of a specific protein. The phenotype is the final, expressed result of the genetic instructions.
The relationship between the two is not always a simple one-to-one correspondence, as the environment plays a role in modifying the final outcome. A person may have the genetic potential (genotype) to be tall, but poor nutrition during childhood could result in a shorter actual height (phenotype).
When a trait is controlled by a single gene, the link between genotype and phenotype is more direct. For example, a dominant allele for dark hair color determines the phenotype, regardless of whether the genotype is homozygous dominant or heterozygous. However, for most complex traits like intelligence or weight, the phenotype is influenced by multiple genes acting in combination with environmental factors.
Real-World Examples of Genotypes
Simple Mendelian traits provide clear illustrations of how a specific genotype leads to a predictable phenotype. One example is the ability to bend the thumb backward, called “hitchhiker’s thumb,” which is a recessive trait. Using ‘h’ for the recessive allele and ‘H’ for the dominant allele (straight thumb), an individual with the genotype hh exhibits the hitchhiker’s thumb phenotype. Conversely, a person with the genotype HH or Hh will have a straight thumb phenotype, as the dominant allele masks the recessive one.
Another common illustration is the ABO blood group system, which involves multiple alleles and codominance. The gene for blood type has three possible alleles: $I^A$, $I^B$, and $i$. $I^A$ and $I^B$ are dominant over $i$. Genotypes $I^A I^A$ or $I^A i$ result in Type A blood, while $I^B I^B$ or $I^B i$ results in Type B blood. The genotype $i i$ results in the recessive Type O blood. If the genotype is $I^A I^B$, the person will have Type AB blood, because both dominant alleles are fully expressed simultaneously.