A genotype refers to an organism’s unique genetic code, containing instructions inherited from parents. This internal blueprint determines many aspects of an organism’s biology. Understanding genotypes provides insight into why individuals exhibit a wide range of traits and how these characteristics are passed down through generations. This genetic makeup remains constant throughout an individual’s life.
Genotype Versus Phenotype
The concept of genotype is distinct from phenotype. Genotype is the specific combination of alleles an organism possesses for a particular gene. In contrast, phenotype refers to the observable physical characteristics or traits that result from the genotype’s expression and environmental influences. For instance, eye color is a phenotype, while the specific genetic code dictating that color is the genotype.
A genotype is an internal genetic code and cannot be observed directly. Phenotypes, however, are visible and measurable attributes, such as height, hair color, or even behavioral traits. While genotype provides the instruction manual, the phenotype is the actual product built from those instructions, often modified by external factors. Organisms with identical phenotypes might possess different genotypes.
The Blueprint: How Genotypes are Written
Genotypes are symbolically represented to illustrate the genetic makeup for specific traits. Each gene has different versions, known as alleles, inherited from each parent. These alleles are denoted using letters, where a capital letter signifies a dominant allele and a lowercase letter represents a recessive allele. For example, ‘A’ might represent a dominant trait, while ‘a’ represents its recessive counterpart.
When an organism inherits two identical alleles for a gene, its genotype is described as homozygous. This can be homozygous dominant (e.g., ‘AA’) if both alleles are dominant, or homozygous recessive (e.g., ‘aa’) if both are recessive. If an organism inherits two different alleles for a gene, with one dominant and one recessive, its genotype is termed heterozygous (e.g., ‘Aa’). In a heterozygous genotype, the dominant allele’s trait will be expressed, masking the presence of the recessive allele.
Passing On the Code: Genotype Inheritance
Genotypes are transmitted from parents to offspring through specialized reproductive cells called gametes. During their formation, gametes receive only one allele for each gene from the parent’s pair. When fertilization occurs, a gamete from one parent combines with a gamete from the other, forming a new individual with a unique combination of alleles. This means offspring inherit half of their genetic information from each parent.
The probability of offspring inheriting specific genotype combinations can be visualized using a Punnett square. This diagram shows all possible allele pairings from the parents, allowing for the prediction of potential genotypes in the next generation. For instance, if both parents are heterozygous (Aa), a Punnett square can illustrate the 25% chance of homozygous dominant (AA), 50% chance of heterozygous (Aa), and 25% chance of homozygous recessive (aa) genotypes in their offspring. This tool highlights the probabilistic nature of genetic inheritance and how variations arise across generations.
Genotypes in Action: Real-World Examples
Genotypes determine many observable traits in living organisms. For example, human blood types are determined by specific genotypes involving the A, B, and O alleles. An individual with an ‘AA’ or ‘AO’ genotype will have A blood type, while ‘BB’ or ‘BO’ results in B blood type, and ‘AB’ leads to AB blood type. Only the ‘OO’ genotype results in O blood type.
Another example is the presence or absence of dimples. If the allele for dimples is dominant (D) and the allele for no dimples is recessive (d), a person with genotypes ‘DD’ or ‘Dd’ will have dimples. Conversely, only individuals with the ‘dd’ genotype will lack dimples. Similarly, for earlobe attachment, a dominant allele for unattached earlobes and a recessive allele for attached earlobes dictate the phenotype.