Heredity, the process by which characteristics are passed from one generation to the next, fundamentally shapes life on Earth. Genetics, the study of this transmission, explains why organisms resemble their parents yet display variations. It forms the blueprint for development, function, and reproduction, revealing the instructions that govern life’s attributes.
Defining Genetic Building Blocks
The fundamental unit of heredity is called a gene. A gene is a specific segment of deoxyribonucleic acid (DNA) that carries the instructions for a particular trait or characteristic, such as eye color or blood type. These genes are organized and located on larger structures within the cell’s nucleus, known as chromosomes. Each chromosome contains numerous genes arranged along its length.
Different versions of the same gene are called alleles. For instance, the gene for eye color can have an allele for brown eyes and another allele for blue eyes, accounting for the variations seen in a population. The specific combination of alleles an individual possesses for a particular gene is referred to as their genotype. This genetic makeup represents the inherited instructions, distinct from the observable physical trait, which is known as the phenotype.
Counting Alleles in a Genotype
For most organisms, including humans, a genotype for a given gene typically consists of two alleles. These two alleles occupy the same specific position, or locus, on homologous chromosomes. Homologous chromosomes are pairs of chromosomes that are similar in structure and carry genes for the same traits.
The Role of Inheritance
The presence of two alleles in a genotype stems from the process of sexual reproduction and the diploid nature of many organisms. Diploid organisms, such as humans, have cells that contain two complete sets of chromosomes. One set is inherited from the maternal parent, and the other set comes from the paternal parent.
During the formation of reproductive cells, called gametes (sperm and egg), a specialized cell division process occurs. This process ensures that each gamete receives only one chromosome from each homologous pair, and consequently, only one allele for each gene. When fertilization takes place, a sperm and an egg combine, restoring the diploid state in the offspring.
Common Allele Combinations
The two alleles that make up a genotype can combine in different ways. When an individual inherits two identical alleles for a specific gene, their genotype is described as homozygous. This can occur if both inherited alleles are dominant (e.g., represented as ‘AA’) or both are recessive (e.g., represented as ‘aa’). In a homozygous genotype, the trait associated with those identical alleles will be expressed.
Alternatively, if an individual inherits two different alleles for a particular gene, their genotype is termed heterozygous. This combination is often represented with one dominant and one recessive allele (e.g., ‘Aa’). In a heterozygous genotype, the dominant allele typically masks the effect of the recessive allele, meaning only the dominant trait is outwardly observed. Even though the recessive trait is not expressed, the individual still carries the recessive allele and can pass it on to future generations.