Genetics is the study of heredity, exploring how characteristics are passed from parents to offspring. At the core of this process lies deoxyribonucleic acid, or DNA, which serves as the instruction manual for all living organisms. These instructions are organized into functional units called genes, which dictate specific traits. Genes are located on chromosomes, thread-like structures found within the nucleus of nearly every cell.
Understanding Alleles
A gene can exist in different versions, called an allele. These variations arise from slight differences in the DNA sequence at a particular location. For most genes, an individual inherits two alleles, one from each parent. Each allele occupies a specific, fixed position on a chromosome, known as a gene locus. This paired inheritance means that for every trait, an organism carries two copies of the gene, which may be identical or different.
Homozygous and Heterozygous Alleles
The terms homozygous and heterozygous describe the combination of alleles an individual possesses for a particular gene. When an individual inherits two identical alleles for a specific gene, they are considered homozygous, such as two copies of a dominant allele (often represented as “AA”) or two copies of a recessive allele (“aa”). Conversely, an individual is heterozygous when they inherit two different alleles for a specific gene, represented as “Aa,” indicating one dominant and one recessive allele. In a heterozygous state, their interaction determines which trait is expressed.
How Genotypes Influence Traits
An organism’s genetic makeup, the combination of alleles it carries, is called its genotype. This genotype directly influences its phenotype, the observable characteristics or traits. The relationship between alleles often involves dominance, where one allele can mask the effect of another. A dominant allele expresses its trait even when only one copy is present in a heterozygous individual, while a recessive allele only expresses its trait when two copies are present. A heterozygous individual will display the dominant trait, while still carrying the unexpressed recessive allele.
Common Examples in Genetics
Earlobe attachment in humans is a simple Mendelian trait, with unattached earlobes typically being a dominant trait. A person with two alleles for unattached earlobes is homozygous dominant. If they have one allele for unattached and one for attached earlobes, they are heterozygous and will still have unattached earlobes. Only individuals with two alleles for attached earlobes are homozygous recessive and display that trait. Eye color, while more complex, also offers examples where blue eyes are a recessive trait, requiring two copies of the blue-eye allele to be expressed.