Genetics is the study of heredity, exploring how characteristics are passed from parents to offspring. Understanding this process involves specific terminology, and this article clarifies the fundamental term “homozygous.”
Understanding Homozygous: The Core Definition
A gene is a basic unit of heredity made of DNA, providing instructions for making proteins or influencing traits. Humans inherit two copies of each gene, one from each biological parent. Different versions of the same gene are called alleles; for example, a gene for eye color might have an allele for brown eyes and another for blue eyes. The term “homozygous” describes a genetic condition where an individual has inherited two identical alleles for a particular gene. These identical alleles are located at the same position, known as a locus, on homologous chromosomes, which are pairs of chromosomes (one from each parent) that carry genes for the same traits at corresponding loci.
Homozygous in Action: Examples and Trait Expression
When an individual is homozygous for a gene, the trait associated with those identical alleles will be expressed, depending on whether the alleles are dominant or recessive. For instance, if someone inherits two dominant alleles for a trait, they are considered homozygous dominant. An example is having two alleles for brown eyes (often represented as ‘BB’), which results in brown eyes.
Conversely, if an individual inherits two recessive alleles, they are homozygous recessive. A recessive trait only appears when both alleles are recessive, as there is no dominant allele to mask its effect. For example, blue eye color is a recessive trait; therefore, a person with blue eyes is always homozygous recessive for the eye color gene (represented as ‘bb’). Some genetic conditions, such as cystic fibrosis or sickle cell anemia, are expressed only when an individual is homozygous for specific recessive mutated alleles.
Homozygous vs. Heterozygous: A Key Distinction
To understand homozygous, it is helpful to contrast it with heterozygous. While homozygous refers to having two identical alleles for a gene, heterozygous means having two different alleles for a particular gene. An individual inherits one allele from each parent, leading to this combination. For example, a heterozygous individual might have one allele for brown hair and one for blonde hair. In a heterozygous genotype, the dominant allele typically masks the effect of the recessive allele. This means the dominant trait will be observed, even though the recessive allele is still present. For example, a person with one brown eye allele (dominant) and one blue eye allele (recessive) will have brown eyes, demonstrating the dominant allele’s influence.