All living organisms possess a unique set of characteristics, known as traits, which are inherited from their parents. These traits are determined by genes, which are segments of DNA carrying the instructions for building and operating an organism. An organism’s genetic makeup, or the specific combination of genes it possesses, is called its genotype. The observable physical or biochemical characteristics that result from this genetic blueprint are referred to as its phenotype.
Understanding Heterozygous
Organisms that reproduce sexually typically inherit two copies of each gene, with one copy coming from each parent. These gene copies are known as alleles, and they can be identical or different versions of the same gene. A heterozygous state occurs when an individual inherits two different alleles for a specific gene. This means that while both alleles contribute to the genetic information, they carry slightly varied instructions. In contrast, a homozygous individual has two identical alleles for a particular gene.
How Heterozygous Traits Appear
The appearance of a heterozygous trait, or its phenotype, depends on how the two different alleles interact. In many cases, one allele exhibits complete dominance over the other. This means that the dominant allele’s characteristic will be fully expressed in the phenotype, completely masking the recessive allele. A heterozygous individual will display the same trait as someone with two copies of the dominant allele.
Incomplete dominance occurs when neither allele is completely dominant, and the heterozygous phenotype appears as an intermediate blend of the two homozygous phenotypes. For example, if one allele produces red pigment and another produces white, a heterozygous individual might exhibit a pink color.
Codominance occurs when both alleles are expressed equally and distinctly in the heterozygous individual. Rather than blending, the characteristics of both alleles are simultaneously visible in the phenotype. This means that both traits associated with the individual alleles are present, not as a mixture, but as separate and identifiable components.
Real-World Examples of Heterozygous Traits
A common example of complete dominance in humans involves eye color, where the allele for brown eyes is dominant over the allele for blue eyes. An individual inheriting one allele for brown eyes and one for blue eyes (heterozygous) will have brown eyes. Similarly, certain genetic conditions like Huntington’s disease are inherited through a dominant allele, meaning a person with one copy of the affected allele and one normal allele will develop the condition.
In plants, incomplete dominance is illustrated by the snapdragon flower. If a snapdragon plant inherits one allele for red flowers and another for white flowers, the resulting heterozygous plant will produce pink flowers.
Human blood types provide a clear example of codominance, particularly with the ABO blood group system. An individual with an ‘A’ allele and a ‘B’ allele (heterozygous) will have AB blood type, meaning both A and B antigens are present on their red blood cells. Another instance can be observed in roan cattle, where a heterozygous animal possesses both red and white hairs appearing together, not as a blend, but as distinct patches of color.