What does it mean if an organism is heterozygous?

When an organism is described as heterozygous, it means that it possesses two different alleles at the same locus on homologous chromosomes. For example, if we consider a gene that governs flower color, a heterozygous organism might have one allele for red petals and another allele for white petals. This genetic diversity at the locus allows for a combination of traits that can influence an organism's phenotype, possibly leading to traits that are intermediate between those of the two alleles, or to the expression of one allele over the other (dominance).

In contrast, the alternatives emphasize different genetic configurations. Having identical alleles at loci refers to a homozygous state, which contradicts the definition of heterozygosity. The option regarding lacking variation suggests an absence of genetic diversity, which does not apply to heterozygous individuals, as they inherently have variances in their alleles. Lastly, being completely homozygous reinforces the notion of having identical alleles, further distinguishing it from heterozygosity. Understanding these distinctions is crucial for grasping fundamental genetic concepts.