What is an example of a single-gene variation?

A single-gene variation, also known as a Mendelian trait, is a trait that is influenced by a single gene, where specific alleles of that gene can lead to distinct phenotypes. The difference in color in the two-spot ladybug serves as a prime example of this. This color variation can often be traced back to specific alleles that affect pigmentation, demonstrating how a change in one gene can result in a noticeable change in phenotype.

The other options describe traits influenced by multiple genes or environmental factors, making them less clear examples of single-gene variations. For instance, the differences in size among species of birds or the variation in height among different populations of plants are typically polygenic traits, where multiple genes contribute to the overall characteristics observed. Similarly, behavioral differences among various animal species are usually complex traits influenced by both genetic makeup and environmental interactions, rather than a single gene. Thus, the clear and direct link between a single gene and the observable color variation in the two-spot ladybug confirms it as an example of single-gene variation.