Biology STAAR Practice Test 2025 - Free State of Texas Assessments of Academic Readiness Practice Questions and Study Guide

The ABO blood type system is a classic example of multiple alleles because it consists of three alleles: A, B, and O. In this system, the presence of these alleles leads to four possible blood types: A, B, AB, and O. Each individual inherits one allele from each parent, which means that the combinations of these alleles dictate the individual's blood type.

For instance, if a person inherits an A allele from one parent and a B allele from the other, their blood type would be AB, demonstrating co-dominance where both alleles are expressed. Similarly, if an individual inherits two O alleles, their blood type will be O. The concept of multiple alleles is essential in understanding genetic diversity and how different traits are expressed in populations.

In contrast, focusing solely on a single blood type, such as A or O, does not highlight the presence of multiple alleles since these types are just specific outcomes of the ABO system. Type AB blood represents only one possible combination of alleles and does not encompass the entirety of the genetic variation provided by the three alleles.