So far we have only examined characters with two traits that are controlled by two alleles. This is easy to visualize because diploid organisms can only have two alleles. Within a population, however, more than two alleles can exist (although any given individual still only has two alleles).
The human ABO blood group provides an example of multiple alleles , and the structure of the cell surface antigens for the three blood type alleles is shown in this figure. There are four possible blood types for this gene: A, B, AB, and O. The letters refer to two specific carbohydrate molecules on the surface of red blood cells. Individuals can have the A carbohydrate (blood type A), the B carbohydrate (blood type B), both the A and B carbohydrates (blood type AB), or neither carbohydrate (blood type O).
Please note that the molecules depicted above are not the same as those in the MN blood groups. In fact, MN and ABO are just two of nearly thirty different blood group systems. Collectively, over 990 alleles at approximately 39 genes control for the different surface proteins which determine the compatibility of blood type.
The ABO blood groups are formed by various combinations of three different alleles; IA (codes for carbohydrate A), IB (codes for carbohydrate B), and i (codes for the lack of any carbohydrate). This figure shows the relationship between genotype and phenotype for the ABO alleles.
A Punnett square can be used to predict the genotype frequencies resulting from multiple allele crosses. However, one cannot be certain of an individual's genotype if they are blood type A or B because there are two possible genotypes for each of these blood types. Therefore, many cross problems that examine blood types are similar to test crosses; that is, the parental genotype is uncertain. A few examples will aid in your understanding.
At the following Web sites, find the correct answer to the multiple-choice monohybrid cross questions. Work out each problem. To view an explanation of the problem, select the "TUTORIAL" button. After viewing the correct answer, close the Monohybrid Cross Problem Set window to return to this page. (These sites are a part of the Monohybrid Problem Sets provided by The Biology Project at the University of Arizona.)
Problem 11 : Co-dominant alleles: The Human ABO markers - This problem is a part of the Monohybrid Cross Problem Set.
Problem 13 : Predicting human blood types - This problem is also a part of the Monohybrid Cross Problem Set.