Most of the genetic systems that we have examined so far consist of two alleles. In Mendel’s peas, for instance, one allele encoded round seeds and another encoded wrinkled seeds; in cats, one allele produced a black coat and another produced a gray coat. For some loci, more than two alleles are present within a group of individuals—the locus has multiple alleles (which may also be referred to as an allelic series). Although there may be more than two alleles present within a group of organisms, the genotype of each individual diploid organism still consists of only two alleles. The inheritance of characteristics encoded by multiple alleles is no different from the inheritance of characteristics encoded by two alleles, except that a greater variety of genotypes and phenotypes is possible.

THE ABO BLOOD GROUP A multiple-allele system is found at the locus for the ABO blood group, which determines your ABO blood type. This locus, like the MN locus, encodes antigens on red blood cells. The three common alleles for the ABO blood group locus are I^A, which encodes the A antigen; I^B, which encodes the B antigen; and i, which encodes no antigen (O). We can represent the dominance relations among the ABO alleles as follows: I^A > i, I^B > i, I^A = I^B. The I^A and the I^B alleles are dominant over i and are codominant with each other; the AB phenotype is due to the presence of an I^A allele and an I^B allele, which results in the production of A and B antigens on red blood cells. A person with genotype ii produces neither antigen and has blood type O. The six common genotypes at this locus and their phenotypes are shown in Figure 4.16a.

The body produces antibodies against any foreign antigens (see Figure 4.16a). For instance, a person with blood type A produces anti-B antibodies because the B antigen is foreign to that person. A person with blood type B produces anti-A antibodies, and a person with blood type AB produces neither anti-A nor anti-B antibodies because neither A nor B antigen is foreign to that person. A person with blood type O possesses no A or B antigens; consequently, that person produces both anti-A antibodies and anti-B antibodies. The presence of antibodies against foreign ABO antigens means that successful blood transfusions are possible only between persons with certain compatible blood types (Figure 4.16b).

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The inheritance of alleles at the ABO locus is illustrated by a paternity suit against the movie actor Charlie Chaplin. In 1941, Chaplin met a young actress named Joan Barry, with whom he had an affair. The affair ended in February 1942, but 20 months later, Barry gave birth to a baby girl and claimed that Chaplin was the father. Barry then sued for child support. At this time, blood typing had just come into widespread use, and Chaplin’s attorneys had Chaplin, Barry, and the child blood typed. Barry had blood type A, her child had blood type B, and Chaplin had blood type O. Could Chaplin have been the father of Barry’s child?

Your answer should be no. Joan Barry had blood type A, which can be produced by either genotype I^AI^A or genotype I^Ai. Her baby possessed blood type B, which can be produced by either genotype I^BI^B or genotype I^Bi. The baby could not have inherited the I^B allele from Barry (Barry could not carry an I^B allele if she were blood type A); therefore, the baby must have inherited the i allele from her. Barry must have had genotype I^Ai, and the baby must have had genotype I^Bi. Because the baby girl inherited her i allele from Barry, she must have inherited the I^B allele from her father. Having blood type O, produced only by genotype ii, Chaplin could not have been the father of Barry’s child. Although blood types can be used to exclude the possibility of paternity (as in this case), they cannot prove that a person is the parent of a child, because many different people have the same blood type.

In the course of the trial to settle the paternity suit against Chaplin, three pathologists testified that it was genetically impossible for Chaplin to have fathered the child. Nevertheless, the jury ruled that Chaplin was the father and ordered him to pay child support and Barry’s legal expenses. TRY PROBLEM 32

COMPOUND HETEROZYGOTES Different alleles often give rise to the same phenotype. For example, cystic fibrosis, as we saw earlier in this chapter, arises from defects in alleles at the CFTR locus, which encodes a protein that controls the movement of chloride ions into and out of the cell. Over a thousand different alleles at the CFTR locus that can cause cystic fibrosis have been discovered worldwide. Because cystic fibrosis is an autosomal recessive condition, one must normally inherit two defective CFTR alleles to have cystic fibrosis. In some people with cystic fibrosis, these two defective alleles are identical, meaning that the person is homozygous. Other people with cystic fibrosis are heterozygous, possessing two different defective alleles. An individual who carries two different alleles at a locus that result in a recessive phenotype is referred to as a compound heterozygote.