Page 57 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

3.1 Extensions to Mendel for Single-Gene Inheritance 49

Figure 3.5 ABO blood types are determined by three second allele. In other words, dominance relations are
A
alleles of one gene. (a) Six genotypes produce the four blood unique to a pair of alleles. In our example, I is completely
B
group phenotypes. (b) Blood serum contains antibodies against dominant to i, but it is codominant with I . Given these
foreign red blood cell molecules. (c) If a recipient’s serum has dominance relations, the six genotypes possible with I , I ,
B
A
antibodies against the sugars on a donor’s red blood cells, the and i generate four different phenotypes: blood groups A,
blood types of recipient and donor are incompatible, and
coagulation of red blood cells will occur during transfusions. In this B, AB, and O. With this background, you can understand
table, a plus (+) indicates compatibility, and a minus (−) indicates how a type A and a type B parent could produce a type O
B
A
incompatibility. Antibodies in the donor’s blood usually do not cause child: The parents must be I i and I i heterozygotes, and
problems because the amount of transfused antibody is small. the child receives an i allele from each parent.
(a) Corresponding Phenotypes: An understanding of the genetics of the ABO system
Genotypes Type(s) of Molecule on Cell has had profound medical and legal repercussions. Match-
A A
I I A ing ABO blood types is a prerequisite of successful blood
A
I i transfusions, because people make antibodies to foreign
blood cell molecules. A person whose cells carry only A
B B
I I B molecules, for example, produces anti-B antibodies; B peo-
B
I i ple manufacture anti-A antibodies; AB individuals make
A B
I I AB neither type of antibody; and O individuals produce both
anti-A and anti-B antibodies (Fig. 3.5b). These antibodies
ii O cause coagulation of cells displaying the foreign molecules
(Fig. 3.5c). As a result, people with blood type O have his-
torically been known as universal donors because their red
(b) Blood Type Antibodies in Serum blood cells carry no surface molecules that will stimulate
A Antibodies against B an antibody attack in a transfusion recipient. In contrast,
B Antibodies against A people with blood type AB are considered universal re-
cipients, because they make neither anti-A nor anti-B anti-
AB No antibodies against A or B
bodies, which, if present, would target the surface molecules
O Antibodies against A and B of incoming blood cells.
Information about ABO blood types can also be used
(c) Blood Type Donor Blood Type (Red Cells) as legal evidence in court, to exclude the possibility of pa-
of Recipient A B AB O ternity or criminal guilt. In a paternity suit, for example, if
the mother is type A and her child is type B, logic dictates
A + – – +
that the I allele must have come from the father, whose
B
B – + – + A B B B B
genotype may be I I , I I , or I i. In 1944, the actress Joan
AB + + + + Barry (phenotype A) sued Charlie Chaplin (phenotype O)
O – – – + for support of a child (phenotype B) whom she claimed he
fathered. The scientific evidence indicated that Chaplin
could not have been the father, since he was apparently ii
Note in Fig. 3.5a that the A phenotype can arise from and did not carry an I allele. This evidence was admissible
B
A
two genotypes, I I or I i. The same is true for the B blood in court, but the jury was not convinced, and Chaplin had
A A
B B
B
type, which can be produced by I I or I i. But a combina- to pay. Today, the molecular genotyping of DNA (DNA
tion of the two alleles I I generates blood type AB. fingerprinting, see Chapter 11) provides a powerful tool to
A B
We can draw several conclusions from these observa- help establish paternity, guilt, or innocence, but juries still
tions. First, as already stated, a given gene may have more often find it difficult to evaluate such evidence.
than two alleles, or multiple alleles; in our example, the
B
A
series of alleles is denoted I , I , and i.
Second, although the ABO blood group gene has three Lentil seed coat patterns
alleles, each person carries only two of the alternatives— Lentils offer another example of multiple alleles. A gene
A A
B B
A
B
I I , I I , I I , I i, I i, or ii. Thus six possible ABO geno- for seed coat pattern has five alleles: spotted, dotted, clear
A B
types exist. Because each individual carries no more than (pattern absent), and two types of marbled. Reciprocal
two alleles for each gene, no matter how many alleles are in crosses between pairs of pure-breeding lines of all patterns
a series, Mendel’s law of segregation remains intact, be- (marbled-1 × marbled-2, marbled-1 × spotted, marbled-2 ×
cause in a sexually reproducing organism, the two alleles of spotted, and so forth) have clarified the dominance relations
a gene separate during gamete formation. of all possible pairs of the alleles to reveal a dominance
Third, an allele is not inherently dominant or recessive; series in which alleles are listed in order from most dominant
its dominance or recessiveness is always relative to a to most recessive. For example, crosses of marbled-1 with