Page 20 - Applied Probability

1. Basic Principles of Population Genetics
they end up in regions of different color in a gamete. This event occurs
whenever the two loci are separated by an odd number of crossovers along
the gamete. Chapter 12 will elaborate on this brief, simplified description
of the recombination process.
3
1 2

A O
A 1 /A 1 A 2 /A 2

3 4

A O
A 1 /A 2 A 2 /A 2

5

O
A 1 /A 2

FIGURE 1.1. A Pedigree with ABO and AK1 Phenotypes

As a concrete example, consider the locus AK1 (adenylate kinase 1) in
the vicinity of ABO on chromosome 9. With modern biochemical techniques
it is possible to identify two codominant alleles, A 1 and A 2 , at this enzyme
locus. Figure 1.1 depicts a pedigree with phenotypes listed at the ABO locus
and unordered genotypes listed at the AK1 locus. In this pedigree, as in
all pedigrees, circles denote females and squares denote males. Individuals
1, 2, and 4 are termed the founders of the pedigree. Parents of founders
are not included in the pedigree. By convention, each nonfounder or child
of the pedigree always has both parents included.
Close examination of the pedigree shows that individual 3 has alleles A
and A 1 on his paternally derived chromosome 9 and alleles O and A 2 on
his maternally derived chromosome 9. However, he passes to his child 5 a
chromosome with O and A 1 alleles. In other words, the gamete passed is
recombinant between the loci ABO and AK1. On the basis of many such
observations, it is known empirically that doubly heterozygous males like
3 produce recombinant gametes about 12 percent of the time. In females
the recombination fraction is about 20 percent.
The pedigree in Figure 1.1 is atypical in several senses. First, it is quite
simple graphically. Second, everyone is phenotyped; in larger pedigrees,
some people will be dead or otherwise unavailable for typing. Third, it is
constructed so that recombination can be unambiguously determined. In
most matings, one cannot directly count recombinant and nonrecombinant