Evolution__3rd_Edition

..
For linked genes, inheritance
depends on recombination ...
. . . and Mendelian ratios can be
calculated
CHAPTER 2 / Molecular and Mendelian Genetics 37
The offspring ratios for other crosses can be worked out by the same principle. The
segregation of unlinked genotypes is called independent segregation.
When the loci are linked on the same chromosome, they do not segregate independently.
At meiosis, when haploid gametes are formed from a diploid adult, an additional
process called recombination occurs. The pairs of chromosomes physically line
up and, at certain places, their strands join together and recombine (Figure 2.9).
Recombination shuffles the combinations of genes. If an individual inherited AB from
its mother and ab from its father, and recombination occurred between the two loci, it
will produce Ab and aB gene combinations in its gametes.
Recombination is a random process; it may or may not “hit” any point in the DNA.
It occurs with a given probability, usually symbolized by r, between any two points on a
chromosome. r can be defined between nucleotide sites or genes. If the A locus and the
B locus are linked, the chance of recombination between them in an individual is r and
the chance of no recombination is (1 − r). In any one individual, recombination either
does or does not happen, but the chance of recombination determines the frequencies
of genotypes in the gametes produced by a population. If we consider a large number of
AB/ab individuals, they will produce gametes in the following proportions:
Gamete AB Ab aB ab
Proportion 1 /2(1 − r) 1 /2r 1 /2r 1 /2(1 − r)
These fractions can be used in the standard way to calculate the Mendelian ratio for a
cross involving an AB/ab individual. The principle is logically easy to understand, but
the ratios can be laborious to work out in practice. The case of independent segregation
corresponds to r = 0.5. That is, when the A and B loci are on separate chromosomes,
r = (1 − r) = 0.5 and the Ab/aB parent produces Ab, aB, AB, and ab gametes in the ratio
1:1:1:1. For genes on the same chromosome, the value of r ranges from just above 0
for two sites that are next to each other up to 0.5 for two sites at opposite ends of the
chromosome.
For any two genes, recombination can “hit” more than once between them in an
individual (this is called “multiple hits”). If two recombinational hits occur between a
pair of loci, they cancel each other. The chromosome has the same combination of
genes at these two loci as if recombination had not occurred. It is more exact to say that
the probability of recombination r equals the probability of an odd number of hits, and
the probability (1 − r) is the chance of no hits plus the chance of an even number of hits.
The Mendelian ratios, in which paired diploid genes segregate into haploid gametes
and the gametes of different individuals then combine at random, is the basis of all the
theory of population genetics that is discussed in Chapters 5–9.
2.9 Darwin’s theory would probably not work if there was
a non-Mendelian blending mechanism of heredity
As Chapter 1 described, Mendel’s theory of heredity plugged a dangerous leak in
Darwin’s original theory, and the two theories together eventually came to form the