Evolution__3rd_Edition

36 PART 1 / Introduction
Figure 2.9
Recombination seen at the level
of: (a) chromosomes, (b) genes,
and (c) nucleotides. At
recombination, the strands of a
pair of chromosomes break at
the same point and the two
recombine. The postrecombinational
sequence of
genes, or nucleotides, combine
one strand from one side of the
break-point with the other
strand from the other side.
In (b) the gene sequence in
chromosome 1 changes from
ABC to ABc; in (c) the
nucleotide sequence of the
chromosome with bases A
and T (stippled nucleotides)
changes to A and A. (For the
nucleotide sequence only one
of the strands of the double helix
is shown: each of the pair of
chromosomes has a full double
helix with complementary base
pairs, as in Figure 2.2.)
(a) Recombination, at chromosome level
Twin
chromatids
1 2 3 4
(b) Recombination, at gene level
A locus B locus
Allele A Allele B
Chiasmata of
chromatids 1 and 3
1 2
Chromosomes
A B C D
Recombination
Allele a Allele b Allele c
A B c
a b C
(c) Recombination, at nucleotide level
1 2 3 4 1 2 3 4 1 2 3 4
C locus
Allele C
G G A G C T T G A T C A T C
Recombination
G G G G C T T G A A C A T C
G G A G C T T G A A C A T C
G G G G C T T G A T C A T C
Double helix, only one
strand shown
Chromosome 1
Chromosome 2
..