Biology Reference
In-Depth Information
5'
A
B
B
S
Number of B + B
S
repeats
8
7
7
8
9
10
Whites
3'
Blacks
Figure 8.18.
Polymorphism of the repeat pattern of the late region of the M segment of
human involucrin. Each column of rectangular boxes indicates an allele, and each box
denotes a 10-codon repeat. There are three different kinds of repeat: A (diagonal lines), B
(shaded) and B
s
(chequered) which differ with respect to the sequences of the first three
codons (AAGCACCCG, GAGCTCCCA, and GAGCTCTCT respectively). The two
columns on the left denote the variable number of B
s
repeats in whites whilst the four
columns on the right denote the variable numbers of B and B
s
repeats found in blacks.
Arrowheads indicate the position of the hotspots for repeat addition in the white and
black populations (redrawn from Green and Djian, 1992).
regions of the M segment although these were relatively few in number as com-
pared to the additions (Green and Djian, 1992). These deletions have occurred
independently of the vectorial process of repeat addition both spatially and tem-
porally (Green and Djian, 1992).
In addition to the inter-specific differences in the size of the involucrin mole-
cule, the higher primates also exhibit size polymorphisms which result from vari-
ation in the number of repeats in the late region. In humans, the polymorphism
comprises variable numbers of B and B
S
repeats within the late region (
Figure
8.18
). The most common allele in white Caucasians contains nine repeats (3B
S
,
5B, 1A) but a second 'Mormon' allele (2B
S
, 5B, 1A) has also been described (Simon
et al
., 1989; Urquhart and Gill, 1993). Blacks possess the 'Mormon' allele plus
three other alleles containing 6, 7, or 9 B repeats (Simon
et al
., 1991; Urquhart and
Gill, 1993;
Figure 8.18
). Repeat number polymorphisms in the late region have
also been reported in
Aotus trivirgatus
,
Macacca mulatta
and
Gorilla gorilla
(Green
and Djian, 1992). The repeat pattern observed in anthropoid apes is explicable in
terms of the presence in the involucrin gene of a hotspot at which repeats are gen-
erated by unequal recombination (Green and Djian, 1992). In the human lineage,
the location of this hotspot varies between racial groups such that in whites it is
located within the B
S
repeat region whilst in blacks, it is within the B repeat
region (
Figure 8.18
). Thus, although the process of vectorial repeat addition has
operated in both blacks and whites, there has been a difference between these
racial groups in the sites of repeat addition, a finding that is consistent with an
endogenously controlled mechanism.
Attempts have been made to relate the evolution of a larger involucrin molecule
with a different repeat structure to the trend in anthropoid apes towards relative
