Biology Reference
In-Depth Information
species. This approach allows the rapid construction of chromosome maps from
the primate species in question which can reveal cytogenetic homologies with the
human karyotype (Wienberg and Stanyon, 1997). By these means, a high degree
of synteny has been found between human and baboon chromosomes (Rogers
et
al
., 1995). By contrast, numerous translocations are apparent in the gibbon
(
Hylobates lar
, 2n = 44) genome as compared to human and the great apes (Jauch
et al
., 1992; Arnold
et al
., 1996); the 22 human autosomes have to be divided into
51 elements in order to recombine them into the 21 gibbon autosomes. Similarly,
in the Concolor gibbon (
Hylobates concolor
, 2n = 52), the 22 human autosomes
have to be divided into 63-67 segments in order to recombine them into the 25
gibbon autosomes (Koehler
et al
., 1995).
Despite the degree of genetic similarity between the great apes, the chim-
panzee genome is approximately 10% larger than that of the human or gorilla
(Pellicciari
et al
., 1982). Nevertheless, the structure of the human and chim-
panzee genomes is highly conserved at both chromosomal and sub-chromosomal
levels (Jauch
et al
., 1992; Ried
et al
., 1993). Even at lower resolution, the extent of
evolutionary conservation is readily apparent. Human microsatellite DNA
sequences are sufficiently well conserved in chimpanzees that human PCR
primers can be used to amplify (CA)
n
repeats in the chimpanzee (Blanquer-
Maumont and Crouau-Roy, 1995; Deka
et al
., 1994; Garza
et al
., 1995).
Differences in microsatellite allele length between humans and other primates
have however been noted (Ellegren
et al
., 1995; Garza and Freiner, 1996;
Rubinztein
et al
., 1995). Crouau-Roy
et al
. (1996) studied microsatellites within a
30 cM region of human chromosome 4p and found that all informative loci
which are linked in human were also linked in the chimpanzee, indicating that
evolutionary conservation extends to the locus level. In general, heterozygosity
was found to be greater in chimpanzees, a reflection perhaps of the greater
genetic diversity in chimpanzee populations. Some loci, however, appeared to be
less heterozygous than in human, a phenomenon that appears to be caused by
interruptions of the repeat elements at these loci.
Human chromosomes are not always identical. Indeed many exhibit hetero-
morphism, especially in the centromeric and satellite regions of the acrocentric
chromosomes. Chromosomes 1, 9, 13, 14, 15, 16, 19, 21, 22, and Y are the most het-
eromorphic whilst chromosomes 2-8 and X are the least heteromorphic (Park
et
al
., 1998; Samonte
et al
., 1996; Trask
et al
., 1989a). Inter-chromosomal variation
can be substantial; two homologues of chromosome 21 having been noted to vary
in healthy individuals by as much as 21 Mb or 40% of the length of the chromo-
some (Trask
et al
., 1989a). Family studies have shown that such heteromorphisms
are not artefactual and can be inherited in mendelian fashion (Trask
et al
., 1989b).
It would appear that this variation can be largely ascribed to variation in the size
of repeat sequence arrays and probably results from unequal crossing over
between different classes of repetitive element. Bivariate flow karyotyping has
been used to study the relative DNA content of homologous chromosome pairs in
individuals from different racial groups (Mefford
et al
., 1997). Significant varia-
tion in DNA content, ranging from 10 to 40%, was found for chromosomes 1, 13,
14, 15, 16, 19, 21, 22, and Y. However, the spectrum of variation observed in the
different racial groups was very similar.
