Genes and gene families - Human Gene Evolution - page 142

protein only evolved later (and independently) in birds, eutherian mammals and
in some marsupials (Schreiber and Richardson, 1997).
Troponin I, together with troponins C and T comprise the three subunits of the
troponin complex of the thin filaments of vertebrate striated muscle. Since inver-
tebrates and ascidians possess a single troponin I gene (Hastings, 1997), at least
two duplications must have taken place during the vertebrate lineage to generate
the three troponin I genes evident in the human genome ( TNNI1 , 1q31; TNNI2 ,
11p15; TNNI3 , 19q13).
The human ABO blood group gene ( ABO ; 9q34) has a common evolutionary
origin with the chromosomally linked and now inactive

-1,3-galactosyltrans-
ferase ( GGTA1 ; 9q33-q34; see Chapter 6, Section 6.2) gene; the two genes
emerged by duplication and divergence about 400 Myrs ago during the evolution
of the early vertebrates (Saitou and Yamamoto, 1997).
Olfactory receptors are extremely important to mammals; dogs, rats and mice
may have as many as 1000 genes encoding them. Although the human genome may
contain fewer functional genes than other mammals, pseudogenes abound. Indeed,
it has been estimated that >0.1% of the human genome is composed of olfactory
receptor genes and pseudogenes dispersed between at least 13 different chromo-
somes (Trask et al ., 1998). [The classification of this gene family is still in its
infancy but known members include OR1A1 , 17p13; OR1D2 , OR1D4 , OR1D5 ,
17p13; OR1E1 , OR1E2 , 17p13; OR1F1 , 16p13; OR1G1 , 17p13; OR2D2 , 11p15;
OR3A1 , OR3A2 , OR3A3 , 17p13; OR5D3 , OR5D4 , 11q12; OR5F1 , 11q12;
OR6A1 , 11p15; OR10A1 , 11p15]. The origin and emergence of the present-day
size of the olfactory receptor gene family probably preceded the divergence of the
mammals (Ben-Arie et al ., 1993; Buettner et al ., 1998; Issel-Tarver and Rine, 1997).
However, at least in the primates, the olfactory receptor genes appear to have still
been in a considerable state of flux with numerous translocations, duplications and
deletions having occurred during the evolution of the great apes (Trask et al ., 1998;
see section 4.2.3, Olfactory receptor genes ).

4.1.5 Human genes whose origin preceded the divergence of the
vertebrates

One example of a human gene whose origin preceded the advent of the vertebrates
~500 Myrs ago is the c- myc proto-oncogene (Atchley and Fitch, 1995). Present in
all vertebrates, a homologue of the human gene ( MYC ; 8q24) is detectable in
echinoderms but not in Caenorhabditis or Drosophila (Walker et al ., 1992). Other
examples include the insulin-like growth factor genes ( IGF1 and IGF2 ; 12q22-q24
and 11p15.5 respectively) which emerged during the evolution of the protochor-
dates more than 600 Myrs ago (McRory and Sherwood, 1997; see Section 4.2.3).

4.1.6 Human genes whose origin preceded the divergence of the metazoa

Human genes with homologues in the fruitfly, Drosophila , must have originated
prior to the divergence of the deuterostomes from the protostomes ~700 Myrs
ago (Doolittle et al ., 1996; Ayala et al ., 1998). A large number of Drosophila genes
have been shown to have human homologues and vice versa (see FlyBase at