Biology Reference
In-Depth Information
2
Evolution of the human
genome
2.1 Ancient genome duplications at the dawn of vertebrate
evolution
2.1.1 Evidence for an ancient genome duplication
In evolution, novel genes have arisen either by whole genome duplication or by
regionally localized duplication events (see Chapter 9, section 9.5). Both mecha-
nisms give rise to paralogous genes, genes that occur within the same species and
which have a common ancestor. Thus, members of multigene families and super-
families are paralogous and are distinct from orthologous genes which are found in
different species and have diverged from their common ancestor over evolution-
ary time. In practice, the consequences of whole genome duplication and region-
ally localized gene duplication are often very hard to distinguish. Skrabanek and
Wolfe (1998) elegantly explained the problem by analogy:
'Take four, or maybe eight, decks of 52 playing cards. Shuffle them all together
and then throw some cards away. Pick 20 cards at random and drop the rest on
the floor. Give the 20 cards to some evolutionary biologists and ask them to
figure out what you've done. For encouragement, tell them they can have the
cards on the floor in 2005 (the estimated date of completion of the human
genome sequence)'.
Ohno (1970) first proposed that the vertebrate genome had evolved to its present
size through an ancient tetraploidization event. Evidence supporting this hypoth-
esis now comes from a variety of different sources. Comparative nuclear DNA
measurements in higher organisms are compatible with the idea of successive
genome doublings (Ohno, 1970; Sparrow and Nauman, 1976). So are cytogenetic
data, since the human karyotype can be divided into similar pairs of chromo-
somes on the basis of structure and banding patterns viz. chromosomes 1 and 2, 4
and 5, 7 and 8, 11 and 12, 14 and 15, 16 and 17, 19 and 20, 21 and 22 (Comings
1972; McKusick, 1980). The postulated ancient tetraploidization event also
appears to be reflected in the genetic information content of these chromosome
pairs. This is exemplified by the distribution of the insulin and RAS oncogene
 
 
 
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