Figure 1. Evolution of the globin genes. The numbers in parentheses represent the estimated number of nucleotide changes needed to account for the observed amino acid differences. Adapted from <http://www courses/b315f99/ sessions/phylogeny/ globinPhylogeny.jpg>.

basal lowest level

Millions of years ago

700 600

500 400 300


100 0

1 1

1 1 1


1 1

Alpha chains


(a1, a2)

Zeta chain ©

/ (76)



Epsilon chain (e)

1 (49)


Gamma chains


(AX g7)

Prevertebrate or

early vertebrate


(9) /

Delta chain (8)





Beta chain (g)


genetic elements with no known function. For genes that encode proteins, duplicate copies of genes have been found for over 2,000 proteins in a variety of genomes.

Members of gene families may be located in contiguous clusters on one chromosome, or they may be scattered throughout a genome. Homeotic genes, which lie in contiguous clusters on a few chromosomes, provide the best example of evolutionarily preserved gene order within a gene family. These genes play a role in the spatial development of the anterior-posterior axis of vertebrates and invertebrates. Four binary axes are laid down early in the basal body plan of most metazoans, including us: anterior-posterior, dorsal-ventral, left-right, and inside-outside. They affect locations on the body axis in roughly the same order as they are arranged along a chromosome, even though different clusters appear on different chromosomes. Amazingly, these genes work about the same in a fruit fly as they do in a human in establishing linear arrangements.

In some gene families, related genes have stayed together over long periods of evolution, while in other gene families, members have become widely distributed within genomes. In ribosomal RNA genes, tandem arrays, in which multiple copies of the same gene occur one after another, have been observed. On the other hand, in the globin gene family, both the order and distribution of the genes, which are not identical, vary widely, even within one taxonomic group such as the mammals.

Members of a gene family usually have similar structures, but they may have diverged evolutionarily to such an extent that they are expressed in different ways. They may be expressed at different times in the development of multicellular organs or in different cells and tissues, and they may have acquired different functions. They may even have been transferred between organisms that are not closely related by evolution.


i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i i i I

6 G7 A7

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