Gene family

(genetics) A group of related genes of similar sequence and function resulting from multiple duplications and subsequent mutational variation among the different copies.

Gene families are groups of DNA segments that have evolved by common descent through duplication and divergence. They are multiple DNA segments that have evolved from one common ancestral DNA segment that has been copied and changed over millions of years.

The members of a gene family may include expressed genes as well as nonexpressed sequences. Such nonexpressed sequences include promoters, operators, transposable genetic elements, and pseudogenes, which are genes that are no longer functionally expressed.

Pseudogenes resemble other family members in their linear sequence of nucleotides. However, they usually either lack the signals that would allow them to be expressed or have significant deletions or rearrangements that prevent successful transcription or translation.

One well-studied gene family is that of the globins, shown in both Figures 1 and 2. The globin family contains many pseudogenes as well as many functional genes, including the genes coding for hemoglobins (α, β, γ, δ).

Gene families vary enormously in size and number, ranging, in the human genome, from just a few copies of very closely related sequences to more than a half-million copies of Alu sequences, which are transposable 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.

One controversy about gene families involves whether they have arisen primarily by polyploidy or via tandem gene duplications. Polyploidy means that full genomes in an organism are duplicated either by mitosis or meiosis without cytokinesis or by matings between organisms with unequal numbers of chromosomes. This is followed by full copying of both parents' full genomes so each haploid set of chromosomes is now diploid. In tandem duplication, one or more copies of a gene lie on the same chromosome adjacent to one another. Polyploidy has been invoked to explain the evolution of complex new functions in taxa. Researchers give five reasons. Polyploids:

1. have "higher levels of heterozygosity than do their diploid parents";
2. "exhibit less inbreeding depression than do their diploid parents";
3. "are polyphyletic … [which] … incorporates genetic diversity from multiple progenitor populations" and, thus, they have higher genetic diversity "than expected by models of polyploid formation involving a single origin";
4. have genome rearrangements that are common; and
5. are like duplicated genes, freed from intense selection pressure, which allows frequent evolution of new functions (Soltis and Soltis 2000, p. 310).

Austin Hughes has been the major critic of the often invoked polyploid hypothesis for origin of major animal groups because of the major substitutional load that would be involved and molecular phylogenetic evidence against it (1999, pp. 205-212). However, the results of most molecular evolutionary studies are more consistent with the gradualist view that new functions are generated primarily by tandem gene duplication and divergence of both sequence and function, spread over a long time.

Bibliography

Henikoff, Steven, et al. "Gene Families: The Taxonomy of Protein Paralogs and Chimeras." Science 278 (1997): 609-614.

Holmes, Roger S., and Hwa A. Lim, eds. Gene Families: Structure, Function, Genetics and Evolution. Singapore: World Scientific Publishers, 1996.

Hughes, Austin L. Adaptive Evolution of Genes and Genomes. New York: Oxford University Press, 1999.

Page, Roderic D. M., and Edward C. Holmes. Molecular Evolution: A Phylogenetic Approach. Malden, MA: Blackwell Science, 1998.

Patthy, Laszló. Protein Evolution. Malden, MA: Blackwell Science, 1999.

Soltis, Pamela S., and D. E. Soltis. "The Role of Genetic and Genomic Attributes in the Success of Polyploids." In Variation and Evolution in Plants and Microorganisms, Francisco J. Ayala, et al., eds. Washington, DC: National Academy Press, 2000.

Thorston, J. W., and R. DeSalle. "Gene Family Evolution and Homology: GenomicsMeets Phylogenetics." Annual Review of Genomics and Human Genetics 1 (2000): 41-73.

Internet Resources

"Phylogenic Relationships between Globin-Type Proteins." Concordia College.http://www.cord.edu/faculty/landa/courses/b315f99/sessions/phylogeny/globinPhylogeny.jpg.

"Proposed Evolution of Globin Genes." Portland State University. www.irn.pdx.edu/~newmanl/GlobinGeneEvolution.GIF.

—John R. Jungck

Genes are organized into groups called gene families. Many genes have overlapping sequences with each other. Those that share between 30 and 90 percent of their sequences are grouped together in families. Gene families may range in size from just a few genes to several thousand. An example of a gene family is the group of genes that code for histones. Histones are proteins important for maintaining DNA in a particular shape.

It has been suggested that Multigene family be merged into this article or section. (Discuss)

This article does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2007)

A gene family is a set of genes with a known homology. They are generally biochemically similar. Genes are categorized this way into families, depending on shared nucleotide or protein sequences.

Phylogenetic techniques can be used as a more rigorous test. The positions of introns within the coding sequence can be used to infer common ancestry. Knowing the sequence of the protein encoded by a gene can allow researchers to apply methods that find similarities among protein sequences that provide more information than similarities or differences among DNA sequences. Furthermore, knowledge of the protein's secondary structure gives further information about ancestry, since the organization of secondary structural elements presumably would be conserved even if the amino acid sequence changes considerably.^{[citation needed]} These methods often rely upon predictions based upon the DNA sequence.

If the genes of a gene family encode proteins, the term protein family is often used in an analogous manner to gene family.

The expansion or contraction of gene families along a specific lineage can be due to chance, or can be the result of natural selection. To distinguish between these two cases is often difficult in practice. Recent work uses a combination of statistical models and algorithmic techniques to detect gene families that are under the effect of natural selection. The Evolution of Mammalian Gene Families

See also

• List of gene families
• protein family

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