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Schematic illustration of a gene tree created using Y chromosome polymorphisms. Each modern population (a-n) is descended from a single ancestral populate living 100,000 years ago. Each branching point represents a mutation event, which is then faithfully inherited.

polymorphisms DNA

sequence variant include forensics (criminological investigations, such as determining whether or not an individual has been involved in a crime) and genealogical reconstruction (verifying membership in a particular family's ancestry).

DNA polymers (such as chromosomes) are composed of a four-letter alphabet of chemicals called nucleotide bases. Random unique event mutations in DNA sequences can change the identity of a single base in the DNA molecule. These "spelling changes" are the essential currency of genetic anthropological research.

What is central is the assumption that a particular mutation arose just once in human history, and all men that display such a mutation on their Y chromosome descend from a common forefather on whom the mutation first appeared. The sequential buildup of such mutational events across the generations can be readily determined and displayed as a gene tree. Informally, the last known mutation to accumulate on a particular chromosome can be used to define a particular lineage or branch tip in the tree. As long as the mutational change does not affect the individual's ability to reproduce, it may be preserved and handed down to each succeeding generation, eventually becoming widespread in a population. Such mutations are called polymorphisms or genetic markers.

Since most of the Y chromosome has the special property of not recom-bining during meiosis, no shuffling of DNA from different ancestors occurs. As a consequence, any Y chromosome accumulates all the mutations that have occurred during its lineal life span and thus preserves the paternal genetic legacy that has been transmitted from father to son over the generations. The discovery of numerous Y chromosome polymorphisms has allowed us to deduce a reliable genealogy composed of numerous distinctive lineages. This concept is analogous to the genealogical relationships maintained by the traditional transmission of surnames in some cultures, although the gene tree approach provides access to a prehistorically deeper set of paternal relationships.

Molecular anthropologists have exploited this knowledge in an attempt to understand the history and evolutionary relationships of contemporary populations by performing a systematic survey of Y-chromosome DNA sequence variation. The unique nature of Y-chromosome diversification provides an elegant record of human population histories allowing researchers to reconstruct a global picture, emblematic of modern human origins, affinity, differentiation, and demographic history. The evidence shows that all modern extant human Y chromosomes trace their ancestry to Africa, and that descendants left Africa perhaps less than 100,000 years (or approximately 4,000 generations) ago.

While variation in any single DNA molecule can reflect only a small portion of human diversity, by merging other genetic information, such as data from the maternally transmitted mitochondrial DNA molecule, and nongenetic knowledge derived from archeological, linguistic, and other sources, we can improve our understanding of the affinities and histories of contemporary peoples. see also Molecular Anthropology; Poly-mophisms; Sex Determination; X Chromosome.

Peter A. Underbill

Bibliography

Cavalli-Sforza, Luigi L. Genes, Peoples, and Languages, Mark Seielstad, trans. New York: North Point Press, 2000.

Jobling, Mark, and Christopher Tyler-Smith. "New Uses for New Haplotypes: The Human Y Chromosome, Disease, and Selection." Trends in Genetics 16 (2000): 356-362.

Strachan, Tom, and Andrew P. Read. Human Molecular Genetics. New York: Wiley-Liss, 1996.

Yeast

Yeast are single-celled eukaryotic organisms related to fungi. The baker's yeast Saccahromyces cerevisiae and the distantly related Schizosaccharomyces pombe are favored model organisms for genetic research. The interest in yeast research stems from the fact that, as eukaryotic organisms, the subcellular organization of yeast is similar to that of cells of more complex organisms. Thus, understanding how a particular gene functions in yeast frequently correlates to how similar genes function in mammals, including humans.

Yeast Genetics

Yeast have many advantages as a genetic research tool. First, yeast are nonpathogenic (they do not cause diseases) and are therefore easy and safe to grow. Yeast can divide by simple fission (mitosis) or by budding and, like bacteria, they can be rapidly grown on solid agar plates or in liquid media. After just a few days in culture, a single yeast cell can produce millions of identical copies of itself, giving scientist a large supply of a genetically pure research tool.

Second, yeast grow as either haploids (having only one set of chromosomes) or diploids (with two chromosome sets). Thus, genetically recessive mutations can be readily identified by phenotypic (visually observable) changes in the haploid strain. In addition, complementation can be performed by simply mating two haploid strains, where one does not contain the mutation. The resulting diploid strain contains both the functional and nonfunctional version of a gene responsible for a phenotype. The addition agar gel derived from algae media nutrient source phenotypic related to the observable characteristics of an organism

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