Tetrad Analysis
Tetrads, the four haploid cell products of a single meiotic division, can be used to study the behavior of chromosomes and genes during meiosis. This kind of analysis is only possible in haploid organisms, such as some fungi or single-celled algae, in which the products of each meiosis are held together in a kind of bag (or ascus). Thus, the four products of a single meiosis are recoverable and testable. Tetrads can take different forms in different organisms. In some species, four sexual spores containing the four nuclei are formed as products of meiosis e.g. in baker's yeast, Saccharomyces cerevisiae. In other species, e.g. the red bread mould, Neurospora crassa, eight sexual spores are formed: in these cases each of the four meiotic product nuclei undergoes a further mitotic division, producing eight nuclei that are then enclosed in eight spores. These groups of eight may be called octads, but most geneticists also call them tetrads because they simply represent double tetrads.
Tetrad analysis has proved very powerful in testing some of the assumptions of the chromosome theory of heredity directly. In many experiments in Mendelian genetics, individuals are studied on the basis of random meiotic product analysis. For example, a testcross of Aa to aa produces a 1:1 ratio, from which the equal segregation of A and a in a single meiosis is inferred. The use of tetrads provides a more direct test of this notion because the products of meiosis can be examined directly.
In some species, tetrads can be jumbled and unordered within the ascus while in others they can be arranged linearly. The latter are particularly interesting because they can be used to estimate the distance of the locus under study from the centromere (the region of the chromosome where spindle fibers attach). The linear array of sexual spores is a result of the lack of spindle overlap in the meiotic divisions (or in any subsequent mitotic divisions). Because no spindle overlap occurs, the nuclei do not pass each other in the long ascus and a linear array of haploid spores is produced. Using marker genes A and a, it is possible to cross A and a cultures and then isolate the tetrads. Typical results for a cross A X a are shown below:
Tetrad analysisAaAaAaAaAaAaAaaAaAAaaAaAaAAaaAaAAaaAaAaAAaaA
aAAa
The first two types on the left are called first division segregation, or MI, patterns. These asci result from meioses in which there has been no detectable crossover between marker locus and the corresponding centromere locus of that chromosome. Because of this, the two alleles A and a segregate into separate nuclei at the first meiotic division. The four ascus types on the right, however, show second division segregation, or MII, patterns. These asci arise from meioses in which there has been a crossover between the marker locus and centromere locus. As a result of the crossover, A and a alleles appear in the same nucleus at the end of the first meiotic division, and they do not segregate until the second division. The frequency of second meiotic division segregation spore patterns for a particular gene locus is proportional to distance of that locus from the centromere. It is possible to calculate this distance in map units by dividing the percentage of asci showing a second-division segregation pattern for that locus by two. The analysis can be further extended to map more than one locus.
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