The meiotic products of ascomycetes (occasionally some other organisms) stay together as the four products of single meiosis, as a tetrad. In some organisms, tetrad formation is followed by a post-meiotic mitosis within the ascus, resulting in spore octads. If the four spores are situated in the same linear order as produced by the two divisions of meiosis it is an ordered tetrad.
In the ordered tetrad, considering two genes A and B, three arrangements of the spores (parental ditype [PD], tetratype [TT], non-parental ditype [NPD]) can be distinguished as seen in the Figure T35. The parental ditype (PD) indicates no crossing over; tetratype (TT) reveals one recombination between the two genes and the second division segregation of the B/b alleles reveals recombination between the B/b gene and the centromere (see Fig. T36).
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(2008). Tetrad Analysis. In: Encyclopedia of Genetics, Genomics, Proteomics and Informatics. Springer, Dordrecht. https://doi.org/10.1007/978-1-4020-6754-9_16853
Publisher Name: Springer, Dordrecht
Print ISBN: 978-1-4020-6753-2
Online ISBN: 978-1-4020-6754-9