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The term "tetrad" in the context of meiosis specifically refers to a structure formed during prophase I of meiosis when homologous chromosomes pair up. Each homologous chromosome consists of two sister chromatids; therefore, when the two homologous chromosomes align, they create a total of four chromatids. This grouping of four chromatids is what is referred to as a tetrad. The significance of the tetrad lies in its role during the process of crossing over, where genetic material can be exchanged between non-sister chromatids, contributing to genetic variability.

A pair of homologous chromosomes would not capture the full structure as it does not include the context of the sister chromatids being present. A set of four alleles does not align with the definition of a tetrad, as alleles relate to different versions of a gene, while a tetrad refers to the physical structure of chromosomes during meiosis. Lastly, a group of two chromosomes does not accurately describe a tetrad since a tetrad specifically involves four chromatids from two chromosomes.