Final answer:
In crossover events during meiosis, only two of the four chromatids — the non-sister chromatids from homologous chromosomes — participate in recombination, resulting in genetic variation with the creation of recombinant chromatids.
Step-by-step explanation:
In a cross over event, only two of the four chromatids will participate in the recombination event. This is because the process involves the exchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis.
During prophase I of meiosis, homologous chromosomes pair up gene-for-gene, forming a structure known as a tetrad, which consists of four chromatids. However, only two of these chromatids, one from each homolog, participate in the crossing-over. The result of this reciprocal exchange of equivalent DNA segments is the creation of two recombinant chromatids, each possessing a unique combination of both maternal and paternal genes that did not exist prior to the crossover event. This contributes to the genetic diversity observed among offspring.
The process does not involve all four chromatids because it is specifically the non-sister chromatids that engage in this genetic material exchange. This ensures that some chromatids remain non-recombinant, maintaining a portion of the genetic material unchanged, while recombinant chromatids introduce variation.