On the right are two chromatids, each one represented by a double strand of DNA. The two gene loci are A and B and the numbers are reference points that delineate common loci between the two homologues. According to the White- house hypothesis, the first event in crossing over involves a close pairing of homologous regions on homologous chromosome pairs followed by a break on one of the DNA strands on each of the chromatids. Flip to the next card to see this. [Note that a large protein structure called the synaptonemal complex is built between members of a homologous chromosome pair and is believed to play some role in aligning the chromosomes properly so that the process proceeds without complications.] If you wish details on the synaptonemal complex, click on the appropriate button below. You will be returned to this card after reviewing the data.