Science / Biology / Crossing-Over: During the first meiotic prophase, the process in which part of a chromatid is physically exchanged with another chromatid to form chromosomes with new allele combinations.
Science / Biology / Chiasma: The site where the exchange of chromosome segments between homologous chromosomes takes place (crossing-over) (pl.: chiasmata). MORE
Science / Biology / Sister Chromatids: Chromatids joined by a common centromere and carrying identical genetic information (unless crossing-over has occurred). MORE