When homologous chromosomes pair during the first meiotic division, the chromatids may get tangled with one another, a situation called crossing over. When crossing over occurs, the chromatids may break and reattach to a chromatid from a different chromosome. When this occurs, the result in a new combination of alleles along the chromosome. If a gamete containing a chromosome with a recombined set of alleles is involved in fertilisation, the resulting offspring with have the new combination of alleles. This combination of alleles is not the same as that possessed by either of the parents.