 Without proper homologous recombination , chromosomes often incorrectly align for the first phase of cell division in meiosis . This causes chromosomes to fail to properly segregate in a process called nondisjunction . In turn , nondisjunction can cause sperm and ova to have too few or too many chromosomes . Down 's syndrome , which is caused by an extra copy of chromosome 21 , is one of many abnormalities that result from such a failure of homologous recombination in meiosis . 
