Only rarely are the effects of an allele and its pseudoallele additive. A pseudoallele does thus not replace an allele. This and other observations lead to the discovery that the genes of a genome are no independent unities. Their activities are actually controlled by neighbouring DNA domains. This phenomenon was originally called the position effect .
Molecular analyses begin to elucidate how such control mechanisms of single genes work.
After the fraction do two homologous fragments join so that one chromosome without a centromere forms, while the other chromosome has two. The chromosome without a centromere is lost. During anaphase do the two centromeres of the other chromosome segregate to opposite poles, thus causing a break of the in-between domain. Since the exact place of the breaking is subjected to chance, will one of the daughter cells get an elongated, the other one a shortened fragment. In the elongated one is the additional domain inverted.