Each type of isolating mechanism is based upon differences in respect to many different genes or chromosomal segments. The progeny of hybrids do not sort out in clear cut Mendelian ratios meaning thereby, the action of multiple genes. The sterlity of hybrids may be due to following causes:
(i) If the parent species have different chromosome numbers, the chromosomes cannot usually be arranged in homologous pairs as some of them will be left unpaired. This irregularity of chromosome number in the gametes and consequently in the zygote should be fatal.
(ii)The pairing chromosomes (synapsis) involve pairing of homologous genes. If the two species crossed are not very much alike, synapses may not take place in the hybrids.
(iii)Gene mutations and chromosomes aberrations such as duplications and deletions are almost fatal as they will cause difficulty in pairing of chromosomes. Inversions and translocations may not be fatal but reduce fertility of hybrids to a great extent.
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(iv) Another means of producing sterility of hybrids is through combined action of certain genes. Thus, the genes S, (Sj Sj) and S2 (S2 S2) alone may not produce any untoward effect but when both (S, S2) occur together in the same individual, they may result in abnormal development of the hybrid with reduced fertility.