Are the alleles of a gene essentially originated one from the father and the other from the mother? Are there exceptions?
It is natural so that alleles have come one from the mother and the other from the father but it is not obligatory. In a "clone" generated by the nucleus transplantation technology, for instance, the alleles come from a single individual. In the polysomies (as in trisomy 21) each gene of the affected chromosome has four, in tetrasomies or three alleles in trisomies.