parent - of - origin effects is a broad term that encompasses two distinct phenomena parent - of - origin effects on transcription , and parent - of - origin effects on mutation rates . a parent - of - origin effect on transcription , or genomic imprinting , results from epigenetic modification of the genome which , in turn , results in unequal transcription of parental alleles . for these imprinted genes , expression of the alleles is dependent upon the sex of the parent from which they were inherited ( 1 ) . a parent - of - origin effect on mutation rate , however , refers to the preferential occurrence of some spontaneous mutations in either the father 's or the mother 's germ line . the mechanisms by which these spontaneous mutations arise depend upon the parental germ line in which the mutation occurred . for example , base substitutions , arising from errors during replication , tend to be paternal in origin , owing to the greater number of cell divisions in spermatogenesis as compared with oogenesis ( 2 ) . oocytes are arrested in prophase of meiosis i until sexual maturity , when one oocyte per month is selected to resume the cell cycle . it is thought that the longer the oocytes are arrested in meiosis , the greater the chance for a nondisjunction event to occur ( 3 ) . advanced parental age seems to influence the development of some , but not all , of these mutations ( also referred to as the paternal or maternal age effect ) ( 2 ) . in 1998 , the catalogue of imprinted genes and parent - of - origin effects was first published ( 4 ) . this catalogue served as the basis for the development of a more comprehensive , searchable , online database , made publicly available in 1999 . the original database included 41 imprinted genes , and other parent - of - origin effects , including some records on the parental origin of spontaneous mutations ( 5 ) . we have added recently a comprehensive section on spontaneous mutations that show a bias with respect to their parental origin . this new part of the database can be searched according to mutation type , disorder , chromosomal location , gene name and inheritance pattern . outcomes of the search are presented in a tabular format with the following information : disorder , inheritance pattern , incidence of disorder , gene name , chromosomal location , evidence of a paternal or maternal age effect , mutation type and any recurrent mutations associated with a parent - of - origin effect , number of paternal mutations , number of maternal mutations and pubmed reference ( e.g. table 1 ) . in the case of base substitutions , data are separated according to the type of base substitution ( missense mutation , nonsense mutation or splice site mutation ) , whether the mutation is a transition or transversion mutation , and whether the base substitution falls within a cpg dinucleotide . for deletions and insertions , the distinction is made between large deletions and insertions ( > 20 bp ) and small deletions and insertions ( < 20 bp ) . this size distinction is made based upon the possibility of different mechanisms contributing to these different types of mutations , and therefore potentially different parental origins ( 2 ) . in general , large deletions do not appear to have a parent - of - origin effect , whereas small deletions tend to be more paternal in origin . currently , > 1700 mutations with a parent - of - origin effect are catalogued in this database . the other major section of the database includes known imprinted genes and observations of other putatively imprinted genes . of the 464 database entries , 152 entries describe 85 unique imprinted genes in humans , mice , cattle , sheep , pigs , rats and marsupials , as well as 14 genes for which the evidence of imprinting is conflicting or provisional . an additional 186 entries report parent - of - origin effects in the transmission or linkage of simple and complex genetic conditions including human diseases and animal quantitative traits . the imprinted gene and parent - of - origin effect database is housed at the university of otago in dunedin , new zealand and can be accessed at . the database is maintained by the corresponding authors who welcome submissions and comments and is updated as new literature is published . submissions to the imprinted gene database should be directed to i.m.m . and submissions to the parental origin of de novo mutations database example of report for parental origin of de novo mutations showing base substitutions within a cpg dinucleotide ad , autosomal dominant ; xd , x - linked dominant ; xr , x - linked recessive ; p , point mutation ; ms , missense mutation ; ns , nonsense mutation ; cpg , mutation in a cpg dinucleotide ; ts , transition mutation ; tv , transversion mutation .
the imprinted gene and parent - of - origin effect database ( ) consists of two sections . 
 one section catalogues the current literature on imprinted genes in humans and animals . 
 the second , and new , section catalogues current reports of parental origin of de novo mutations in humans alone . 
 the addition of a catalogue of de novo mutations that show a parent - of - origin effect expands the scope of the database and provides a useful tool for examining parental origin trends for different types of spontaneous mutations . 
 this new section includes > 1700 mutations , found in 59 different disorders . 
 the 85 imprinted genes are described in 152 entries from several mammalian species . 
 in addition , > 300 other entries describe a range of reported parent - of - origin effects in animals .