In dioecious dwarf surfclam Mulinia lateralis (Say, 1822), it has been also found that its gynogenetic progeny are all females indicating that they must be the homogametic
sex (carrying 2X-chromosomes), and triploids showed both genders, males and females, such that it was proposed that they were XXX and XXY, with the Y-chromosome being dominant over the X-chromosome.
Females are heterogametic (Z0 or WZ) and males are homogametic
(ZZ), although sporadic rearrangements created multiple sex chromosome systems in some species (Traut et al.
Previous studies on fluted pumpkin reported an XY system of sex chromosomes with homogametic
XX female and heterogametic XY male .
Higher possible genomic mortality of heterogametic (ZW) females compared with homogametic
males (ZZ) allows the males of these species to offer sacrifice to save the females, which has the responsibility of rearing the brood.
mori, are designated ZW for the heterogametic female and ZZ for the homogametic
male (Yokoyama et al.
In birds, females are the heterogametic sex (ZW) and males are homogametic
(ZZ) (Ellegren 1996).
Normally, the heterogametic sex (males in XY/XX systems, females in ZW/ZZ systems) shows lower chiasma frequencies than the homogametic
one (Trivers 1988, Burt et al.