(4, 40) In addition, the difference in size between male and female gametes in anisogamous and oogamous organisms showed that contribution to the progeny of the male organellar genome (DNA) is small, while that of female gametes is large, suggesting that anisogamy
and oogamy reinforce maternal inheritance.
In biology, "male" and "female" are defined in a very specific manner: (115) "those individuals we label 'female' are individuals that produce relatively large, nutrient-rich, immobile gametes (sex cells); males are those individuals that produce relatively small, nutrient poor, mobile gametes." (116) This division into two distinct gametal types--termed "anisogamy
"--has all manner of repercussions for the organisms that produce them due to the simple definitional fact that "sperm is [relatively] cheap." (117) Furthermore, among mammals and other groups that internally gestate their offspring, it is almost invariably (118) the female of the species that does so.
One possible exception to this rule is the divergence in gamete size between sexes in all animals, but recent theory suggests that sperm limitation coupled with selection on increased zygote size can result in anisogamy
without direct sperm competition or Bateman's principle (Levitan 1996b, 1998).
Maternal effects are a fundamental consequence of anisogamy
; immediately following fertilization, an individual's phenotype reflects the size and composition of the egg or seed it developed from, and thus its mother's physiological state (mediated by her genotype and environment), rather than its own (or its father's) genotype (Wade, in press).
Early models of anisogamy
suggested that small sperm size would be favored when there is a risk of sperm competition (defined as occurring whenever females copulate with more than one male during a single period of female fertility; Parker 1970).