Why is the XX fitter? Evidence consistent with an effect of X-heterosis in the human female from sex ratio data in offspring of first cousin marriages.
D003241:Consanguinity; D005260:Female; D006579:Heterozygote; D006720:Homozygote; D006801:Humans; D007564:Japan; D010375:Pedigree; D012730:Sex Chromosomes; D012744:Sex Ratio