title: Letter: Why is the XX fitter? Evidence consistent with an effect of X-heterosis in women from sex ratio data in offspring of first cousin marriages.
mesh_terms: D003241:Consanguinity; D005260:Female; D005298:Fertility; D005313:Fetal Death; D006579:Heterozygote; D006801:Humans; D007178:Inbreeding; D008297:Male; D009026:Mortality; D010375:Pedigree; D010809:Physical Fitness; D011247:Pregnancy; D012730:Sex Chromosomes; D012737:Sex Factors; D012744:Sex Ratio