The possibility of a flaw underlying Weinberg's differential rule.
There seems rather good evidence that there is a flaw underlying Weinberg's Differential Rule, and that among DZ twin pairs, there are perhaps eight same-sexed pairs to every seven opposite-sexed pairs.
D005260:Female; D006801:Humans; D008297:Male; D008954:Models, Biological; D011247:Pregnancy; D012732:Sex Determination Analysis; D014427:Twins; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic