Monozygotic (identical – MZ) twins raised in a family share both 100% of their genes, and all of the shared environment. Any differences arising between them in these circumstances are random (unique). The correlation between identical twins provides an estimate of "A" + "C". Dizygotic (DZ) twins also share C, but share on average 50% of their genes: so the correlation between fraternal twins is a direct estimate of ½"A"+"C". If "r" is correlation, then "r"mz and "r"dz are simply the correlations of the trait in identical and fraternal twins respectively. For any particular trait, then: