They're saying that if they found the following:The underlined was really hard to understand. I took an example myself in terms of height.
MZ : a: 170cm, b: 170cm => difference = 0
DT : c:170cm, d:180cm => difference =10cm
so, if we subtract from 10, 0, ,it would be 10-0=10cm. Is this assumption correct?
Isn't there any chance that MZ's difference could be
1,2cm like a=170cm, b= 172cm
because the environmental factor is the same?
Of course there is a chance that monozygotic twins will be 170cm and 172cm tall. However, it would make no sense to claim that this difference was "because the environmental factor is the same".
It would make the difference 8 instead of 10. That would make it more likely that environmental determinants existed for height than if the difference between MZ and DZ were 10, or if the difference between MZa and MZb were 0.
ex)How do scientists determine whether hereditary factors influence people's health? Much of the research on hereditary factors has focused on the differences in characteristics shown in monozygotic(MS) twins as compared with and dizygotic(DZ) twins. MZ twins are conceived together and have exactly the same genetic inheritance; DZ twins are conceived separately and are no more genetically similar than singly born siblings. Because the two individuals in an MZ pair are genetically identical, we can assume that differences between them are environmentally determined. Conversely, the greater the similarity between MZ twins, the more likely it is that the characteristic is genetically influenced. Differences between DZ twins, on the other hand, are due to both genetic and environmental factors. If we could assume that both members of each MZ and DZ pair have had equal environmental experiences, we could measure genetic influence simply by subtracting the differences for MZ from the differences for DZ twins.
MZ a = 175, b = 173, diff = 2
DZ a = 174, c = 176, diff = 2
MZ - DZ = 0
then there would be no evidence of a genetic factor in height, on this test.
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