Are there twin-specific effects? Are sibs with large age differences any less similar than those closer together in age? My general impression of the literature is that there are very few twin effects. So, can we have a list please of all those traits, features or diseases where DZ twins are more similar than same sex sibs? This topic has attracted much less attention than it deserves, as it helps narrow down the long list of possible environmental influences which act over different time periods.
This seems to be a nature/nurture question. We all know that monozygotic (identical) twins are supposed to be 100% DNA matches (nature), so any differences in behavior or phenotype (looks, weight, etc) must be due to non-genetic influences (nuture). I will not get into the discussion of epigenetics at this point, but we can imagine how certain gene expressions could be modified by external influences. Assuming MZ twins are not separated at birth, we can also assume that their environmental influences will be more similar for MZ siblings than siblings separated by years. That said, we would know that DZ twins share a temporal closeness, but are genetically no more alike than individual siblings separated by years. However, I believe this shared temporal environment does make DZ twins more similar than individual siblings. The reason for this is that environmental influences that could alter gene expression (NOT genotype) in early development are more likely to be the same for twins housed in the same uterus at the same time than it is for individual siblings who do not share this same space or time.
As for specific traits that indicate the closeness of DZ twins vs same sex siblings from individual births, I can imagine oncology cases would be good examples. For instance the BRCA1 gene expresses DNA repairing protein. If an individual has a mutation on BRCA1 then the expression of this repair protein is turned off, increasing the risk for certain types of cancer, most commonly breast cancer. However, having a mutated BRCA1 genes does NOT mean the individual WILL develop cancer. It simply means the individual is at higher risk when DNA damaging conditions present themselves. Individuals can inherit the mutation (the most common method) or develop the mutation by environmental impacts that damage the BRCA1 gene sequence(exposure to things that like radiation, chemicals, etc). It is possible to imagine that DZ twins who have inherited normal BRCA1 genes would have a higher likelihood of being exposed to the same factors in-utero or in early development that lead to DNA damage than two siblings born at difference times with different risks of exposure. In addition, we know that oocytes are present from birth and age with the female. At the onset, each oocytes should contain a random mixture of genes to be inherited, which is why non-MZ siblings are not identical. The DNA in these oocytes are also bombarded with all kinds of environmental factors that could damage the DNA contained in each as the female ages. This simple effect at the oocyte level increases the likelihood that individual siblings separated by greater time would be more different that siblings born closer in time.
This seems to be a nature/nurture question. We all know that monozygotic (identical) twins are supposed to be 100% DNA matches (nature), so any differences in behavior or phenotype (looks, weight, etc) must be due to non-genetic influences (nuture). I will not get into the discussion of epigenetics at this point, but we can imagine how certain gene expressions could be modified by external influences. Assuming MZ twins are not separated at birth, we can also assume that their environmental influences will be more similar for MZ siblings than siblings separated by years. That said, we would know that DZ twins share a temporal closeness, but are genetically no more alike than individual siblings separated by years. However, I believe this shared temporal environment does make DZ twins more similar than individual siblings. The reason for this is that environmental influences that could alter gene expression (NOT genotype) in early development are more likely to be the same for twins housed in the same uterus at the same time than it is for individual siblings who do not share this same space or time.
As for specific traits that indicate the closeness of DZ twins vs same sex siblings from individual births, I can imagine oncology cases would be good examples. For instance the BRCA1 gene expresses DNA repairing protein. If an individual has a mutation on BRCA1 then the expression of this repair protein is turned off, increasing the risk for certain types of cancer, most commonly breast cancer. However, having a mutated BRCA1 genes does NOT mean the individual WILL develop cancer. It simply means the individual is at higher risk when DNA damaging conditions present themselves. Individuals can inherit the mutation (the most common method) or develop the mutation by environmental impacts that damage the BRCA1 gene sequence(exposure to things that like radiation, chemicals, etc). It is possible to imagine that DZ twins who have inherited normal BRCA1 genes would have a higher likelihood of being exposed to the same factors in-utero or in early development that lead to DNA damage than two siblings born at difference times with different risks of exposure. In addition, we know that oocytes are present from birth and age with the female. At the onset, each oocytes should contain a random mixture of genes to be inherited, which is why non-MZ siblings are not identical. The DNA in these oocytes are also bombarded with all kinds of environmental factors that could damage the DNA contained in each as the female ages. This simple effect at the oocyte level increases the likelihood that individual siblings separated by greater time would be more different that siblings born closer in time.
"This seems to be a nature/nurture question."
Not really, it is aimed at weeding out environmental no-hopers. If one can pin down the time interval or critical periods over which environmental effects occur, this greatly narrows down the long list of potentially relevant environmental factors.
"I believe this shared temporal environment does make DZ twins more similar than individual siblings."
And so do a lot of people, as it sounds so eminently reasonable. However, I have two reasons for doubting this, or, if true, that it is of more than trivial importance. I do not recall coming across any evidence or data to back up this belief, and I suspect it may be untrue given that the environmental factors identified by behavior genetic twin studies are overwhelmingly found to be child-specific not family-wide, from unique not common environment.
"I will not get into the discussion of epigenetics at this point"
Unless there are strong twin or age-dependent effects, the rug will surely be pulled from underneath the epigenetic bandwagon.
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