Nitpick: homogeneous, but I fear I’m losing this battle.
I blame autocorrect!
Well autocomplete more precisely.
I asked my DIL about the boys weight.
She said they’re 2oz different in them.
I haven’t been around them as much as I’d like. They seem really identical in pictures. I don’t see how she can tell them apart.
At first she painted different colors of nail polish on a toe nail.
That lasted about 2 week.
I wonder why any one worries about it at home.
Frank and Hank don’t know the difference. One will eventually shows his difference and he’ll end up up as Hank. Or Frank.
I know one set of identical twins where one is straight and the other is gay.
The correlation on a trait between identical twins is the upper bound on a trait’s heritability. So for any trait which identical twins are not always identical, then we know that there is some sort of environmental influence on that trait.
For example, height and schizophrenia are two traits that each correlate between about 0.70 to 0.90 in identical twins (if one is short, the other is likely short; if one is diagnosed with schizophrenia, the other is likely diagnosed with schizophrenia). But because the correlation is less than 1.00, we know that there must be an environmental influence on the trait. (It could also be measurement error.)
Which traits identical twins differ on is very interesting, and offers insight into the human animal.
The zygote generally splits between 2 and 14 days after fertilization. The exact timing of the split can influence how many placentas and amniotic sacs are present.
Sorry that the question was not clear.
When do their epigenomes begin to diverge?
I suspect from day one of separate zygotes or possibly before in a sense but wonder if animal model research sheds any light.
And it’s only an upper bound because identical twins usually have a lot of other things in common besides genetics: They’re likely to be raised in the same house, with access to the same food, medicine, and schools, subject to the same parental upbringing, and so on.
There are some studies of twins who were separated at birth (one or both adopted), which help shed light on the purely-inborn influence, but pairs of identical twins separated at birth aren’t particularly common, so there’s not much statistics there. And even given that, “inborn” isn’t the same as “genetic”: The prenatal environment in the womb can also influence a lot of things, and identical twins share that, too.
Alternately, you can study identical twins, and fraternal twins, and non-twin siblings, and adopted siblings, and so on, and look for correlations within each of those categories. If a trait is highly correlated between identical twins, but nearly as highly correlated between adopted siblings, then that suggests that genetics has very little impact on that trait, compared to upbringing. If it’s highly correlated between fraternal twins, but much less correlated between non-twin siblings, then that might suggest that the pre-natal uterine environment is relevant, and so on.
It’s an upper bound because they share (nearly) 100% of their genes the same. Identical home environment, age, and such results in environmental variation which also makes them more similar on some traits.
Environment that makes two people more similar is referred to as “shared environment.” Environment that makes people less similar is referred to as “non-shared environment.” Those are both psychological/sociological phenomenon, and a mathematical phenomenon.
Which leads to:
Generally we use structural equation modeling and maximum likelihood, instead of just looking at correlations, but for simple things, you can just look at correlations to get an estimate of the proportion of the variance due to genes, shared environment, and non-shared environment.
With modern genetics, we can measure shared genes between unrelated individuals to do similar type things with more than just twins and siblings. Such analyses are generally called genome-wide complex trait analysis (GCTA).
And if you’ve made it this far, this gets super complicated super fast. The womb environment is influenced by Mom’s genetics. Each fetus shares half of their genes with Mom, so some of the genes creating the womb environment are also present in the fetuses.
Shared and non-shared environments are defined by their impacts? That seems odd.
I would expect that some parts of the environment are in common or not in common and neutral in impact. They are still shared or non-shared, aren’t they? Some of those shared features might even impact one genome one way and another differently, the difference only showing up with exposure?
Sometimes I forget just how fast genetic science is advancing these days. Thanks for the reminder.
And of course, everything in human biology is made more complicated by the inability (within ethical bounds) to run controlled experiments. In most sciences, if we want to see what effect something has, we can just change it (while keeping everything else the same) and see.
Which proves gays are not born gay surely?
Why would you think that? While identical twins are very similar, they are not, in fact, identical. There are many in utero factors that could contribute to different developmental paths that include sexual preferences.
All it means is partner preference is not 100% determined by genetics.
Exactly. Numerous twin studies have shown that identical twins are, in fact, more likely to share the same sexual orientation than fraternal twins, but it’s not 100%. The conclusions from these studies indicate that genetics do play a significant role in sexual orientation, but that genetics are not the sole determining factor.
Identical twins don’t have the same fingerprints. Do you also have an interest in using that information to prove that you are not born with fingerprints?
This gift link to a recent New York Times article is sort of related. A Canadian artist had a photography project in which he photographed pairs of “doppelgängers”, or unrelated people who are surprisingly similar in appearance. Later, a Spanish researcher compared the DNA of these pairs of people, finding that some pairs have similarities in their DNA. (The New York Times article has photos of the pairs, and they really do look like twins.)
I can’t find any papers right now, but I assume you’re right. There have been studies on human twins showing that their epigenomes are very similar when they are born. Then they diverge as they age. That makes sense since the environment in utero will still be pretty similar even if they have different placentas/amniotic fluid. It’s still the same mom.
I remember watching something on PBS about the epigenome and autism. Autism has a genetic component and twins have a 90% chance of both of them having autism. However, their severity can be quite different. This was attributed to differences in methylation patterns on the DNA in their neurons. I’m going by memory but that stood out to me.
I wonder if that was because the clone was hand raised? See:
ETA: Berserker Llama Syndrome would be a massively cool band name.
Thank you for trying!
Turnabout being fair play I found a study about methylation patterns in monozygotic twins concordat and discordant for autistic spectrum diagnosis and severity:
https://www.nature.com/articles/mp201341
This is fascinating stuff.
The questions raised go beyond those in that article’s discussion section, although it is likely premature to ask them. When do these epigenetic changes occur and what either triggers or avoids them? Are they reversible? Do they have a different course with early identification and intervention that impacts phenotypic severity?
I haven’t read the article, so maybe they discuss this. Another extremely important question, is the difference in site specific methylation a cause of autism or a result of autism (or unrelated, obviously, but let’s assume the finding is real)? Having autism can be very stressful, and stress does result in epigenetic changes.
The discordant twin design is an useful technique. It is a natural experiment of finding instances when the same genetic background produces different disease states, or investigating the same genetic background in different environments. Some people I’m working with are looking at things like do members of a twin pair have a different relationship with marijuana when they live in states with differences in marijuana legalization.