Quote from: JulieOnHerWay on January 29, 2018, 09:33:21 PM
I am no genetics person and so maybe I am completely off the reservation here, BUT if I got my info right, Roll, all your siblings that share the same mother and father should have the identical genetic make up. you know the half from mom and half from dad thing.
Maybe your younger sib has a different parental source but they are half yours and will only be different in your step-parent's contribution.
My kids and their mother thought for several generations that they were Native American. One of those tests came back, "nope".
My brother did his and it was Scottish, a little Irish and a bunch of Dutch. Yea, him and by proximity, me.
You may be surprised how much you vary from your siblings. (Here is a quick article about it:
http://genetics.thetech.org/ask-a-geneticist/same-parents-different-ancestry)
Only identical twins have the exact same genetic makeup. What happens is that you get half of one parent and half of another parent of course, but it's not always (well, never except for the aforementioned twins) the same half as your siblings. It will be close, and there are certain things that will almost always pass along, but they won't be identical.
So say that you have different genes represented like this(all else being equal):
Dad - 1 2 3 4 5 6 7 8 9 10
Mom - A B C D E F G H I J
You may get 13489ABEFG while your sibling gets 23478ACFIJ. (This is a massive over simplification of course, and I'm just using this as example of genes not even getting into chromosomal pairings.) Hence the reason siblings can have different color eyes, hair, etc., and even vary from their parents. (But that deals with dominant and recessive, and not really relevant.)
The way 23andme and the other testing services work is they match up against known genes. So let's say that my dad has genes that are associated with different ethnic backgrounds at the following proportions: 50% scottish, 25% german, 25% native american. For my half from him, it could be all scottish, or half german and half scottish, or half german and half native american (or any proportion therein). If the genes I received were split between those associated with scottish and those associated with german, I wouldn't show evidence of the native american genes (in these specific tests) at all. Though it is extraordinarily unlikely you show no genetic heritage at all for any remotely recent ancestor. (Again, all super over simplification.)
Unfortunately, it is also imprecise based on the sample method, and some people have said you would have to run the test at least 8 times for a reliable average. However, any overwhelmingly large numbers are most likely accurate to a reasonable. (Ie: I'm definitely extremely northwestern European, and a retest isn't going to suddenly drop that to 10%.

But dropping to 80% and revealing other stuff, possible.) With the lower numbers, you need the average though, and they can and will vary by low double digits or reveal previously unrevealed associations (and not show previously shown ones). So this time I was 1.4% west african, next time maybe I'm .8% mongolian and .6% pacific islander. But that is why they always label them for entertainment purposes only.