[allmendream]

Why wouldn't this work for Down’s syndrome? They have just as many great^nth power grandparents as anyone else on average - they just got an extra copy of chromosome 21 during cell division of their initial gametes.

And you don't get chromosomes unchanged from a grandparent or great grandparent. The chromosomes you inherited from mom are almost exactly a 50/50 inheritance split between her mom and dad - and the chromosomes you inherited from dad are almost exactly a 50/50 inheritance split between his mom and dad.

Recombination frequency is measure in “centimorgans” which correspond to a % chance that a crossover event will happen between two chromosomal markers (mixing and matching mom and dad's DNA along the chromosome to make reproductive cells). It is about a 1% chance to have a cross over every 15,000 nucleotides.

The human genome is on 46 chromosomes and consists of 3 billion base pairs. That makes the scale of 15,000 nucleotides pretty small.

[allmendream]

Correct and amend my remarks....

“and the chromosomes you inherited from dad are almost exactly a 50/50 inheritance split between his mom and dad.”

Other than the Y chromosome in males - that you get from your father much the same as he got it from his father; with only a small section that does recombination with the X chromosome he got from his mother.

[SunkenCiv]

Something *similar* would work -- there would be a doubling of 21 from one or the other side (which might be determinable if the parents are available for testing and money were no object); beyond that, where the extra one fell down through would very likely be guesswork, unless one had genetic samples going back that many generations.

Half of our chromosomes come from mom, half from dad.

*On average* one quarter come from each grandparent, but obviously there is no integer solution to one quarter of 46, so the concentration of genetic origin begins with the grandparental generation, and it turns into something analogous to the old "war" card game -- an early advantage means eventual victory. Here's some arbitrarily and randomly assigned values, showing the pathway to the 46 of the current hypothetical person (but entirely possible, and given the 2^23 squared possibilities in each generation, this is bound to have happened at least once by now, to someone, somewhere):

---

0	0	0	1	2	1	0	1	1	0	1	2	0	1	0	0	3	1	2	2	1	0	0	1	0	0	1	0	1	1	0	0	0	0	0	0	0	1	1	0	1	3	0	2	1	1	0	1	0	1	2	1	1	0	1	0	0	1	0	0	1	2	2	0
0		1		3		1		1		3		1		0		4		4		1		1		0		1		2		0		0		0		1		1		4		2		2		1		1		3		1		1		1		0		3		2
1				4				4				1				8				2				1				2				0				2				6				3				4				2				1				5
5								5								10								3								2								9								6								6
10																13																11																12
23																																23
46

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I started from 46 and worked back, but if we wanted to maximize the number of genetic sources from the 4ggrand generation (46 of 'em, of the 64 on the tree), we need only put zeroes in 18 of them at random, then carry the totals additively downward. Probably would be a lot easier than the pain in the neck I just went through with that. Hope I didn't miss any typos. And of course, the other way to do that is to do the actual family tree and see if there are any duplicate ancestors. :')

[SunkenCiv]

Here's an example, from the top down:

---

1	1	0	1	0	0	1	1	1	0	0	1	1	1	1	1	1	1	1	0	1	1	1	0	1	1	1	0	0	1	1	1	0	1	1	1	0	0	1	1	1	1	1	1	1	1	1	0	1	0	1	0	1	1	1	0	1	1	1	0	0	1	1	1
2		1		0		2		1		1		2		2		2		1		2		1		2		1		1		2		1		2		0		2		2		2		2		1		1		1		2		1		2		1		1		2
3				2				2				4				3				3				3				3				3				2				4				3				2				3				3				3
5								6								6								6								5								7								5								6
11																12																12																11
23																																23
46

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