Hi David,
I wrote the above DNA explanation and I also am not from a scientific background but have done entensive reading on DNA transference; the reference comes from
HERE and I agree that it is an overly simplistic way of explaining the possible percentages of genetic relationship since it doesn't properly illustrate the level of randomness involved in the transfer of autosomal chromosomes. You are entirely correct in your summation of chromosome transference and the fact that these percentages are a very vague approximation since the only fact that can be stated with absolute certainty is that you inherit exactly 50% of your DNA from your father (i.e. 23 of his 46 chromosomes of which 44 are autosomal and 2 are sex-determining) and the other 50%, with the exact same breakdown, from your mother. Which 22 of your father's 44 autosomal chromosomes you inherit and which 22 of your mother's 44 you inherit, however, is completely random. As far as the two sex-chromosomes are concerned you inherit one from each parent; which sex chromosome you inherit from your mother is also random since a female possesses two X-Chromosomes and can, therefore, only pass one of those down but which one you inherit from your father is not random at all since he possesses one Y-Chromosome and one X-Chromosome so if you are male you must have inherited his Y-Chromosome and if you are female you must have inherited his X-Chromosome (i.e. since you are male you definitely inherited your father's Y-Chromosome and one of your mother's two X-Chromosomes but if you were female you would have definitely inherited your father's one X-Chromosome and one of your mother's two X-Chromosomes).
The percentage of DNA each person shares with any relation other than a parent is unique and highly variable due to the way DNA is passed down in this mixture of chromosomes (i.e. it is highly possible you may have inherited a higher percentage of chromosomes from your paternal grandfather than you did from your paternal grandmother as, although it can be categorically stated that you received 50% of your DNA from your father, that does not necessarily mean you will have inherited exactly 25% from each of his parents.) In the case of parents the existence of obligatory genes mean a positive result can be achieved without the need for statistical probabilty but for all other relationship tests a
Likelihood Ratio is used and the more distant the relationship the more approximate the percentages are likely to be. You are also correct to note that Y-Chromosome and MtDNA testing do not have such probabilities attached to them due to the fact they are passed down virtually unchanged for hundreds of years.
Since only the 50% figure for parental inheritance is entirely accurate this means even the approximate 50% you share with a sibling is highly variable and the actual relatedness could theoretically range from 0% to 100% so it is statistically possible for a person to share more DNA with a sibling than they do with a parent. It is, as you say, conceivable, although virtually impossible, that one sister could end up with an entirely different set of 44 autosomal chromosomes to another sister but two full sisters have to share at least one chromosome as they both receive the exact same X-Chromosome from their father since he only has one X-Chromosome and the same applies to full brothers sharing their father's Y-Chromosome. A sister and a brother, however, could hypothetically share no chromosomes at all if they inherited the exact opposite sets of autosomal chromosomes from their parents and the boy then received the father's Y-Chromosome and one of the mother's two X-Chromosomes as his sex determining chromosomes and the girl received the father's X-Chromosome and the mother's other X-Chromosome but they would still share MtDNA. You would be hard-pressed to find this pair though and indeed I have seen no evidence that any instances of this phenomenon have ever been documented. Conversely siblings could share more than 50% of chromosomes with each other but the only set of siblings that you can state a percentage for categorically are identical twins who share 100% of all their chromosomes; if non-identical twin siblings were found to share 100% of their DNA then I would suspect some radical scientist had succeeded in an attempt at human cloning as the odds on this occurring are less than one in a trillion (see
HERE) and, once more, no examples that I know of have ever been uncovered. In reality I would expect most siblings to share somewhere between 40%-60% of chromosomes with 50% as a decent guide average and those who share significantly more or less to be relatively rare.
I included the approximate percentages in my explanation above as I felt they were useful as a rough guide for relatives to see how unlikely an autosomal match beyond the grandparent generation is and I did not expand on it as I did not want to make the explanation even longer or deviate too much from the main objective of helping relatives understand which distant relations may make the best DNA matches. I have since altered this paragraph slightly on my master copy, however, to include more of an emphasis on the high level of approximation involved as after re-reading it I felt, like you, that this needed to be made clearer but I forgot to ask Mel to alter it on here so apologies for any confusion. I have read a few articles which state that although autosomal testing is currently not possible beyond the grandparent generation scientists are currently working on broadening its scope but I'm sure, as you say, that would simply create a set of circumstances in which we would find even lower percentages of probability than we currently have for closer generations.
Mutations also have to be taken into consideration but I won't go into those complexities here. Ultimately I think autosomal testing will not be a highly used method in relation to Fromelles and, considering the latest press releases, I feel it is more and more likely MtDNA will be the main DNA testing method employed.
Regards
Michelle
Edit: I see Mel has replied with an excellent explanation above while I was writing this anyway!