X-chromosome inhertance problem.

[Roaring]

So i've tested all of my grandparents, and currently have results for 3 of them, as far as i understand i should've inherted half X from m. grandfather, and half form m. grandmother, but that doesn't seem to be the case.

Here are my results: Tatiana- m. g-mother, Raisa- p. g-mother, Vladimir-m. g-father.





Can somebody explain this situation to me?

[geebee]

The same thing happened to you that happened to my brother Bernie and my sister Kim. The X chromosome our mother passed on to each of them contains DNA only from her father, with no recombination* with her maternal X chromosome.

Unlike you, I'm not actually able to compare Bernie's and Kim's results directly to our grandfather. However, I've been able to map matches across the full length of the chromosome -- and all of them trace to our grandfather's side, none to our grandmother.

In addition, I match both Bernie and Kim across the entire X chromosome, except for a 20 cM region surrounding the centromere. Outside of this region, my matches are the same as Bernie's and Kim's. But I have several matches within this region, as well. All are traceable to our grandmother's side.

The bottom line is that a mother's two X chromosomes may recombine, but they do not always have to do so. In fact, this isn't just true for the X chromosome. Even the autosomes will sometimes be passed on without recombination.*

Based on the genome comparison between my father and my daughter Kathryn, I passed on a total of eight chromosomes that are from just one of my parents. Two came from my father: That is, Kathryn matches her grandfather across the entire chromosome. These are chromosomes 9 and 21.

Six chromosomes I passed on to my daughter came only from my mother. That is, they have no matching with my father. These include the X chromosome, of course; but they also include chromosomes 7, 15, 17, 18, and 22.

*Technically, the chromosomes always recombine. However, recombination sometimes takes place between what are called sister chromatids. Since they're identical, the result is the same as no recombination occurred. Only, if I try to explain any further, I'll just get bogged down. So here's a link (or two) that hopefully will give you a better picture:

https://www.nature.com/scitable/definition/meiosis-88
https://en.wikipedia.org/wiki/Sister_chromatids

[lwa714]

You do have a reasonable question. Most of the time, as in my case, a mother's X-chromosomes get shuffled around and recombine to make one X chromosome that she passes down to her child. In some cases, though, The mother doesn't recombine her X's making you slightly more related to one maternal grandparent to the other.

[geebee]

You do have a reasonable question. Most of the time, as in my case, a mother's X-chromosomes get shuffled around and recombine to make one X chromosome that she passes down to her child. In some cases, though, The mother doesn't recombine her X's making you slightly more related to one maternal grandparent to the other.

Well, you're certainly right that the X chromosome a mother passes on to a son or daughter is more likely to have DNA from both of her parents than from only one. But this isn't quite the same thing as saying that it is more likely to have an even split -- half and half -- than an uneven one. This is actually true for every chromosome.

Here's my daughter's inheritance from my father:

• Chromosome 1. Length of chromosome, 284 cM. Matching cM: one segment of 234.26 cM. Matching percent: 82%
• Chromosome 2. Length of chromosome, 269 cM. Matching: two segments, one of 16 cM and one of 61, for a total of 77 cM. Matching percent: 29%.
• Chromosome 3. Length of chromosome 223 cM. Matching: two segments, one of 6 cM and one of 42 cM, for a total of 48 cM. Matching percent: 21%.
• Chromosome 4. Length of chromosome, 214 cM. Matching: one segment of 117 cM. Matching percent: 55%.
• Chromosome 5. Length of chromosome, 204 cM. Matching: two segments, one of 39 cM and one of 28 cM, for a total of 67 cM. Matching percent: 33%.
• Chromosome 6. Length of chromosome, 192 cM. Matching two segments, one of 25 cM and one of 15 cM, for a total of 40 cM. Matching percent: 21%.
• Chromosome 7. Length of chromosome, 187 cM. Matching: none. Matching percent: 0%.
• Chromosome 8. Length of chromosome, 168 cM. Matching: one segment of 26.69 cM. Matching percent: 16%.
• Chromosome 9. Length of chromosome, 166 cM. Matching: one segment of 166 cM. Matching percent: 100%.
• Chromosome 10. Length of chromosome, 181 cM. Matching: one segment of 173 cM. Matching percent: 96%.
• Chromosome 11. Length of chromosome, 158 cM. Matching: one segment of 123 cM. Matching percent: 78%.
• Chromosome 12. Length of chromosome, 175 cM. Matching: one segment of 60 cM. Matching percent: 34%.
• Chromosome 13. Length of chromosome, 126 cM. Matching: one segment of 110 cM. Matching percent: 87%.
• Chromosome 14. Length of chromosome, 119 cM. Matching: Two segments, one of 49 cM and one of 14 cM, for a total of 63 cM. Matching percent: 53%.
• Chromosome 15. Length of chromosome, 141 cM. Matching: none. Matching percent: 0%.
• Chromosome 16. Length of chromosome, 134 cM. Matching: one segment 95 cM. Matching percent: 71%.
• Chromosome 17. Length of chromosome, 128 cM. Matching: none. Matching percent: 0%.
• Chromosome 18. Length of chromosome, 117 cM. Matching: none. Matching percent: 0%.
• Chromosome 19. Length of chromosome, 108 cM. Matching: One segment of 105 cM. Matching percent: 97%.
• Chromosome 20. Length of chromosome, 108 cM. Matching: One segment of 85 cM. Matching percent: 79%.
• Chromosome 21. Length of chromosome, 62.7 cM. Matching: One segment of 62.7 cM. Matching percent: 100%.
• Chromosome 22. Length of chromosome, 72.7 cM. Matching: none. Matching percent: 0%.
• X Chromosome. Length of chromosome, 182 cM. Matching: none. Matching percent: 0%.

As you can see, there really are only two chromosome that she can be said to have inherited more-or-less evenly from both of her paternal grandparents: chromosome 4, where the split was actually 55% to 45%; and chromosome 14, where the split was 53% to 47%. Overall, my daughter actually inherited 47.5% from my father, which would make her inheritance from my mother 52.5% (though my mother was not actually tested).

I don't think this is actually unusual. Recombination doesn't mean that a person will inherit half of each chromosome from each grandparent on a side; it only means that the overall average will probably be close to half.

Intuitively, this actually makes sense. Most chromosomes end up with only one or two crossovers, where a crossover is the point at which the DNA shifts from being from one grandparent to being from the other grandparent on that side. There are more places a crossover can occur that isn't near the middle of the chromosome, that is near it.

So the bottom line is, it's more likely that for any given chromosome, the split will not be 50-50 than that it will be.

EDIT:

I forgot to include my source for the total length of each chromosome:

https://isogg.org/wiki/CentiMorgan#c...per_chromosome