Meiosis, Schmeiosis

In an earlier post, I mentioned in passing that AncestryDNA’s Match White Paper (31 March 2016) misused the word meioses in their Figure 5.2. A full explanation would have distracted from the points I wanted to make at the time, but a few astute readers asked about it, so I thought I’d devote a post to explaining myself. My ulterior motive is, as always, to teach biology in a way that will help with genetic genealogy.

 

Meiosis

First, what does meiosis (the plural is meioses) mean? Meiosis is a scientific term for the special type of cell division that occurs to form eggs or sperm (also known as gametes) in a parent’s body. Meiosis results in a gamete cell getting half of the parent’s DNA; the parent’s cells have two copies of each chromosome — 22 autosomes in total (plus two sex chromosomes) — while the gamete only has one copy of each autosome (plus one sex chromosome). When the father’s sperm fuses with the mother’s egg, the offspring is restored to a full complement of DNA: 22 pairs of autosomes plus two sex chromosomes.

The relationship between a mother and her child involves a single meiosis event, the one that formed the egg that made the child. Same goes for the relationship between a father and child: one meiosis to produce the sperm cell. That between a grandparent and grandchild involves two meiosis (one in the grandparent, one in the parent). Similarly, half siblings are separated by two meioses, one in the shared parent to produce the first child, and a second in that same parent to produce the second child.

This is where AncestryDNA misuses the term. In their figure, they label the group that includes half siblings and grandparents/grandchildren (forest green in the figure) as three meioses, not two. (Note: I earn a small commission if you purchase through the links in this post. The cost is the same for you. Click here for more information.)

 

AncestryDNA labels full siblings as being separated by two meioses, but that’s not the right way to look at it. They are separated by two meioses through a given parent, but they’re also separated by two meioses through the other parent. This isn’t equivalent to two meioses total, nor is it quite the same as 2 × 2 meioses, or 2 + 2 meioses, or even 2 ÷ 2 meioses. A simple count of meiotic events doesn’t apply. Essentially, full siblings are related twice over; they are double half-sibs.

 

(I’m sure AncestryDNA made a conscious decision to try to make the concept easier for the novice to understand rather than an error out of ignorance. Unfortunately, in doing so, they’ve used the term incorrectly.)

 

Crossing over

Meiosis doesn’t just halve the number of chromosomes in a gamete. It also creates the segments that we use in genetic genealogy. Simply put, the pairs of chromosomes that we inherited from our own parents line up with one another and swap bits —literally — before the cell divides. The process is called crossing over. That means the copy of chromosome 1 that I passed on to my daughter wasn’t an exact copy of the one I inherited from my mother or my father; it was a random mixture of the two. (The same applies to the other chromosomes I gave my daughter.) You can’t tell when you compare my daughter to me — we are half identical across the entire chromosome, as shown by the yellow in the GEDmatch comparison below.

But you can tell when you compare my daughter to my parents. On some segments, she matches my mother and on others my father. The points where the matching switches between them is where crossing over happened when my body made the egg that created her.

 

When she has children, crossing over will happen again to break up some of the segments she shares with my parents. For example, her child will share fewer and smaller segments with my mother (the child’s great grandmother).

 

The special case of avuncular relationships

The fact that shared segments, on average, get smaller with each meiotic event has an interesting consequence for avuncular relationships (aunt/uncle/niece/nephew). A full aunt/uncle is separated by three meioses. However, they are expected to share the same amount of DNA as a grandparent or half sibling, who is separated by two meioses. The reason they share in that closer range is that a full aunt/uncle is a double relative (i.e., double half-aunt/uncle). Based on the total amount of shared DNA, we can’t tell the difference between an aunt/uncle and a grandparent or half sibling, but the extra round of meiosis in an avuncular relationship means that the shared segments will be smaller, on average.

As part of a larger study, scientists from 23andMe simulated the numbers and sizes of segments shared in different relationships to produce this nifty graph (their Figure 3A).

 

On the x axis (left–right), they plotted the total amount of shared DNA for each simulated pair of relatives, and on the y axis (up–down), they plotted the number of shared segments. The results for avuncular relationships (in yellow) compared with grandparent–grandchild (in orange) are exactly what we’d expect: an aunt/uncle shares roughly the same amount of total DNA as a grandparent (plotted on the x axis) but with more segments (the y axis). If the same amount of DNA is spread out over more segments, those segments must be smaller, which we predicted based on the greater number of meioses. Although there’s a small zone of overlap between the yellow and orange circles on the graph, in most cases, we should be able to distinguish an aunt/uncle from those other two possibilities. A match who shares between 1550 cM and 2000 cM of DNA is most likely an aunt/uncle if there are more than 35 segments and most likely a grandparent/child or half sibling if there are fewer than 30 segments.

A deeper understanding of meiosis can sometimes be useful after all!

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