DNA Briefly Explained

A brief lesson on terminology and genetics.

We are “diploid” organisms because we each have 2 copies of most chromosomes, one from our mother and one from our father (some organisms like worker bees only have one of each chromosome, and they are referred to as “haploid”). When considering the orgin of only one of the chromosome pairs, it is termed “haplotype”.

For chromosomes that are paired (1 from mom and 1 from dad, most of them), there is recombination (“crossing over”) that takes place as the egg develops, resulting in shuffling of genes between the maternal and paternal chromosomes. Then, the chromosomes segregate to form haploid eggs (similar development occurs for “haploid” sperm, which then join with the egg during fertilization to form a diploid organism). Each gene on a chromosome is likely to vary in small ways from other unrelated members of the species because of mutation, but the closer related 2 people are, the better chance that they have at least one identical gene, or at least fewer mutations that distinguish them.

However, because of crossing over, one can have genes from ones mother and ones father in one chromosome – that becomes a challenge to sort out if one is trying to determine heritage (but it can be done, and is done).

Y chromosomes are used to track male lineage because females don’t have a Y chromosome.

Also, Y chromosomes don’t cross-over because there is not a maternal Y chromosome to cross-over with. So, any differences between the Y chromosomes of 2 given males is due only to mutation. The more “different” they are, the more distant is the relationship between the males.

The rates of mutation are fairly well known, so the “distance” between their relatedness can be calculated. In order to get more usable and reliable results, these sorts of tests typically look at many different “marker” regions. So, if 2 males have identical markers all along the Y chromosome, one can assume direct relatedness. If they differ by, say, 1 marker out of 50, one can use that frequency (1/50) to calculate how long ago they were likely to have shared a common male ancestor. With only 1/50 difference, it would likely be in the not-too-distant past. If there were 3/50 differences, that common male ancestor would have been that much further in the past.

Also … be sure you can actually trust the lab you choose to test and store your family DNA. Here’s why I say this: https://gizmodo.com/what-dna-testing-companies-terrifying-privacy-policies-1819158337. The lab who tends the Parr family DNA project is 100% trustworthy!

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