Jim Sims

Making genetic genealogy more understandable...

Y-chromosome Inheritance
Only males have a Y-chromosome, so you need a male to be the donor for a Y-chromosome (also called a Y-DNA) test. A Y-DNA test returns information only about the direct male biological ancestors of the male DNA donor. That portion of the Y-chromosome analyzed by Y-DNA tests does not undergo recombination during reproduction. This DNA is inherited as a large block of DNA without recombination.


ydna_pedigree

In the four generation pedigree chart above, the generation at the bottom of the chart is the youngest generation, and the generation at the top of the chart is the oldest generation. Assuming that we sample one of the males at the bottom of the chart, we see that his Y-chromosome, denoted by the salmon color, was inherited from his father, who received that Y-chromosome from his father and so on backward in time. We get no information about other male ancestors shown in this chart. We only get the direct male line. For full siblings, you only need to test one male to get all the information about the direct male ancestors Y-chromosome.


mtDNA Inheritance
Everyone has mitochondrial DNA (mtDNA) and everyone inherits this type of DNA only from their mother. Anyone can be a DNA donor for a mtDNA test. Below is a four generation pedigree chart showing the pattern of mtDNA inheritance. mtDNA does not undergo recombination during reproduction. This type of DNA in inherited as a unit without recombination.

mtdna_pedigree


In the four generation pedigree chart above, the generation at the bottom of the chart is the youngest generation, and the generation at the top of the chart is the oldest generation. Assuming that we sample one of the people in the youngest generation, we get information from a mtDNA test only about their mother, their mother's mother and her direct maternal ancestors going backward in time. Notice that for each pair of mother and child in this chart, the child inherits mtDNA from the mother (as denoted by colors for each distinct mtDNA). For full siblings, you only need to test one sibling to get all the information about the direct maternal ancestral mtDNA. Testing mtDNA for the more recent generation, shown at the bottom of the chart, gives no information about ancestors who are not in the direct maternal line.


Autosomal DNA Inheritance
Everyone has autosomal DNA and everyone can be a donor for an autosomal DNA test. Autosomal refers to chromosomes 1 through 22 in the human genome. Autosomes undergo recombination at the time of reproductions, and this means that each human is a unique recombination of their parents. Children are not simply half Mom and half Dad, they are unique recombinations of Mom and Dad's DNA.


autosomeal_pedigree

The colors in the four generation pedigree chart above try to convey the fact that we are each a unique recombination of Mom and Dad. Because siblings will inherit different combinations of DNA from Mom and Dad, testing more than one sibling gives better coverage of the autosomal inheritance. Generally speaking, autosomal DNA tests give information for all of the ancestral lines going back to 3rd and 4th great grandparents. Autosomal DNA is not very good at detecting very distant cousin matches.

Autosomal DNA is the most popular form of DNA testing today, and it is also the most complicated to use effectively. The International Society of Genetic Genealogy has a glossary of genetic genealogy terms. My web site has a number of tutorials on how to use vendor web sites after testing autosomal DNA. It is suggested that you work through the tutorials in this order, according to your needs: autosomal DNA part 1, autosomal DNA part 2 and autosomal DNA part 3.

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