All people have 23 pairs of chromosomes
22 pairs of autosomes
1 pair of sex chromosomes.
The chart below from FTDNA shows how we inherit various kinds of DNA. At the bottom of the chart are three types of DNA. Across the top great grandfathers are in blue, green, gray and light blue. Great grandmothers are in yellow, pink, orange and salmon. Each great grandparent is shown with a “O.” (More on these “O”s below.)
The Sex Chromosome Pair: is chromosome 23
Men inherit their X from their mothers and the Y from their father (XY=male)
Women inherit one X from their mother and one X from their fathers. (XX=female)
On the chart the Y-chromosomes are illustrated as a short chromosome for the males only. Each man inherits their Y-chromosome from his father. In our chart the Y is passed down only through the males on the far left to our “son” at the bottom, even though all males inherit their Y from there fathers the Y’s for the green, gray and light blue do not pass to the son at the bottom.
Because each man inherits this Y-DNA virtually unchanged from his father and his father from his father and so forth Y-DNA is the easiest DNA to use for genealogical purposes. In the chart you can see how all the information in the Y-chromosome is passed strictly from father to son in the short blue chromosome. In this example this means anything passed in this chromosome does not get recombined (or get mixed up—it stays completely blue all the way from great-grandfather to great-grandson). So if for instance a trait that had to do with some visual appearance came down along the Y-chromosome it would be more prevalent in all the male offspring than something that was passed down through the autosomes. Another advantage of the Y is that males tend to retain the surnames of their fathers so the Y-chromosome is very helpful in surname studies. If we have two men named Philbrick and Filbrick and we want to see if they are related testing the Y-Chromosome will help determine if or how closely they are related. Even though the chart shows only back as far as great-grandparents the Y-DNA stretches back in time for hundreds and thousands of years.
Not shown on the chart, and trickier to work with than the Y-DNA is the X. Everyone inherits one X from our mothers and a X or Y through our fathers (Male=XY Female=XX). The X-chromosome is included in some autosomal DNA tests. It has a unique pattern of inheritance that can be useful but X-DNA is not being explored as much as the Y-DNA for genealogical purposes.
Just as we can follow the father’s father’s father’s line, we can do the same for anyone’s mother’s mother’s mother’s line using Mitochondrial DNA (abbreviated mtDNA) The mtDNA is shown on our chart by a “O” but you will notice many of the “O”s have an X through them. This is because mtDNA is inherited only through our mother’s so each father’s “O” is not passed on thus is crossed out. Like the Y-DNA, mtDNA does not go through recombination (mixing up) so we inherit our mitochondrial DNA intact from our mothers. We all, both men and women, inherit our mtDNA from our mothers. The chart illustrates in the salmon “O” how we inherit our mitochondrial DNA from our mothers’ mothers’ mothers……line. In our chart it is the great-grandmother on the far right who passes her “O” to the son at the bottom. Even though our chart shows only back as far a great-grandparents the mtDNA stretches back hundreds if not thousands of years just like the Y-DNA. Mitochondrial DNA is not used as much for genealogical purposes as Y-DNA or autosomal DNA but still can be useful in some circumstances.
Autosomes: Chromosomes 1-22
On the chart at the top we see an illustration of one pair of autosomes representing each of our 8 great grandparents. Remember this shows the inheritance pattern for only one pair of chromosomes but we actually have 22 pairs (plus the sex chromosomes in chromosome 23). Below each set of great-grandparents is a representation of how those great grandparents DNA combined to create your grandparents DNA. In the illustration the blue and yellow great grandparents each contribute half of their DNA for your paternal Grandfather who is shown with a blue and a yellow half of his pair. When that paternal Grandfather combines his DNA with your paternal Grandmother the result is your Father. You will see that although he received half of his DNA from your paternal Grandfather and half from your paternal Grandmother the amounts he received from his paternal great Grandparents are not equal. In the diagram he has received a greater share of his paternal Grandmother represented in yellow than he has from his paternal Grandfather. Welcome to the DNA Lottery! Taking it one step further the son has the highest share of his orange Great Grandmother, followed by his yellow Great Grandmother, then his green Great Grandfather and so forth until we get to his blue great grandfather from whom he received very little of his autosomal DNA (in this example). This would be repeated for each of your 22 pairs of chromosomes. So when someone says you favor Grandmother Jones or Great-grandfather Smith this may well be true.