The Chromosomes are the nuclear DNA in humans. We have 23 pairs of chromosomes. 22 autosomes that are labeled 1 to 22 from the longest to the shortest and occur in pairs. And the Somal (or Sex) chromosomes X and Y that, in some combination, form the final pair. Mitochondria is the only DNA that is NOT a chromosomes and exist outside the cell nucleus in humans.

What can be confusing is the term chromosome is the term more properly for each DNA strand in the pair. There is an analogy here to the word “twin” where it refers to the singular person of a known pair. (But not implying these are identical twins but more appropriately fraternal twins.) To avoid confusion and improper use of more strict biological terms, we try to use the term strand when referring to a single chromosome of the pair of chromosomes (that is, just one of the pair).

A chromosome structure is not really important. Suffice it to say there are genes that are areas defined to code or develop specific proteins, and there are inter-gene or junk areas between. The ends of the chromosome are termed telomere and are like end caps that are known to slowly degrade and shorten over time. The centromere is a region where the chromosome pair, during replication with sister chromatids, link up. We only mention these locations and the structure because they are highly volatile and thus are not looked at during testing for inheritable markers.

This introduced definition for strand is unconventional but needed to help avoid confusion when describing certain aspects of genetic genealogy clearly. As a result, use of the term in this website links back to this page on chromosomes and the term does not appear in our glossary. The biological term chromatid is not a strand as chromatid only applies to a chromosome during certain phases of replication in the cell. A chromatid and a strand as defined here are analogous in content of DNA for our purposes. But to avoid misusing a well defined biological term, we introduce strand instead. (Many terms co-opted in genetic genealogy are misused, redefined biological terms. But we do not want to start another one here.) As the mitochondria does not occur in a pair, it is also referred to as a strand of DNA here.

For the most part, in our cells, we have unrelated pairs of chromosomes as we obtained one from our mother and one from our father. The are more properly termed homologous chromosomes but that knowledge is not necessary for understanding here. The chromosomes in the pair do not match each other identically (with the caveat that 99.999% of the DNA between any two people is the same as define by the similarity of the chromosomes.) Due to recombination, we more rarely share an exact copy of a chromosome with a sibling or even a parent. The only exception is identical twins from the same sac that usually have near identical DNA throughout. Although we always get 1/2 our chromosomes from a parent, that parent may not give us half from each of their parents (i.e. your grandparents). So although you (normally) have four grandparents, you may only have 15% of your paternal grandfather’s DNA and 35% of your paternal grandmothers. The amount of your paternal or maternal grandparents DNA will always add up to 50% as you get 50% of your DNA from each parent, always. The variance of how much you inherit from which grandparents starts to widen quickly with great grandparents and beyond.

The autosomes range in approximate size from 250 million base-pairs (for chromosome 1) down to 50 million base-pairs (for chromosome 22). This represents from 280 cM to 80 cM; the more common unit of measure in genetics. X and Y are approximately 155 and 60 million base-pairs; respectively with the X coming in around 195 cM. The Y chromosome is never really compared and reported as matching segments like the other chromosomes. This because it is so nearly identical as passed down, to start with, and because the ends of the Y chromosome actually recombine with the X. Biology is messy and such nuances are found if you desire to look. (For example, part of a male’s Y SNPs are reported as the second value in the pair of his X result. Thus giving rise to what appears as a second X chromosome in the male tester.)

See Also

DNA Somal Autosomes

External References