It means a “close genetic relative” but this relationship could actually be more than hundreds of years apart. That is, the common ancestor of the two living relatives may have lived hundreds of years ago.
The genetic cousins in DNA Nation are identified on the basis of similarity in only one chromosome, (the mitochondrial DNA) out of all the chromosomes in the human body. There are 22 pairs of chromosomes all up, plus the sex chromosomes and the mitochondrial DNA).
The mitochondrial DNA, known as mtDNA, is DNA taken from the mitochondria that are found outside the nucleus of the cell, where all other genetic material is found.
The mitochondrial chromosome is tiny in size - comprising only ~15,500 nucleotides, all the other chromosomes dwarf it. The small genes on it are involved in cell respiration activities.
The mitochondrial chromosome is a single copy (unlike other chromosomes) and is transmitted in a linear fashion across the generations but only from female to female so reflecting an individual’s maternal ancestry.
The only way changes occur on the mtDNA is via mutation which is a relatively rare event.
So if 2 people have the same mtDNA sequence over the 15,500 nucleotides, clearly they share a recent common ancestor. And if there is only one difference in the sequence, then also they share a recent common ancestor.
How recent is difficult to determine because the answer depends upon which nucleotide differs between them. Some we think mutate quite rapidly and others very, very slowly.
Because the mtDNA is only a tiny part of our overall DNA composition, this is why we shouldn’t expect a physical lookalike among our “genetic cousins,” just because we have the same mtDNA. This is exactly what happened with Ian Thorpe and his genetic cousin on DNA Nation.
Full siblings are also the perfect example of this, as they will have identical mtDNAs (because they share their Mum) and yet look nothing alike. This is because of the different mix of other genes they get from their parents.
How do we trace ‘genetic cousins’ using mtDNA?
To help date the era when the common ancestor of the mtDNA of two individuals may have been alive, we examine the variation in DNA. We look at how many times the DNA has mutated at a particular position or nucleotide, as well as how many people have the same mtDNA (population size).
By doing this, we can calculate very approximately when that ancestor lived. The smaller the differences between the two persons’ mtDNA, the closer they are in their maternal ancestry, and the closer ‘genetic cousins’ they are. Of course this does not influence the remainder of their genetic ancestry
Dr John Mitchell is a human geneticist who is also the series consultant for DNA Nation and will act as a guide to intrepid adventurers Ernie Dingo, Julia Zemiro and Ian Thorpe throughout the show.
DNA Nation airs Sundays, 8.30pm on SBS.