Y-Chromosome DNA or Y-DNA is the DNA carried down the direct paternal line, and the DNA that belongs to males only. Y-DNA is inherited from a man’s father, who got his Y-DNA from his father, all the way back to the end of the line. Y-DNA is only inherited through that direct line, and therefore not inherited from an individual’s mother’s father.
In humans, the Y-DNA chromosome spans around 58 million base pairs, which results in it representing approximately 2% of the total DNA in a male cell. Traits that are inherited via the Y Chromosome, called holandric traits, are inherited from the 86 genes that make up the human Y chromosome.
The most common ancestor on the Y-DNA side is known as Y-Chromosomal Adam. Unlike his biblical namesake, Adam was not the only living man of his time; he’s not even the only man of his time to have descendants living today. Adam is however the furthest back a direct, unbroken paternal line can be tracked, with that line said to travel back between 237,000 and 581,000 years ago.
In an earlier article, regarding the most common ancestor on the maternal line, MitoChondrial Eve, Adam was discussed, and it was deciphered that the name is where the biblical similarities end for Y-Chromosomal Adam and mt Eve. It is incredibly unlikely that mt Eve and Y-Adam would have lived in the same time in history, let alone ever met. They are just the furthest back a direct ancestor has been traced for mtDNA and Y-DNA.
Y-Chromosomal DNA differs from other forms of DNA; for example, in comparison to Autosomal DNA, Y-DNA does not re-combine with mtDNA during meiosis (cell splitting during reproduction), but is transferred intact from father to son. As with other forms of DNA, mutations occur periodically, and these mutations are then passed on to males every other generation. Then, if you were to plot these mutations into a family tree, with each mutation serving as a branching point and followed it to the top, Y-Chromosomal Adam would sit at the top of this ‘family’ tree.
Y-Chromosomal Adam is believed to have originated in East or Southern Africa, with the basis of this theory being that Haplogroups A and B achieve their highest frequencies here. Y-DNA itself however is thought to have originated no more than 166million years ago, at the time of the monotremes (mammals that lay eggs such as the platypus or achidna) split from other mammals.
You may wonder why we’ve decided to focus an article on Y-DNA; after all, Y-Chromosomes play a limited role in our make-up, with very few functional genes, they also aren’t even found in women. Well, all of our chromosomes come in pairs, and everyone gets one of each pair from their mother and from their father. Although each chromosome derives entirely from them, they are never passed down exactly, and with each chromosome comes changes.
The reason for this is that when parent’s chromosomes are passed on to one of their children, the DNA is re-assembled in a completely new, shuffled way. In effect, a little bit of chromosome A and a little bit of chromosome B are tagged together along the entire length of the genome. The reason for this recombination occurring is still largely unknown, but given its regular occurrence in nature there must be a reason for it.
Clean lines of descent within ancestral lines are very quickly lost when a mutation occurs. Some very short regions of shuffled chromosomes called Haplotype blocks do occur, but they only tend to last several generations before they are broken, making the ancestral line very difficult to track. Geneticists ideally want to study a large piece of DNA with no recombinations, and it is this that makes Y-DNA so important.
The Y is found inside cells like any other regular chromosomes, but due to the inexplicable changes in genetics it is incredibly unique. If reproduction occurs, and a Y chromosome is matched with an X Chromosome, the gender of that embryo is determined. If a Y chromosome is present, the child will be a boy, if a pair of X’s are found, it’ll be a girl. This mis-match with the X chromosome is what makes Y-DNA so important. There is no recombination with Y-DNA, and the Y-DNA gets passed down the paternal line intact. This then provides geneticists with an incredible opportunity to trace genetic lineage far back in time, all the way back to our most common paternal ancestor, Y-Chromosomal Adam.