Michael Cooley's Genetic Genealogy Blog GEN • GEN
19 Apr 2016

Y Chromosome Inheritance

I've taught several genetic genealogy seminars learning that, although it is relatively easy for people to understand that we receive half of our DNA from each parent—each child in different combinations—or that everyone receives their mitochondrial DNA from their mothers, it is much more difficult for many of us to understand Y-DNA inheritance. This diagram illustrates it in its simplest form and demonstrates that a woman's lineage has no influence over the Y:



Here's a more complicated explanation as to why roughly half of us have a Y and women do not. Some of you might want to skip this part.


Meiosis

Apart from spiritual considerations, no one denies that our bodies are mortal. Every life experiences death. But our DNA is close to being immortal: it is passed on, largely intact, to our progeny. Our cells, which include our DNA makeup, not only make duplicate copies throughout our lives, they make copies throughout the reproductive process. The results are passed to the embryo which becomes the child.

Most of us understand that women have two X chromosomes, one from each parent, and men have an X and a Y. Women are born with all of their eggs. Only about 400 mature and are released during her reproductive years, each containing one of her two Xes (her father's and one of her mother's). So when it comes to understanding the Y, which carries the male sex gene, we need to look exclusively at the male side of reproduction, a process that results in his spermatozoa.

Men get their Y chromosomes from their fathers and their Xes from their mothers—of course. The male germ cells, those cells that reside in the testes, undergo a specialized cell division called meiosis. Each cell replicates its DNA, exchanges genetic material between the mother-half and the father-half (a process called recombination), and twice divides. The process results in four cells (gametes), each having a unique half, thanks to recombination, of the father's DNA. Millions upon millions of sperm are thus produced during a man's lifetime, each one differing from one another. Only one male gamete is lucky enough to unite with a mother's egg at any one time and, therefore, with a copy of one of her two X chromosomes (along, of course, with the other genetic components). There's roughly a 50/50 chance its single sex chromosome will be an X or a Y. Here's my overly-simplified illustration, a sequence of slides that changes frames every three seconds:



The process is especially important for the recombination of autosomes (chromosomes 1-22) but it also determines sex, since 50% of the male gametes will hold the father's Y.

There is, by the way, a widely-held misconception that recombination happens with the joining of the male and female gametes (sperm and egg). In fact, the recombination occurs while making the gametes. The two strands—the mother's recombined version and the father's recombined version—remain distinct.


Y-DNA Clones

Because both men and women have X chromosomes—and three between the two of them—X inheritance is rather convoluted and is not be discussed. But the Y chromosome is straight-forward. Although recombination determines whether a gamete inherits a Y or an X, it doesn't change the genetic makeup of the Y. (There isn't a mother-part to recombine with.) This means that any son born to a man will have an exact copy of the father's Y. That Y has passed from a germ cell, through recombination to a gamete, and unites with a mother's egg. The chain is unbroken. The Y continues, unchanged, from one generation to the next until a particular lineage has "daughtered out," as will happen with my grandfather's Y with the deaths of his grandsons. The mortality of that Y lineage will become extinct.

One could suggest that we are only the carriers for what really counts.

Well, I did just tell one itsy-bitsy lie. I'll discuss mutations in a subsequent post.