The biology of heredity.
MtDNA
Every one of us has mitochondria in every single one of our 50 trillion or so cells. Mitochondria generate additional energy to power those cells. Mitochondria aren’t just organelles in human cells, either: they are found in nearly all species of plants and animals. Mitochondria are intriguing because they contain their own DNA. Mitochondrial DNA (MtDNA) in human beings is a small circular string of 16 thousand base-pairs, much simpler than the DNA found in our cell nuclei. Another important difference between MtDNA and nuclear DNA is that our MtDNA comes only from our mothers.
When the male’s sperm fertilizes the female’s egg, their genetic material recombines, the egg and the sperm each contributing 23 chromosomes to the new zygote. Both the egg and the sperm contain their own mitochondria, but the sperm’s few mitochondria are doomed to die: the egg locates sperm mitochondria and destroys them before they get a chance to replicate. At birth, human infants have nuclear DNA which is a fusion of their mother’s and their father’s nuclear DNA but their cells contain only their mother’s MtDNA.
Each of us had a biological mother and biological father: going back a single generation, each of us inherited genetic material from 2 different human individuals, mom and dad. If we regress 2 generations, to our grandparents, we find 4 different individuals contributing to our genetic heritage. The number of people who contributed to our individual nuclear DNA grows exponentially as we go back in time: a mere 5 generations ago, about 125 years ago, 32 different people contributed to our nuclear DNA. One of those 32 people, the woman who was your mother’s mother’s mothers’ mother’s mother had the exact same MtDNA as you do, because you got your MtDNA in an unfused line of descent from her. Twenty generations back, about 500 years ago, as many as 1 million different people contributed to your genetic inheritance. One of those million individual people, a woman, passed her MtDNA unaltered down to you in matrilineal descent.
When MtDNA gets copied, rare mistakes and mutations occur, some of which result in still-functional mitochondria. Your MtDNA is almost certainly identical to your mother’s MtDNA and your mother’s mother’s MtDNA, but the further we recede in matrilineal descent, the more likely we are to encounter mutations. Geneticists examine those rare mutations to calculate “genetic distance” between the now 100s of thousands of analyzed MtDNAs. They then use those calculations to trace, organize, and analyze human ancestry and migration. An MtDNA haplogroup is a set of haplotypes, statistical groupings of closely related DNA. Different haplogroups are found in different geographical regions and are due to distinct population histories.
Y-chromosome: patrilineal haplogroup
Every one of our 50 trillion or so cells has a nucleus which contains chromosomes housing our nuclear DNA. Human beings have 46 chromosomes organized in 23 joined pairs and containing about 3 billion base-pairs, which function to organize each individual’s biology. One of those chromosome pairs is sex-differentiating: men have an XY-chromosome-pair and women have an XX chromosome-pair. When females produce eggs and males produce sperm, those 23 pairs of chromosomes, diploids, are split into 23 single chromosomes, haploids. Any given sperm has either that man’s X chromosome or his Y chromosome. At fertilization, the haploid cells, sperm and egg, recombine their nuclear DNA to produce a diploid, the zygote. If a Y-chromosome bearing sperm fertilizes an egg, the zygote will almost certainly differentiate into a male and if an X-chromosome bearing sperm fertilizes an egg, the resulting zygote will almost certainly differentiate female.
Every person’s nuclear DNA is a recombination of the nuclear DNA of their biological parents. The exception is the male Y chromosome, which is almost certainly identical to the father’s Y chromosome. That men inherit their father’s Y chromosome unaltered by their mother’s DNA allows researchers to trace patrilineal descent. A mere 5 generations ago, about 125 years ago, for example, 32 different people contributed to a man’s nuclear DNA. One of those 32 people, the man who was the father’s father’s fathers’ father’s father had nearly the exact same Y chromosome as the man does, unaltered by any other ancestor. And 20 generations back, about 500 years ago, as many as 1 million different people contributed to that man’s genetic inheritance. One of those million individual people, a man, passed his Y chromosome unaltered down to him in patrilineal descent.
When the Y chromosome gets copied, rare mistakes and mutations occur, some of which result in still-functional chromosomes. A man’s Y chromosome is almost certainly identical to his father’s and his father’s father’s Y chromosome, but the further we recede in patrilineal descent, the more likely we are to generate mutations. Geneticists examine those rare mutations to trace, organize, and analyze human ancestry and migration. A Y chromosome haplogroup is a set of haplotypes, statistical groupings of similarity in DNA: they allow geneticists to organize human descent. Different haplogroups are found in different geographical regions and are due to distinct population histories.