This tutorial is Part 2 of a 3 part series
Part 1, Autosomal DNA 101
Part 2, Inheritance pattern of Autosomal DNA <<== You are here
Part 3, Applications of Autosomal DNA
We are all unique
No two individuals who ever lived is the same... we are all unique. With the exception of identical twins, no two individuals have the same autosomal DNA profile. That's why autosomal DNA testing is also known as "DNA Fingerprinting".
How does this differ from mtDNA and Y-DNA?
All individuals who share the same maternal line will have the same mtDNA profile. That's because your mtDNA was inherited from your mother. Your siblings, your mother, your grandmother, your cousins and nephews and nieces and aunts and uncles on your mother's side of the family will all have exactly the same mtDNA profile as you. However, no two individuals, with the exception of identical twins will share the same Autosomal DNA profile.
Likewise, all males who descended from the same paternal lineage will have exactly the same or a very similar Y-DNA profile. That means that your Y-DNA profile will be the same as your brother's, your father's, your grandfather's and all other male descendents who shared the same paternal lineage as you. Again, this differs from autosomal DNA, as no two males, not even brothers, will have the same autosomal DNA profile.
Why are we all so unique?
The answer lies in the inheritance pattern of autosomal DNA. Several key events happen during reproduction that ensures that no two individuals are ever the same:
1. Random inheritance pattern of 22 autosomal chromosomes from each parent.
2. Crossing over.
Let's go over these events in further detail.
Random Inheritance pattern of Autosomal Chromosomes
Humans have 22 pairs of Autosomal Chromosomes. We inherit autosomal chromosomes from both of our parents. One chromosome from each pair comes from our mother and the other is from our father.
Each parent will only give us one of their two chromosomes
Each of our parents also carry 22 pairs of autosomal chromsomes, one from each of their own parents (our grandparents). For example, our mother carries one copy from her own mother (our maternal grandmother) and one copy from her own father (our maternal grandfather). Likewise, our biological father carries one copy from his own mother (our paternal grandfather) and one copy from his own father (our paternal grandfather).
Which one to pass down?
While each of our parents have two copies of each autosomal chromosomes (one their own mother and one from their own father), they can only pass one of their two copies down to us. For example, for each chromosome, our mother has one copy from our paternal grandfather and one from our paternal grandmother. Which one will she pass down to us? The answer is "it's random". For each one of the 22 autosomal chromosomes, we receive only one of the two copies from our mother and one from our father.
Thus, for each chromosome that we inherit from our parents, there are 4 different possible combinations "options".
The 4 possible genes that we can inherit from our parents is also known as a "gene pool". For each gene that is found on autosomal chromosomes, the gene pool consists of 4 possibilities.
The example above shows the four possible combinations for a single chromosome pair as it is passed down from parent to child. Our parents each carry two copies of each chromosome. However, they do not pass both copies down to us. We will randomly receive one chromosome from each parent. That means that there are 4 possible combinations that we can receive from our parents for each chromosome!
Next, multiply this scenario by the power of 22....
The example above only takes into consideration the possible combinations for one chromosome. Next, imagine the possible number of combinations that we can have given the fact that we have 22 pairs of Autosomal chromosomes, one copy of each pair coming from each parent. The math would be as follows:
Each pair has 4 possible combinations, and there are 22 pairs of autosomal chromosomes. That means that we can have one of 7 x 10 to the power of 13 possible combinations from our parents!
This large number of unique combinations is so far based on the assumption that the chromosomes are passed down unchanged from generation to generation. However, another remarkable event happens during sexual reproduction called "crossing over" which ensures that our autosomal chromosomes are even more random.
Crossing over is an event that occurs during meiosis (during the formation of the egg or sperm). Before the pairs of chromosomes split up to form the gamete, they "cross over" and swap little pieces of DNA. It's almost like shuffling a deck of cards before dealing them out to ensure randomness.
The combined effects of crossing over and random inheritace results in a staggering number of possibilities, ensuring that no two individuals, not even siblings, can possibly have the same genetic profile.
The uniqueness of each individual's autosomal DNA is the reason why the genetic profile that is obtained after Autosomal DNA testing is called a "DNA Fingerprint".
Our Autosomal DNA is a mixture of DNA from many of our ancestors
The completely random inheritance pattern and shuffling of Autosomal DNA results in each individual carrying a unique mixture of DNA from many of their ancestors, with a random mixing of chromosomes in each generation. Unlike Y-DNA testing, which is preserved along the paternal lineage (a male individual's Y-DNA is the same as his brother's, father's, grandfather's..... etc.), no two people living in the world have identical Autosomal DNA (except identical twins).
Next, in Part 3 of this tutorial, we will discuss the Autosomal DNA STR marker test and some of its most popular applications.