Mitochondrial Eve and Y-chromosomal Adam Diagrams

How to read the diagrams

Personal Characteristics

Each person is represented by the characteristics we want to look at enclosed in a black outline, like this:

Here are the elements, in order:

• 1  Id number, just to keep everyone identifiable.
• B  X/Y Chromosome. Uppercase letters indicate Y chromosomes (males), lowercase indicate X (females). Always with a blue background in a blue box.
• b  Fingerprint variation. This is a non-sex-linked variation not possessed by anyone else in the group. This is one of thousands of genes that vary among the people in our group. This could be anything - immune factors, hair color, etc., but we'll refer to it as a fingerprint just to emphasize the personal touch. Always with a green background.
• b  Mitochondrial DNA. Always inherited from the person's mother. Always with a pink background in a red box.
• s  Shared DNA. These are genes that don't vary among the people in our group. This is to act as a reminder that most of our genes are shared in common with most people. Always a lowercase 's' with a light brown background.

As a mnemonic device, we use consonants B, C, D, and F to represent males and vowels A, E, I, and U for females.

Pairing up and producing new people.

Each new generation is created by the previous generation pairing up and producing offspring. In these simplified diagrams, every pairing produces two children, and altogether 4 boys and 4 girls are born in each generation. We represent one pairing and offspring combination in a blue box:

Note the inheritance rules shown here:

• All offspring get the mother's mitochondrial DNA.
• All males get the father's Y chromosome.
• The fingerprint gene is transmitted at random - each offspring may get the father's or the mother's.
• The shared DNA is always transmitted unchanged. Parts of this may come from the father and part from the mother, but since it is all identical it doesn't matter which.

Note that we don't have any new mutations in these diagrams. The only genes present in generation 8 are ones that were present in generation 1.

However - this is the main point of the diagram - many genes that are present in generation 1 are not present in generation 8. Some are lost, the diagrams show how.

Generation 1: Persons 1-8

Our founding population. 4 men and 4 women, each with unique variations of mitochondrial DNA, X and Y chromosomes, and fingerprint.

Now our population pairs up, each pair having two children

Generation 2: Persons 9-16

After 1 generation:

• All the male mitochondria (inherited from their mothers) are gone.
• F's Y chromosome is gone, because he had only female children. However, F's fingerprint gene is still present.
• A's mitochondria is present but doomed, because it is only carried by male children.

Generation 3: Persons 17-24

After the next generation:

• C's Y chromosome is gone, because his only male descendant, 11, had only female children.
• A's mitochondria is gone. As it happens, A's fingerprint gene is also gone, and A's unique variations have disappeared from our gene pool. In a more realistic example, A would have many other variations that would still be present, as long has she still had descendants. Also, the shared DNA that she possessed is still present, whether she has descendants or not.

Generation 4: Persons 25-32

• B's fingerprint gene is gone.

Generation 5: Persons 33-40

• B's Y chromosome is gone. All the males in this generation have D's Y-chromosome, so D is Y-chromosomal Adam.

Generation 6: Persons 41-48

• E's mitochondria is present but doomed, because it is only carried by male children.

Generation 7: Persons 48-56

• E's mitochondria is gone
• I's mitochondria is present but doomed, because it is only carried by male children.

Generation 8: Persons 57-64

• I's mitochondria is gone. All people in the new generation have U's mitochondria, so U is 'Eve'.

Discussion

I'm a chartmaker, not a geneticist, so I will stick to the highlights I wanted to show in the diagrams.

• The genes that disappeared by generation 8 did so by chance, not because they were selected out. With some different 'rolls of the dice' someone else could have been mitochondrial Eve or Y-chromosomal Adam. Also, with a little tweaking I could have arranged the generations so that nobody was Eve or Adam by the end of 8 generations.

  The random determination sex of the offspring and the random mixing of the fingerprint genes eventually result in some genes become more common and others becoming less common, or disappearing altogether.

  In a more realistic environment, some people would have fewer offspring and others would have more. This would allow natural selection to work and speed the process. However, the process occurs even without natural selection.

• Not being mitochondrial Eve does not mean all your genes have vanished. It only means your mitochondrial genes are gone. In this example U is mitochondrial Eve but E's original fingerprint gene is still present. Similarly, D is Y-chromosomal Adam but C's fingerprint gene is still present.

• It is not necessary for mitochondrial Eve and Y-chromosomal Adam to be determined in the same generation. In fact, it is very unlikely.

Although it was not the point of the diagrams, you can also see the effect of inbreeding in a small population. The total number of genes is much reduced over time. A recessive genetic defect that existed in the 1st generation would be unnoticed. However, if it survived to generation 8 it would be a serious problem because the chance that both parents possessed it would be high.

Condensed Version

Here we've removed all the pairing information and just show the new generations. You can't tell which children belong to which parents in this version, but you can still see how the presence of the mitochondrial genes and Y chromosome changes with each generation.

We've highlighted the eventual Eve and Adam to make it easier to track their genes across the generations.

Highlighted Mitochondrial Eve

Highlighted Y-chromosomal Adam