You DON’T Descend From All Your Ancestors

Summary

This script delves into the complexities of genetic inheritance, illustrating that we do not inherit equally from all ancestors due to the randomness of meiosis. It explains how the human genome's 3 billion base pairs are divided among fewer ancestors than might be expected, given the exponential growth of ancestors over generations. The script highlights the genetic lottery, where some ancestors contribute significantly to our DNA, while others contribute nothing, and how this pattern will affect our descendants in the future.

Takeaways

• 🧬 The human genome consists of over 3 billion base pairs, with adenine pairing with thymine and cytosine with guanine.
• 🌐 The number of ancestors doubles with each generation, leading to more ancestors than DNA base pairs in our genome.
• 🔄 Due to cousin marriages, the same individuals may appear multiple times in family trees, reducing the actual number of ancestors.
• 🌍 The global population in 1200 AD was estimated at only 450 million, far fewer than the theoretical number of ancestors.
• 🧵 Genetic material from ancestors is inherited in a non-uniform manner, influenced by the process of meiosis and chromosome recombination.
• 👦 Men inherit a Y chromosome from their fathers, allowing for the tracing of direct paternal lineage.
• 👧 Women inherit an X chromosome from both parents, complicating the tracing of direct maternal lineage, but mitochondrial DNA is inherited solely from mothers.
• 🔄 Each person has 47 distinct pieces of DNA, with 23 pairs of chromosomes and one set of mitochondrial DNA.
• 🧬 Chromosome recombination during meiosis results in offspring inheriting a mix of DNA from both parents, with varying proportions.
• 🎰 Genetic inheritance is a lottery; some ancestors contribute significantly to a descendant's DNA, while others contribute nothing.
• 📉 Over time, the genetic contributions of ancestors diminish, with a high percentage of ancestors contributing nothing to a person's genome after several generations.

Q & A

How many base pairs make up the human genome?

The human genome is made up of more than 3 billion base pairs.

What are the four types of base pairs in DNA, and how do they pair with each other?

There are four types of base pairs: adenine pairs with thymine, and cytosine pairs with guanine.

How many generations back does one need to go to have over 4 billion 30th-great-grandparents?

We only need to go back 32 generations to roughly 1200 AD to have over 4 billion 30th-great-grandparents.

Why is it impossible to have descended equally from all of our ancestors around the time of 1200 AD?

It is impossible because the number of our ancestors doubles with each generation, surpassing the number of base pairs in our DNA, making it physically impossible to have inherited from every single one.

How does cousin marriage affect the number of actual ancestors we have?

Cousin marriage causes some of the same people to appear in different places in our family tree, reducing the number of actual ancestors we have compared to the number of base pairs of DNA.

What is the estimated number of people living on the planet in 1200 AD, and how does it compare to the number of 30th-great-grandparents?

It is estimated that only 450 million people lived on the planet in 1200 AD, which is far fewer than the 4 billion 30th-great-grandparents.

How many chromosomes do we inherit from each parent, and what determines our sex chromosomes?

We inherit 23 chromosomes from each parent, making 46 in total. Men receive an X chromosome from their mothers and a Y chromosome from their fathers, which determines their sex.

Why can't we trace a woman's direct maternal ancestor using the same method as for a man's direct paternal ancestor?

It's impossible because women receive X chromosomes from both parents, and the method relies on the Y chromosome, which is only passed from father to son.

How many distinct pieces of DNA does each person have, and what are they?

Each person has 47 distinct pieces of DNA: 23 chromosomes from each parent and 1 complete set of mitochondrial DNA from the mother.

How does the process of meiosis affect the transmission of chromosomes to the next generation?

During meiosis, the pairs of chromosomes are interspliced, creating a set of 23 chromosomes with an average of 26 splices in sperm and 45 splices in eggs, mixing the DNA from both parents.

What does the genetic lottery refer to, and how does it affect the genetic contribution of ancestors to descendants?

The genetic lottery refers to the random process by which some ancestors contribute more DNA to a descendant, while others contribute nothing at all, due to the random splicing events during meiosis.

How does the number of genealogical ancestors compare to genetic ancestors 15 generations back?

Fifteen generations back, there are 32,768 genealogical ancestors, but only 1,112 genetic ancestors, showing that the number of genealogical ancestors far exceeds the number of genetic ancestors.

What is the chance of contributing to a descendant's genome 400 years into the future?

There is only a 3 percent chance of contributing anything at all to a descendant's genome 400 years into the future, unless you are a direct maternal or paternal ancestor.

How does the genetic contribution of ancestors like Prince Edward of Kent and Strathearn compare to that of more distant ancestors in King Charles III's genome?

Prince Edward of Kent and Strathearn, being a 4th-great-grandfather, contributed an outsized 5.3% of King Charles III's genome, while more distant ancestors like Casimir IV Jagiellon have only a 3% chance of contributing anything and may contribute nothing at all.