Before we look at how much DNA a child shares with parents, grandparents, and other relatives, Let’s first understand how genes propagate in humans.

Reproduction

There are about 8.7 million species of living beings in the world. About 1.2 million of them are animals.

Virtually all of them (> 99.9%) reproduce via sexual reproduction.

During sexual reproduction, both male and female propagate their genes, but they do it in a peculiar way.

Consider human reproduction.
The male sperm is the smallest cell in the human body.
The female ovum is the largest.
During fertilization, a single ovum chooses and selects from millions of sperm.

There are 23 chromosome pairs in both male and female humans.
The first 22 pairs are called autosomes.
The 23rd pair is XX in females and XY in males.
Notice the asymmetry.
A female has XX while a male has XY, and this is what leads to asymmetry in how genes propagate.

Coming back to the ovum, it has only half, that is, 23 chromosomes.
Similarly, a sperm has 23 chromosomes as well.

• All ova have X as the 23rd chromosome
• 50% of sperm have X as the 23rd chromosome
• 50% of sperm have Y as the 23rd chromosome
• If an ovum is fertilized by a sperm that has X as the 23rd chromosome, then it is a female foetus
• If an ovum is fertilized by a sperm that has Y as the 23rd chromosome, then it is a male foetus

Zygote formation is asymmetrical

• When the ovum mates with a sperm carrying X, then in the 23rd pair, the crossover happens between the two X.
• When the ovum mates with a sperm carrying Y, then in the 23rd pair, very little crossover happens, so, the Y passed down is essentially the same.
• And that’s why Y shrinks generation by generation, eventually, it might even disappear.
• Note that this crossover of X is what makes females have fewer genetic diseases.

A female gets 50% of all 23 pairs from each parent. A male gets 50% of the first 22 pairs from each parent. He gets X from the mother and Y from the father. So, while it is about 50% each. The Y is smaller, making it about 49% from the father and 51% from the mother!

The autosomes, the first 22 pairs, are boring since they propagate symmetrically.
So, let’s focus on the 23rd pair for the rest of the blog post.

Female child

A female gets 50% of the 23 pairs from her mother and 50% from her father. The 50% from the mother is a combination of the two X chromosomes the mother has. So, she gets 25% from each of her grandparents on the mother’s side.

However, on the father’s side, her X comes from the father, who came from the paternal grandmother. So, the paternal grandmother contributes 1/2*(11/22 + 1/23) ~ 27% of the DNA. And the paternal grandfather contributes 23%.

Male child

While the story on the mother’s side is the same as above, the story on the father’s side is different. This time, 100% of the 23rd pair (Y) comes from the father, who came from the paternal grandfather. So, the paternal grandfather contributes 1/2*(11/22 + 1/23) ~ 27% of the DNA. And the paternal grandmother contributes 23% of the DNA.

So, you can start noticing the asymmetry here. While the autosomes propagate symmetrically, the 23rd pair does not.

The Y creates a male line

When one focuses only on the 23rd pair, then the Y chromosome is passed down from the father to son as-is (barring mutations). The X chromosome recombines and introduces genetic diversity.

So, a child’s X chromosome from the mother’s side is a combination of the two X chromosomes the mother has. For a female child, the other X chromosome comes from the father, which came from the child’s paternal grandmother.

For a male child, the other Y chromosome comes from the father, which came from the child’s paternal grandfather.

It is estimated that 16 million people in Central Asia and China are descendants of a single male ancestor, likely, Genghis Khan.

The Mitochondrial DNA creates a female line

There is one thing that goes from mother to both son and daughter, and that’s the mitochondrial DNA. The mitochondrial DNA from the sperm is lost in the process. Leaving only the one from the ovum to be passed to the zygote. Orthodox Jews pass their religion via their mother. 40% of Askenazi Jews alive today can trace their ancestry to just four women.

Notice the weird asymmetry, though. The Mitochondrial DNA is passed down to a child regardless of the gender. However, The Y chromosome is passed down from a father to a son.

What about siblings

• A brother and a sister share 0-50% of the X chromosome from the mother’s side
• Two brothers share the Y chromosome from their father and 0-50% of the X chromosome from their mother
• Two sisters share the X chromosome from their father and 0-50% of the X chromosome from their mother

So, a pair of step-sisters (same mother, different fathers) will share 0-50% of the X chromosome from their mother. While a pair of step-sisters (same father, different mothers) will share 50% of the X chromosome from their father and 0-50% of the X chromosome from their mother. So, step-sisters from the same father are more closely related than step-sisters from the same mother.

What about other relatives

• A female child has no 23rd chromosome overlap with her paternal grandfather
• A female child has 50% overall chromosome overlap with her paternal grandmother
• A male child has no 23rd chromosome overlap with his paternal grandmother
• A male child has 50% overall chromosome overlap with his paternal grandfather

It gets pretty interesting when one looks at other relatives.

• A female child has anywhere between 25% to 50% chromosome overlap with her maternal aunt (mother’s sister)
• A male child has 50% chromosome overlap with his paternal uncle (father’s brother)

23rd chromosome overlap for a female

23rd chromosome overlap for a male