Why do siblings end up with different genetics?

Did you already have your answer in mind after reading the title? I know I did; I immediately thought “siblings, right?” Long story short, yes, you can be genetically closer to your siblings than your parents, but you’re considered to be about equally related to both your parents and siblings. To understand why, I looked for the science behind this question.

First, let’s review some genetics. Each person has 23 pairs of chromosomes, 46 in total, receiving one set of 23 chromosomes from each parent. For simplicity, we will exclude the sex chromosomes (X and Y) for now. (More on those at the end!) Each chromosome is broken up into small segments, called genes, that are commonly called the “functional unit of heredity” because they contribute to your unique traits. Because you have two of each chromosome, you can have two slightly different versions of each gene, called alleles.

Doing the math

At first thought, the math may seem easy: you and your siblings have the same 50/50 chance of inheriting one allele or the other from each parent. Therefore, wouldn’t you be equally half-related to each one of your parents and siblings?

However, there is one more component to consider: you are almost exactly half-related to each parent because half of your DNA comes from each of them. On the other hand, you are only half-related to your siblings on average, meaning you could share more or less DNA with your siblings. Statistically, siblings could theoretically be anywhere from 0 to 100% related if they got none or all of the same alleles. Consider the following possible heredity situations, which look at 2 pairs of chromosomes, for simplicity:

The percentages underneath each sibling represent how genetically similar (related) they are, compared to Sibling 9. Each chromosome is represented as an “X”. Different alleles are represented as different colors.

Each sibling has 50% of the same genes as each parent, but the variety of possible allele combinations gives a range of reliability between siblings. Taking an average of the percent relatability between siblings gives you 50%.

Opposite ends of the spectrum

The only example of siblings that share 100% of their DNA are identical twins. This type of twins is also called monozygotic because they originate from one zygote, or fertilized egg, that split in two. Because they came from the same exact sperm and egg combination, they share the exact same DNA.

The odds of siblings being “0%” related is extremely slim, estimated to be about one in ten million if you just consider the 23 chromosomes as is! However, considering other genetic factors, like mitochondrial DNA, the Y chromosome, and genetic recombination, siblings actually cannot be “0%” related.

Looking deeper

The simple examples so far have excluded more complex genetic factors, such as the sex chromosomes, mitochondrial DNA, genetic recombination, and new mutations.

Only the father can pass along his Y chromosome, as the mother only has two X chromosomes. Therefore, brothers and their father share their entire Y chromosome. Although random mutations may happen throughout generations, the Y chromosome remains largely unchanged. In fact, two men separated by 13 generations only had four mutations, which is remarkably low!

Similarly, siblings share their mitochondrial DNA with each other and their mother because their mitochondria only come from the mother’s egg. The mitochondrion, colloquially called the “powerhouse of the cell,” is an organelle found in most human cells that provides energy to the cells. It’s unique in that it has its own DNA – albeit a tiny amount, only coding for 37 genes – which provides evidence that the mitochondrion may have originally been a bacterium that was engulfed by a cell. This resulted in a beneficial relationship for both and stuck around throughout evolution, turning what once may have been a bacterium into what is now an essential part of most eukaryotic cells.

Genetic recombination shuffles alleles between chromosome pairs in a parent’s cells as the pairs get split up into sex cells through a process called meiosis. This creates sex cells with different combinations of alleles in each parent, which can be uniquely combined to create each child.

New mutations can also contribute to differences between generations. It was estimated that there are about 64 new mutations per generation, based on the mutation rate and number of divisions to generate sex cells. These new mutations further contribute to the uniqueness of each person.

Despite being a unique combination of your parent’s genes and new mutations, you are, on average, equally genetically close to both your parents and siblings.

Chelsea Weidman is a biochemist and freelance science writer/editor interested in genetics, gene editing technologies, stem cells, and novel medications. She has written for Massive and Harvard University’s Science in the News.

Her research has focused on a range of topics such as molecular imaging agents, reversible covalent chemistries, and novel antibiotic strategies. She received her Bachelor of Science degree in Biochemistry and her Master of Science degree in Chemical Biology.

Siblings share 50 percent of their DNA. Even though siblings have the same parents, they have unique genomes because the sperm and egg cells they came from had unique genomes as well. Every child receives half of each parent’s DNA. It is random which of the parent’s copies of each chromosome is passed down, made even more random by crossing-over events mixing the parent’s chromosomes together before being passed down.

For any two siblings, there is a 50% chance that the mother will pass down, for example, the same piece of chromosome 4 to both children. This is true for every base along the genome. The mother has two copies, one having been passed down to one child, and which one gets passed down to the other is random. This is also true for the genome passed down by the father. The result is that for any two siblings, their genomes will, on average, be 50% identical.

But I thought any two humans are 99.9% similar?

Since the genomes of any two humans are 99.9% similar, what is really meant by siblings being “50% identical” is that half of the DNA bases that would vary between humans are identical in siblings.

How exactly, do parents pass on their DNA to their children?

The combination of a sperm and egg to create a new life is a very complex process that joins the genetic information from each parent into a totally unique child. What is each parent contributing, and how does this process take place? The genomes for a couple’s children will end up being different, even though they come from the same parents. Their differences arise through the random process of deciding which chromosomes— which parts of are a parent’s DNA— are passed to the child from each parent. This occurs during the process of meiosis.

What happens during meiosis?

Most cells in the human body have two copies of each chromosome. When reproducing, each partner contributes half of their DNA by contributing half of their chromosomes. This requires specialized cells with only one copy of the genome. Meiosis is the process the body uses to produce cells with one copy of the genome from cells that have the typical two copies. Sperm and eggs are the special cells resulting from meiosis that have only one copy of each chromosome. Meiosis only happens in specialized organs: sperm are generated in the testis, eggs are generated in the ovaries.

Meiosis leads to new combinations of DNA in your child

During meiosis, sperm and egg cells first duplicate all their DNA, so that they now contain four copies of the genome in total. At this step, the matching sets of chromosomes physically link up. All of the chromosome 1s join up, all of the chromosome 2s, etc. While all four copies of the chromosomes are intertwined, a random point along the genome will break on two of the copies, and the broken ends will be reattached to the other copy. In essence, pieces of each chromosome will swap places.

Crossing over is the step during meiosis where new combinations are created

This important process is called crossing over, and it means that the resulting chromosomes are no longer the same as when meiosis started. Pieces of the chromosomes have “crossed over” to create new combinations of DNA sequence, since the resulting chromosomes are a mixture of the parent’s maternally inherited DNA and paternally inherited DNA.

After this step, the cells will divide twice, each time splitting the joined chromosomes randomly between the new cells until the resulting cells each have one copy of each of the 23 chromosomes. These cells will go on to become mature sperm or eggs.

Practically, crossing over means that, for example, the chromosome 4 a mother passes down to her child is not the same as either of the chromosome 4s in her own cells, since the new one will contain portions from both. Importantly, the point along the chromosome where crossing over occurs is chosen at random in every cell undergoing meiosis. The specific configuration of mixed-up chromosomes will be different in every resulting sperm or egg cell. This means that every sperm or egg is guaranteed to have a unique genome, adding an element of randomness and diversity to the resulting childrens’ genomes.

Orchid offers advanced genetic testing for couples planning on building their family. We use advanced tools and smart, caring humans to help you give your future children the best shot of a healthy life. Conceive with greater confidence and peace of mind

‍