How genetically similar are siblings?

How genetically similar are siblings?
Written by Dave Gennert, PhD candidate in Genetics at Stanford University

How genetically similar are siblings?

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

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