It’s a common question many siblings have – how can my sister and I look so different if we have the same parents? The answer lies in the fact that siblings can and often do have subtle differences in their DNA. While siblings inherit the same basic genetic blueprint from their parents, the way those genes get expressed can vary between siblings.
How DNA is inherited from parents
Each person receives half of their DNA from their mother and half from their father. Specifically, when egg and sperm cells are formed, the DNA strands from the parents crossover and recombine in a process called recombination. This mixing and matching of genetic material is the reason why siblings are not identical clones of either parent.
Additionally, during reproduction, there are often random mutations that occur in the DNA – these small differences in the genetic code between siblings also contribute to their uniqueness.
Reasons for genetic differences between siblings
There are a few key reasons why siblings can have distinct DNA:
Crossover events
As previously mentioned, when egg and sperm cells form, pieces of the mother’s and father’s DNA strands swap places in a process called crossover. The position of these crossover points is random, leading to new combinations of genetic material being formed.
| Parent 1 DNA | A B C D E F G H |
|---|---|
| Parent 2 DNA | 1 2 3 4 5 6 7 8 |
| Child 1 DNA | A B C 3 4 5 G H |
| Child 2 DNA | A 2 3 4 5 F G H |
As you can see in the example above, the position of the crossover event for child 1 was between DNA segments C and 3, while for child 2 it was between B and 2. This results in the children having different combinations of DNA inherited from the parents.
Random mutations
During the formation of reproductive cells and embryo development, random mutations can occur in the DNA sequence. These changes to the genetic code contribute to the genetic diversity between siblings.
Epigenetics
Epigenetics looks at how external and environmental factors can influence how genes are expressed. Things like diet, stress levels, and exposure to toxins can affect the epigenome (protein markers that attach to DNA and control gene expression). Even though siblings have the same underlying genetic code, differences in their epigenetics lead to differences in physical traits.
How much DNA do siblings share?
On average, biological siblings share about 50% of their DNA. However, the actual amount can vary from as little as 35% to as much as 65%.
Here’s a breakdown of how much DNA siblings typically share:
Full siblings
Full (or whole) siblings have the same biological parents. They share, on average, 50% of their DNA, but results from sibling DNA tests can vary within the range mentioned above.
Half siblings
Half siblings share only one parent. They tend to share about 25% of their DNA, but results can vary between 15-35% due to the random nature of DNA inheritance.
Step siblings
Step siblings are not biologically related – they do not share the same parents. They will share 0% of their DNA (unless their biological parents were related in some way).
What traits differ between siblings?
There are many physical traits that can vary between brothers and sisters due to differences in their genetics. Here are some examples:
Height
Height is largely determined by genetics. DNA differences between siblings mean that it’s common for brothers and sisters to end up with different adult heights.
Facial features
Subtle differences in the shape of facial features like eyes, nose, lips, and jawline often exist between siblings. These small variations stem from distinct genetic influences.
Hair color and texture
Hair color depends on the amount of melanin produced – siblings can end up with different hair colors based on their inherited genes controlling melanin production. Hair texture (straight, curly, etc) is also genetically determined and can vary between family members.
Disease risk
If genetic variants associated with a particular disease are passed down, one sibling may inherit the variant while the other does not. This results in siblings having differing risks for some genetic conditions.
Personality traits
While personality is influenced by many factors, research suggests genetics play a significant role. Small differences in sibling DNA could manifest as noticeable differences in personality over time.
Can siblings have different ancestry results?
Yes, siblings can have varying ancestry composition results from DNA testing companies. Let’s look at how this happens:
Only certain DNA segments are tested
DNA testing companies use SNPs (single nucleotide polymorphisms) to determine ancestry percentages. Since siblings inherit different combinations of parental DNA, the specific SNP variants examined on their DNA can differ – leading to slight variation in ancestry estimates.
Random gene shuffling
During recombination in reproduction, the position at which gene segments cross over is random. This shuffling can concentrate certain ancestry associated DNA in one child over another. For example, one sibling may inherit a greater amount of the DNA from a specific grandparent.
Limitations in reference data
The ancestry algorithms used by testing companies rely on reference population data. Limitations and gaps in the reference panels mean ancestry predictions have a certain degree of uncertainty and variability between close family members.
How can siblings reduce DNA differences for medical reasons?
For certain medical purposes, it can be useful for siblings to minimize genetic discrepancies by using advanced reproductive techniques:
Preimplantation genetic diagnosis (PGD)
PGD is done alongside IVF. Embryos are screened for genetic defects, and only healthy embryos are implanted. This ensures siblings are born without detrimental mutations.
Preimplantation genetic haplotyping (PGH)
PGH tests for haplotypes (blocks of DNA) associated with a genetic condition running in the family. Only embryos lacking the harmful haplotype are selected.
Human leukocyte antigen (HLA) matching
Siblings can be generated via IVF and selected to be a close HLA-match for a sick brother or sister who needs a stem cell transplant. This reduces the chance of transplantation complications.
Conclusion
While siblings share a large portion of their DNA and have the same family ancestry, there are still many differences scattered throughout their genetic code. These discrepancies arise from the random nature of inheritance and mutations acquired as reproductive cells form. Environmental impacts like epigenetics also influence gene expression. Taken together, these effects result in siblings having distinctive appearances, traits, and medical predispositions – making each child unique.