Why Do Some Biological Siblings Look Nothing Alike? (Genetic Explained)

Why Do Some Biological Siblings Look Nothing Alike - Babe in Dreamland

We all know that our genes play a major role in shaping who we are and what we look like.

So, we kind of expect that our biological children will resemble us – and their siblings. After all, they have the same parents, meaning the same genes, right?!

But the truth is:

Siblings CAN look alike, BUT not necessarily.

In fact, some twins may even have different skin colours (like one can turn out with back skin and the other with white skin) if their parents are of mixed race. Check out Lucy and Maria Aylmer’s story if you don’t believe me!

Other siblings may look so similar that parents struggle to tell them apart.

So how on earth is that possible? How can some biological siblings look identical while others are literally on the opposite spectrum?

Let’s discuss why genes from the same set of parents can produce different-looking offspring.

Why Do Some Biological Siblings Look Nothing Alike?

There is no rule when it comes to genetic inheritance. Every biological sibling has unique genes inherited from their parents and many generations before. And that’s because the parent’s genetic material used to make each of the children isn’t genetically identical, due to meiosis. Secondly, because every person has two versions of each gene (called an allele), there is no way to predict which child receives what allele from a parent. And because recessive genes can be masked for some generations and re-emerge in one child, but not the others. Hence the difference in their looks. Besides genetics, environmental factors can influence a sibling’s features, such as maternal diet, activity level and access to nutrients in the uterus.

Humankind Share 99.9% of DNA

The key factor that determines the looks and traits of any human being is genetics.

I guess there is no surprise here.

What may be astounding to learn is that the genes from any two random people from different parts of the planet are 99.9% the same. Which makes all of us almost genetically identical!!

And because of this genetic similarity, we are capable of successful reproduction. Something that isn’t possible between species that share very different DNA.

So, this remaining 0.1% of genes are unique to us and are the reasons for the variation in physical characteristics between people and also risk for certain diseases.

ONLY 0.1% – how crazy (and somewhat fascinating) is that?!

So, (I’ll say that again)—this 0.1% of our genetic makeup will determine our skin, hair and eye colour, body shape, height, and weight.

These, however, are only cosmetic differences because our main bodily functions are the same.

*** As a side note, did you know that the genetic similarity between humans and chimpanzees is a whopping 96%? That’s because chimpanzees are closely related to humans since both species descended from a single ancestor species some 7 million years ago. And for comparison, we also share 60% of our genes with bananas ????

But there is more…

Everyone has a unique mix of genes. Even identical twins (twins who developed from one egg that split into two) will not have 100% of the same DNA. That’s because of the genetic mutations that occur during early development when they are still in the womb.

Every Sibling Has a Unique Mix Of Genes

That is because of the biological material from your parents. Eggs from your mom and sperm from your dad have a different genome (DNA) than the genetic material used to make your siblings.

And there are over 8.4 million unique DNA combinations.

In other words, every sperm cell produced by the same man has a different genetic code! And because of that, his children simply cannot look exactly the same.


Due to meiosis, our bodies create sex cells (called gametes) – eggs and sperm.

In brief, it’s a process where a single cell goes through a few stages that separate and reshuffle chromosomes.

During meiosis, the chromosomes carrying a random set of genes from the mom line up with chromosomes with random genes from the dad. When this occurs, the pair of chromosomes exchange genetic information, producing new genetic information.

These pairs of chromosomes divide twice to form four genetically unique daughter cells containing only a single set of 23 chromosomes. (So it’s exactly half as many chromosomes as the starting gametes).

These 4 daughter cells are genetically different because of all this pairing up, exchanging genetic information, reshuffling and splitting. And that’s why (because of meiosis) the number of genetic combinations that two parents can produce offspring is literally endless.

In other words, it is more likely to find a unicorn than to produce children that are genetically identical.

Meiosis (updated)

Different Versions of The Same Gene

Now, going back to the chromosomes.

During fertilization, when the 23 chromosomes in the egg cell join with 23 chromosomes in the sperm cell, they will form a complete set of 46 chromosomes and start a new genome.

And each pair of chromosomes contains two versions of the same gene. These versions, called alleles, can be different. And the thing is that it is totally random which copy of the allele a parent will pass to their kid.

Depending on the associated trait, alleles can be dominant or recessive.

Dominant alleles reveal their effect and observable trait (and mask recessive allele) even if a person inherited one copy of the dominant allele.

In order for recessive alleles to be observable, they must be present and passed down by both parents.

The curious thing about the masked allele is that it can re-emerge in later generations. The recessive allele can still be passed from generation to generation without showing and reappear several generations later.

That’s why kids often inherit features from their great-grandparents, while their siblings don’t!

*** As a side note: Have you ever heard of the concept of “strong genes” or “weak genes”? For example, when a child inherits curly hair from its father instead of straight strands from its mother? It’s often said that the father had “stronger genes”. But, it is just a colloquialism. There is no such thing as strong or weak genes. The correct terms are dominant or recessive alleles passed down to the kids.

Dominant and Recessive Alleles

A classic example of dominant allele expression is the blood group inherited from parents. There are 3 red blood cell types: A, B, and O. The ABO gene has 3 alleles: A and B – considered dominant and O – considered recessive.

So, if a person inherits the A and B alleles, the blood group will be expressed as AB.

If a person inherits A and O, then the blood group will be A, since A is a dominant allele and O is recessive. If a person received both O, then the blood group will be O.

And the same works with other inherited traits like eye or hair colour, which I already described in my post: Why Are My Baby’s Eyebrows So Light?

Alleles and Genes

Environmental factors

I would like to mention briefly that our appearance is not solely determined by genetics.

Environmental factors can alter our looks, although these are significantly less influential.

Our environment includes how we were raised, our nutrition, physical activity, and stress level.

The condition of the uterus influences the differences in the physical characteristics of twins.

These include the position of each twin in the womb during development, how close they were to the amniotic sac, the length of the umbilical cord and access to nutrition.

Another factor to consider is maternal diet, physical activity, and psychological health during each pregnancy. Because these differences will also affect her children’s looks.

Final Few Lines

If your kids look nothing alike, don’t worry! You don’t need to run a DNA test to check whether they were switched in the hospital. It’s just the genes messing around.

So, to recap:

Every sibling (like everyone on this planet) has a unique mix of genes inherited from their parents and many, many generations before. Also included are identical twins, whose DNA isn’t 100% identical due to genetic mutations during pregnancy.

So, some siblings may not look alike because:

  • The parent’s genetic material that made each of the children is different, thanks to meiosis.
  • Because every person has two versions of each gene (called an allele), there is no rule as to which child will inherit what version.
  • Because recessive genes can be hidden for some generations and re-emerge a few generations later.

The environment, such as the mother’s diet and level of activity during pregnancy, as well as conditions in the womb and absorption of nutrients, will also affect the looks of the child.

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