Why do children who share the same parents have different heights, different eye colors, body builds, personalities, etc.?
Children from the same parents do not always look or act alike. In fact, it can sometimes be hard to tell which children are siblings just by looking at them. This is what makes each one of us so unique. Could you imagine if every child from the same parents looked and acted identically? We would not have nearly as much diversity in our world. Also, you can imagine that families would not be nearly as interesting.
How can you know for certain if you or your children look different because of random genetics or different parents?
Sibling DNA testing, using a high number of alleles to compare each person’s unique genetic profile, is the most accurate and reliable method available today.
Each of us inherits a completely different set of genes from the same set of parents. The explanation as to how this happens has to do with the fact that each parent actually has two different sets of genes and that each parent randomly passes only half of their genes to their child.
All of this together ensures that every person in the world ends up with a different, unique set of genes (with the exception of multiples, aka twins, triplets, etc.).
To explain how this works we are going to use the example of just one gene–MC1R. MC1R is a good gene to look at because it is genetically very simple. It determines whether or not someone will have red hair, pale skin, and/or freckles.
Genes Come In Different Variations
Everyone has two copies of most of their genes. We get one copy from our mother and one copy from our father. This includes two copies of the MC1R gene. Having two copies of a gene wouldn’t matter if each copy was the same, however they are not. Each gene can come in different versions (these different versions are called alleles).
MC1R comes in two different versions, red and not-red. Since we have two copies of MC1R, everyone has one of three possible combinations. They can have two reds, a red and a not-red or two not-reds.
People with two copies of the red version of MC1R have red hair, pale skin and freckles. People with one version of each often have pale skin and freckles and people with two not-red versions can have any shade of hair or skin color (depending on their other genes).
People are not different because they have different genes. As humans, we all share the same genes. What makes us different is that we have different versions of the same genes. These variations are examined closely in sibling DNA testing to determine full or half sibling relationships.
Which Gene Copy You Get is Random
A crucial piece to the puzzle in figuring out how siblings can look so different when they come from the same parents has to do with how genes are passed down. Which of your parents’ two gene copies you end up with is chosen at random.
If your parent has two different copies of a gene, you have an equal chance of getting either one. And the same is true for each of your siblings. It’s like flipping a coin. You might get heads but your brother or sister might get tails.
Let’s put this all together to see how it works using the MC1R gene using the following example:
Imagine two parents that have one red and one not-red copy of the MC1R gene. They can have any hair color and probably have pale skin and freckles. Let’s use an example of a family tree to see what their children might look like:
As you can see, because of this one gene, the parents can have three types of children:
- If both parents pass a red version down, then that child will have red hair, pale skin and freckles.
- If one parent passes a red and the other a not-red, then that child will not have red hair and, most likely, will have pale skin and freckles.
- If both parents pass a not-red version, then that child will have a variety of other hair and skin colors.
Because of the other thousands of genes with their different gene versions, there is a huge variety within each group too. So the red haired group can be tall, short, happy, grumpy, have blue, green, brown, hazel eyes, and so on.
It’s like having two big bags of 50,000 different colored marbles. Imagine pulling 25,000 out of each bag, combining them and then recording the result. When you’re done, you then return the marbles back to their original bags and then take another 25,000 out of each. Odds are you’ll get two very different sets. Just like you’d get two very different children.
Given all of the possibilities for different genes and an individual’s environment, it is no surprise that even children from the same parents are very unique. In fact, your unique genetic combination was never seen before you and will never be seen again.