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Is there any explanation as to why a brother and sister cannot have boys? My sister in law has only been able to have girls and my husband and I can't get pregnant with a boy.

-A curious adult from Ohio

July 11, 2012

No, there isn’t any simple explanation.   If instead you were both males or females, there might be some genetic reason.  But since you are brother and sister, it is most likely simple chance as opposed to some underlying genetic cause.

For it to be genetic, it would probably need to be something bad on the X chromosome being passed down to the sons that causes them to be miscarried.  The problem is that dads don’t pass their X’s to their sons.  And besides, if dad had the bad X, he wouldn’t be alive to pass it on!

What this means is that it is very unlikely for a brother and a sister to not have sons for the same biological reason.  I suppose they could each have a different reason for not having sons but that is pretty unlikely.

This is why the best explanation is simple chance.  Each time a woman gets pregnant, she has a 50% chance for a boy and a 50% chance for a girl.  What came before doesn’t matter.  If she has six girls, her chance for a girl in the next pregnancy is 50%.

Think of it like flipping a coin.  If you get ten heads in a row, your chances for heads on the next flip is still 50%.  The coin doesn’t remember what flip came before.

So this is probably what is happening in your family.  You keep getting heads and so keep having girls.  There is no specific reason why you don’t have boys, you just haven’t had one yet.

What I thought I’d do for the rest of the answer is focus on why a genetic cause is so unlikely in your case.  As you’ll see, it is because dads have an X and a Y chromosome, and moms have two X’s.

X and Y Sex Chromosomes  – What’s the Difference?

As I said, men usually have an X and a Y and women have two X’s.  This is why dad’s sperm usually decides whether a child will be male or female.

See, moms always give their children an X chromosome, while dads can give either an X or a Y chromosome.  If dad gives an X, you get a girl and if he gives a Y, you get a boy.

This explains why it is so unlikely there is a common genetic reason here.  Dad would always give his sons a Y and moms always an X.

X-Linked Lethality

Because girls and boys get different sex chromosomes, there are some differences in the disorders that they can inherit from their parents.  If there is a problem with a father’s Y chromosome, for instance, then all his sons will end up with that same problem.  But none of his daughters will be affected.  However, things are a bit more complicated with the X chromosome.

Sometimes, a mom is less likely to have a boy because one of her X chromosomes is messed up.  If the problem is recessive, meaning that having one normal X chromosome is enough to keep the problem at bay, then either of her X chromosomes is fine for a girl child.  This is because the copy she gets from her dad is normal. 

But when the mom tries to pass the mutated chromosome on to a boy, the resulting embryo can’t survive.  This is because he gets a Y and not an X from dad.  So he doesn’t have a good X to protect him from his bad one.

In these cases, the mom is considered a carrier, meaning that she has one messed up copy of the X chromosome.  Because she also has a normal copy she doesn’t have any problems herself. 

However, this mom might have trouble getting pregnant with a boy.  But remember, she also has another normal X chromosome that she can pass on, so it’s still possible for her to give birth to a healthy son. 

In other words, this wouldn’t keep her from having sons.  She would just be less likely to have one.

I think you can see why a male can’t be a carrier for an X-linked gene. Since he only has one copy of the X chromosome, there is nothing to mask something bad on it.  He would have the lethal disease and not be alive to pass it on! 

So this couldn’t explain why you and your brother can’t seem to have sons.  He couldn’t have a bad X to pass to his son because he wouldn’t be alive to have a child.  And besides, he doesn’t pass an X to his son, he passes a Y. 

Even if we came up with some complicated genetic way for him to have that lethal X and still be alive, he’d pass his Y and not his X.  And if he did manage to pass a bad X, his wife would pass a good X.

Of course it could be that you and his wife happen to share similar problems with your X chromosome.  Since these kinds of things are rare, it would be even rarer for both of you to have the problem.  But it still could happen.

Again, though, this would just decrease the chances for a boy.  A boy could still happen if you or his wife passed on a good X.  This would happen around 50% of the time.

The bottom line is I can’t think of any genetic reason why you and your brother could not have boys.  I even tried to come up with some really complicated and really rare ways it could happen and drew a blank.

This is why it is almost certainly just the luck of the draw.  It just so happens that you and your brother’s wife’s eggs have been fertilized by sperm that carry an X chromosome. 

Odds are that if you keep trying, one of you will have a boy.  But there is no guarantee.  If you keep flipping a coin you might always get heads.

By Amy Johnson, Stanford University


If dad passes an X, he'll have a daughter and if he passes a Y, he'll have a son.


Why some women are less likely to have boys.  (And why boys are more likely to be colorblind!)