Can 2 Rh Positive Parents Have an Rh Negative Baby
-A curious adult from Pennsylvania
March viii, 2016
Not necessarily. You lot might be Rh negative (Rh-) but you could too be Rh positive (Rh+).
This is because Rh- is something called a recessive trait. This means that someone who is Rh+ might take a hidden Rh- in their Dna. If that person and his or her partner both laissez passer an Rh- down to a child, that child will be Rh-. Even though at least 1 parent is Rh+!
If we were to say that the other parent was Rh- and you were Rh+, then for reasons I'll talk virtually below, each child would have a 50% adventure for existence Rh-. From this you might think at to the lowest degree 1 of your 3 kids HAS to be Rh+. But this is not the example.
Think nigh it like flipping a money. With a coin, you lot take a 50% chance for heads and a 50% chance for tails.
If yous flip a money twice and get two tails, what are the chances that y'all will get tails on the adjacent flip? However l%.
The coin doesn't remember what was flipped before. And your body doesn't know whether it passed down an Rh+ or an Rh- with a previous child.
And so you don't necessarily have to be Rh-. In fact not even the other parent has to exist Rh-.
It is possible for both of you to be Rh+ and have three Rh- kids. In this instance, the chances for each child to be Rh- only go downward to 25%. And the chance that all three will be Rh- drops to 1 in 64.
For the rest of the reply, I volition explain why you can be Rh+ with iii Rh- kids. As you'll meet, it has to do with the RHD cistron and how it is passed downwards.
The RHD Gene
Whether or non you are Rh- or Rh+ is decided by the RHD gene. It comes in two versions or alleles. We'll telephone call one version Rh+ and the 2nd Rh-. (Click here to learn why this is a bit of a simplification.)
Like most every other gene, we take ii copies of the RHD factor. I copy came from your mom and the other from your dad.
If both copies are Rh+, information technology is easy to meet why you would be Rh+. That is all you lot have.
The same is truthful for two Rh- copies. In this example y'all will exist Rh-.
But it is less obvious when you have one copy of each. In this case, you are Rh+ because Rh+ is dominant over the Rh-. Another way to say this is that Rh- is recessive to Rh+. (Click here for why the Rh+ allele is dominant.)
And then to be Rh+ you just need 1 re-create of the Rh+ allele. But in this instance you would be conveying a hidden Rh- allele which you could pass down to your child.
How Genes are Passed On
Every bit I said, we get one copy of each gene from mom and one from dad. And so it makes sense that nosotros pass only i copy of each cistron downwards to our kids.
Allow'due south say you are Rh-. Then, since you have two Rh- alleles, you will be passing an Rh- allele to each child. Similarly, if yous have two Rh+ alleles, you lot will pass an Rh+ allele to each child.
At present instead, let'due south say yous have 1 Rh+ allele and one Rh- allele. Which allele will yous laissez passer on to your children?
The reply is simple considering which copy we pass down is random each time nosotros have a child. So in this case, each child has a 50% chance of getting Rh+ and a l% take a chance of getting Rh-.
Both Parents Could be Rh+
Permit'due south say you are Rh+ but carry a hidden Rh- allele and your partner is Rh-. In this case, each child has a 50% take a chance for being Rh-.
Your partner will e'er pass on an Rh- allele merely y'all could laissez passer on either. Each time you take a child, there is a 50% take chances you will laissez passer an Rh+ and a l% chance you will laissez passer an Rh-.
If you pass an Rh+, the child will have 1 of each and so will be a carrier similar you and be Rh+. And if you pass your Rh-, the child will have two copies of Rh- and so exist Rh-. That is where the 50% number comes from.
Now let's say y'all and your partner each have one Rh+ allele and i Rh- allele. With a probability of 50%, you pass on an Rh- allele to your child. Similarly, with a probability of 50%, your partner passes on an Rh- allele.
The total probability the child will have two Rh- alleles is 25%. This ways that in this case, we expect i out of every iv of your children to be Rh-.
This is merely an expectation, though. Because the alleles are passed on randomly, at that place will certainly be cases where all of your children volition be Rh- or none will be. It's up to chance. Like getting iii heads in a row when you flip a money.
Some Groups are More Probable to exist Rh-
Since all three of your kids are Rh-, this makes it more than likely that you both are Rh-. This isn't a guarantee, but if I were a betting man, that's where I would put my money. For sure, it is more than far more likely for you 2 to accept iii Rh- kids if you lot were both Rh-!
At the very to the lowest degree we know that you and your partner both have at least one Rh- allele. Otherwise, information technology would exist extremely unlikely for any of your kids to be Rh-!
The chances y'all are both Rh- volition likewise be dissimilar depending on where you are from. For example, the Rh- allele is much more common in Europeans than it is in Asians (see chart beneath).
(The original table can be found here.)
The second cavalcade shows how common the Rh- allele is in a population. Effectually 40% of the RHD alleles in Europeans is the negative type.
Merely this does not hateful that 40% of Europeans are Rh-. This number is found in the tertiary column. Around 16% of Europeans take ii Rh- copies. (The rest of the Rh- alleles are constitute in people with one of each.)
The fact that you take iii out of 3 kids who are Rh- increases the odds that you are Rh- from 16% to 72.7%. This information is found in the final cavalcade.
All of that was bold you lot were European and that the begetter was also Rh-. The numbers are different depending on your background. For example, for Asians, knowing the blood group of your children simply increases your chances of being Rh- to iii.9%, which is still very small.)
Fifty-fifty with all of this we nevertheless cannot say for certain what your Rh type is. We can only say it is more likely you are Rh- considering you have had three Rh- children.
Cats and quick reflexes
But why are in that location so few copies of the Rh- allele? And why does it vary and so widely betwixt populations?
In that location is no clear reply, but there is a very interesting hypothesis involving cats and quick reflexes.
There is a parasite called Toxoplasma gondii that normally infects cats. When this parasite infects humans, it can cause people to react more slowly. Studies have shown that Rh+ heterozygotes (people who comport the Rh+ and Rh- allele) may exist resistant to this symptom.
Some scientists retrieve that because of this resistance, the Rh- allele has stayed in the population, even though the Rh- trait can cause difficulties for Rh- women during pregnancy.
Different rates of infection by this parasite may have caused unlike frequencies of the Rh- allele. For example, the large number of wild cats in Africa and Asia could have raised the frequency of Rh- at that place.
Although nosotros can't say for certain that you are Rh-, we can say that knowing your children are all Rh- increases your chances of being Rh- besides. To find out your blood type for certain, your doctor can perform a simple claret exam.
By Joe Davis, Stanford Academy
Source: https://genetics.thetech.org/ask-a-geneticist/rh-positive-parents-multiple-rh-negative-children
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