X-linked Inheritance

X-linked Inheritance

So, if you haven’t read Genetics 101, this would be a good time to do so if you need a refresher on the basics. But, to briefly summarize, our bodies are made up of billions of cells. In our cells there are structures called chromosomes that are made up of individual instructions called genes. Genes tell our bodies how to grow and function. We have 46 total chromosomes or 23 pairs; half from mom and half from dad. To grow and function as usual, it is important to have all 46 chromosomes and the corresponding genes working as usual or problems/genetic conditions can arise.

We’ve established that we have 23 pairs of chromosomes and therefore, 23 pairs of genes. Again, genes come in pairs, one from our mom and one from our dad. Typically both copies from the pair are working as usual. However, we all have certain genes that are not working as usual (in genetics, this is referred to as a mutation), this is normal…in fact, scientists estimate that humans probably have about 20 genes that are not working as usual! But, since we have TWO copies of each gene, this is typically not a big deal because the 2nd copy that IS working can compensate for the one that is not working.

So, the whole 2 copies of each chromosome, 2 copies of each gene is true for the first 22 pairs of chromosomes (the autosomes) but for the last pair, and for men specifically, this is not the case. The 23rd pair of chromosomes are called the sex chromosomes. In general, women have two ‘X’ chromosomes (and therefore, 2 copies of all of the genes on the X chromosome) and men have one ‘X’ chromosome and one ‘Y’ chromosome, and therefore, only one copy of the genes on the ‘X’ chromosome. This bit of information comes into play when we learn about another type of inheritance called X-linked inheritance.

X-linked simply means that the gene involved in these conditions is found somewhere along the X chromosome.

X-linked conditions have traditionally been classified as X-linked dominant or X-linked recessive in nature.

X-linked dominant means that if you have one non-working gene on one of your X chromosomes, even if you are a female with another X chromosome that has a working copy, it doesn’t necessarily compensate and you may have the condition. If you are a male with an X-linked dominant condition, you do not have another copy of the X chromosome and you of course have the condition.

X-linked recessive means that in order to have the condition, you either have to be a male who has your only copy of the gene on the X chromosome not working or if you are a female, each of the ‘X’ chromosomes must have the non-working gene for you to have the condition. There are exceptions to this, as some females with X-linked recessive conditions do have symptoms of the condition. An example of this can be seen in a condition called Fragile X, which is considered to be X-linked recessive; however, females can have some symptoms if they have one copy of this non-working gene.

So, how does an X-linked condition play out in a family?

X-linked conditions can be confusing in terms of how they are passed on and who gets the condition within the family. This is due in part because of the whole men have 1 ‘X’ and women have 2 ‘X’ chromosomes thing, but also because the chances for children to inherit X-linked conditions depends on which parent has the non-working copy of the X-linked gene…mom, dad, or it could be both! And, it also depends on whether the condition is typically recessive or dominant in nature. So, it’s not as quick and easy to explain as some of the other patterns of inheritance. Let’s break it up into X-linked recessive and X-linked dominant and go from there….

**Btw, sometimes the x-linked conditions do not fit neatly into recessive or dominant, so these are general rules but they do not necessarily apply perfectly to all situations.

X-linked recessive inheritance

Recap for X-linked recessive inheritance…..

  • Males- since males have 1 ‘X’ chromosome, if they have an X-linked recessive condition it means that their 1 ‘X’ chromosome has the non-working gene.
  • Females- since they have 2 ‘X’ chromosomes and we are talking about X-linked recessive conditions, in order for them to have the condition, both of their ‘X’ chromosomes have the non-working gene.

Scenario 1, X-linked Recessive – Dad has condition, Mom doesn’t

Dad has the X-linked recessive condition, and therefore, his 1 ‘X’ chromosome has the non-working gene. Mom does not have the condition and is not a carrier of the condition, therefore both of her ‘X’ chromosomes have working copies of the gene.

In Scenario 1, when this couple goes on to have children, the chances of their children having the condition depend upon whether or not they have a son or a daughter. None of their sons will get or inherit the ‘X’ chromosome from dad that has the non-working gene, therefore, none or 0% of their sons will have the condition. All of their daughters will inherit the ‘X’ chromosome from dad, so 100% of their daughters will be what we call ‘carriers’ of the condition and have one copy of the non-working gene on one of their ‘X’ chromosomes but are not expected to have symptoms of the condition. See illustration below. Also, to find out what happens when these daughters who are carriers go on to have children, read Scenario 2.

 

xlinked_recessive_inheritance

Scenario 2, X-linked Recessive- Mom is a carrier, Dad doesn’t have the condition

Mom is a carrier (again, not expected to have symptoms) of the X-linked recessive condition, and therefore, she has 1 ‘X’ chromosome with a non-working copy of the gene and 1 ‘X’ chromosome with a working copy of the gene and typically does not have symptoms of the condition. Dad does not have the condition and his 1 ‘X’ chromosome has a working copy of the gene.

In scenario 2, when this couple goes on to have children, the chances of their children having the condition depend upon whether or not they have a son or a daughter. With each pregnancy, mom has a 50% chance of passing on the ‘X’ chromosome with the non-working gene. Dad has a 50% chance of passing on his ‘X’ and a 50% chance of passing on his ‘Y’.

So, for scenario 2 there are 4 possible outcomes with each pregnancy; the chance for each of these is 25% with each pregnancy. This couple could have:

  • A daughter who inherits 1 ‘X’ from dad with the working copy of the gene and 1 ‘X’ from mom with the non-working copy of the gene; she is a carrier of the condition and not expected to have symptoms.
  • A daughter who inherits 1 ‘X’ from dad with the working copy of the gene and 1 ‘X’ from mom with the working copy of the gene; she is NOT a carrier of the condition and does NOT have the condition.
  • A son who inherits 1 ‘Y’ chromosome from dad and 1 ‘X’ chromosome from mom with the working copy of the gene; he does NOT have the condition.
  • A son who inherits 1 ‘Y’ chromosome from dad and 1 ‘X’ chromosome from mom with the non-working copy of the gene; he HAS the condition.

xlinked_recessive_inheritance_mother

Scenario 3, X-linked Recessive Dad has the condition, Mom is a carrier

Dad has the X-linked recessive condition and therefore, his 1 ‘X’ chromosome has the non-working gene. AND, Mom is a carrier of the X-linked recessive condition, and therefore, she has 1 ‘X’ chromosome with a non-working copy of the gene and 1 ‘X’ chromosome with a working copy of the gene and typically does not have symptoms of the condition.

In scenario 3, there are 4 possible outcomes with each pregnancy; the chance for each of these is 25% with each pregnancy. This couple could have:

  • A daughter who inherits 2 ‘X’ chromosomes which both have non-working copies of the gene; she HAS the condition.
  • A daughter who inherits 1 ‘X’ chromosome from dad with the non-working gene and 1 ‘X’ from mom with the working gene; she is a carrier of the condition but does not have symptoms.
  • A son who inherits a ‘Y’ chromosome from dad and the ‘X’ chromosome from mom with the non-working gene; he HAS the condition
  • A son who inherits a ‘Y’ chromosome from dad and the ‘X’ chromosome from mom with the working copy of the gene; he does NOT have the condition.

X-linked Dominant

Just a recap for X-linked dominant inheritance…..

  • For males- since males have 1 ‘X’ chromosome, if they have an X-linked dominant condition it means that their 1 ‘X’ chromosome has the non-working gene.
  • For females- since they have 2 ‘X’ chromosomes and we are talking about X-linked dominant conditions, in order for them to have the condition, only one of their ‘X’ chromosomes needs to have the non-working gene.

Scenario 1, X-linked Dominant Dad has the condition, Mom doesn’t

Dad has the X-linked dominant condition, and therefore, his 1 ‘X’ chromosome has the non-working gene. Mom does not have the condition and is not a carrier of the condition, therefore both of her ‘X’ chromosomes have working copies of the gene.

In this situation, 100% of daughters will inherit the ‘X’ chromosome from dad that has the non-working gene and the ‘X’ chromosome from mom that has the working gene. Since it is a dominant condition, the daughters will all have the condition. All of the sons will inherit the ‘Y’ chromosome from dad and a ‘X’ chromosome from mom with the working copy of the gene and therefore, none of them will have the condition.

xlinked_dominant_inheritance

Scenario 2, X-linked DominantMom has the condition, Dad doesn’t

Mom has the X-linked dominant condition, and therefore, she has 1 ‘X’ chromosome with a non-working copy of the gene and 1 ‘X’ chromosome with a working copy of the gene. Dad does not have the condition and his 1 ‘X’ chromosome has a working copy of the gene.

In this situation, mom has a 50% chance of passing on the ‘X’ with the non-working gene to each of her children. Therefore, regardless of gender, each child has a 50% chance of inheriting the ‘X’ with the non-working gene and therefore having the X-linked dominant condition.

xlinked_dominant_inheritance_mother

Scenario 3, X-linked DominantMom and dad BOTH have the condition

Dad has the X-linked dominant condition and therefore, his 1 ‘X’ chromosome has the non-working gene. AND, Mom has the X-linked dominant condition, and therefore, she has 1 ‘X’ chromosome with a non-working copy of the gene and 1 ‘X’ chromosome with a working copy of the gene.

This situation is likely to be quite rare. However, if this were to occur and this couple decided to have children, all of their daughters would inherit or have the condition and 50% of their sons would, as well.


Questions? Get in touch










Genetic Support Foundation
Copyright © 2015 Genetic Support Foundation, Inc