Nicole Chai: How does X-linked inheritance work?

In this explainer episode, we’ve asked Nicole Chai, Research and
Development Bioinformatician at Genomics England, to explain what
X-linked inheritance is.


You can also find a series of short videos explaining some of the
common terms you might encounter about genomics on
our YouTube channel.


If you’ve got any questions, or have any other topics you’d like
us to explain, feel free to contact us
on info@genomicsengland.co.uk.


You can download the transcript or read it below.


Florence: How does X-linked inheritance work? I'm joined by
Nicole Chai, Research and Development Bioinformatician for
Genomics England, to find out more. So firstly, Nicole, can you
explain a bit about the X and Y chromosomes?  


Nicole: Sure. So, the X and Y chromosomes are what we call sex
chromosomes. And chromosomes are packages of DNA in our cells
that are inherited from our parents, and they contain information
about our physical and biological traits. 


Some examples of traits that are determined by our chromosomes
include what colour our hair is and what colour our eyes are. And
each of these individual traits are determined by smaller
sections on the chromosome called genes. Genes can also determine
what medical conditions we may inherit from our parents. 


As humans, we all typically have 23 pairs of chromosomes in each
of our cells. One of these pairs consists of the sex chromosomes,
and as their name suggests, sex chromosomes determine sex of an
individual. And typically, females will have two X chromosomes
and males will have one X and one Y chromosome. 


Florence: So then, what do we mean by the term X-linked
condition? 


Nicole: So, an X-linked condition means that the condition is
associated with genetic changes on the X chromosome. And what we
mean when we say genetic changes are changes to the normal
sequence of DNA on the gene. And this can sometimes lead to
medical disorders. 


Florence: Do you have a specific example of an X-linked
condition?  


Nicole: Sure. So, an example of an X-linked condition is Duchenne
muscular dystrophy. 


And with this condition you get a progressive loss of muscle due
to the lack of a protein known as dystrophin. Another example of
an X-linked condition is red-green colour blindness. And this is
where people affected with the condition can't see shades of red
and green the way most people see them.  


Florence: Could you explain how X-linked conditions are
inherited?  


Nicole: Sure. So, for many conditions, there are two ways they
can be inherited, either dominantly or recessively. Dominant
inheritance is usually when you just need one copy of the gene to
be affected by the condition, whereas recessive inheritance is
when you need two copies of the gene to be affected by the
condition. 


However, this works slightly differently with X-linked
conditions, and most X-linked conditions are inherited
recessively. 


Florence: So why does inheritance work differently for X-linked
conditions? 


Nicole: So the reason that inheritance works differently for
X-linked conditions is down to the differences between sex
chromosomes, between females and males. As females have two X
chromosomes and males have X and Y, this means that for recessive
excellent conditions, males only need one altered gene to have
the condition. 


So, because males only have one X chromosome, if they inherit a
faulty copy of a recessive gene, they don't have another healthy
copy to compensate. 


On the other hand, as females have two X chromosomes, if they
inherit just one faulty copy, they do have a healthy one that can
compensate for that one. So as a result, what we tend to see is
that males are more commonly affected by X-linked recessive
conditions. 


Florence: That was Nicole Chai explaining the term X-linked
inheritance. If you'd like to hear more explainer episodes like
this, you can find them on our website www.genomicsengland.co.uk.
Thank you for listening.