Where Does Cystic Fibrosis Come From?

So, what exactly is the cause of Cystic Fibrosis?

Well, let’s have a little lesson on genetic inheritance!

Cystic Fibrosis is a recessive trait. When a trait is recessive, as opposed to dominant, the trait will express itself in a person if only recessive genes are present—there can be no dominant genes in their DNA for that trait. If a person has a dominant gene, the dominant trait will be expressed in that person. (We can remember that by thinking of “dominant species” being the stronger one or a person who “dominates” is the one that is most expressive.)

Because Cystic Fibrosis is on one of the autosomal chromosomes, rather than a sex chromosome, both males and females are equally likely to be affected by the disease. This also means that both genes (one will come from the mother and the other from the father) must be recessive to express the trait of Cystic Fibrosis. If both parents have a recessive gene, they can each pass their recessive gene on to their child. This does not mean that a child with Cystic Fibrosis has to have two parents with the disease, though. Remember dominant genes? The dominant trait is to not have Cystic Fibrosis. If the parents have the dominant gene for not having Cystic Fibrosis, that dominant gene will, well, dominate and “cover up” the recessive gene. This is called “carrying” a disease—when a person has both a dominant gene and a recessive gene, only expressing the dominant trait but having the potential to pass down the recessive gene to any children. If one or both parents do not carry the disease, meaning both of their genes are dominant, none of the children will be able to have the disease.

Making sense? I will map out this inheritance pattern to further illustrate what causes a person to inherit Cystic Fibrosis. We do this using a Punnett Square, which is a tool to show each genotype (genetic make-up) that is a possible result when two genotypes (genetic make-ups of parents) are crossed.

Dominant genes are represented with a capitol letter and recessive genes are represented with a lowercase letter. Because we are talking about CF today, I will use the letter F and f. I’ll use F rather than C because generally letters would be used that can’t be confused between capitol and lowercase letters (using N and n rather than U and u, for example).

Let’s first see what happens when two parents carry the disease (remember this means that one gene is recessive, meaning they can pass that on to their children, and the other is dominant, meaning they do not express the disease). Each parent would then give each child exactly one of their genes—in this case, either the dominant or the recessive gene. We place the parents on the outside of the box then create a row or column to each gene—so two rows for the two maternal (from mom) genes and two columns for the two paternal (from dad) genes.

	F	f
F	FF	Ff
f	Ff	ff

This shows that two parents who carry the disease (one capitol letter/dominant gene and one lower case letter/ recessive gene) can have children who either have two dominant genes (FF) meaning they do not express the disease nor do they carry it, children who have a dominant and a recessive gene (Ff) meaning they do not express the gene but they do carry it, or children who have two recessive genes (ff) meaning they do express the gene because there is no dominant trait to cover up the recessive trait. Statistically, this means 3/4 of their children will not express the disease and 1/4 of their children will express the disease.

What if we cross a person who carries CF with a person who does not have or carry CF?

	F	F
F	FF	FF
f	Ff	Ff

This shows that their children will either have one dominant and one recessive gene (Ff) or have two dominant genes (FF). None of their children will have the disease because the parent with two dominant genes will not pass on any recessive genes to build a person who has only recessive genes present. This will happen with any couple where one had two dominant genes because that parent with two dominant genes cannot pass down the necessary second recessive gene to cause CF in their child. (BTW: this is called being homozygous, when both genes are the same—either FF or ff. When they are different—Ff—this is called being heterozygous.)

What other combinations can we think of? Ff and Ff, FF and Ff have been shown already. Other combinations are ff and Ff (a person with the disease and a person who carries the disease), ff and FF (a person who has the disease and a person who does not carry or have the disease), or ff and ff (two people with the disease). Here’s what those would look like:

	F	f
f	Ff	ff
f	Ff	ff

	F	F
F	Ff	Ff
f	Ff	Ff

	f	f
f	ff	ff
f	ff	ff

The inheritance of Cystic Fibrosis is therefore only possible to pass down if BOTH parents either have the disease or carry it.

Recessive Inheritance Pattern

In my case, both of my parents carried the disease. The recessive genes had been hiding in my mother and father’s lineage well enough that we had never heard of Cystic Fibrosis before I was diagnosed. This can happen with Cystic Fibrosis and any other recessive genetic disease because of its ability to hide behind the dominant gene.

If you are concerned about recessive genetic illnesses, especially if any are present in your families, you and your spouse could consider genetic counseling before having children. We should remember, though, that a child is not defined by any disease that they may or may not have in their lives any more than they are defined by the color of their hair or shape of their eyes.

Keep Hope,

-Cystic Sister @----