why some illnesses and diseases “skip” a generation.
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This is a gross simplification but here’s how I teach it in Grade 11 bio:
**Some basic facts:**
– DNA’s major function is to code for proteins in your body. It does a lot of other stuff related to this goal, but at the end of the day it’s [gene -> protein]. Proteins do the vast majority of the biochemical work. You have thousands of different ones, all coded for in your DNA.
– You get half your DNA from your mom and half from your dad in the form of 23 chromosomes from each
– Aside from X and Y, these two sets of chromosomes have all the same genes (same areas of the DNA coding for the same trait), so you have two copies of each gene.
– The two sets can have different versions of the genes, these versions are called alleles.
– Mom and dad also got two copies from their moms and dads and so on and so forth.
**Putting it together:**
I’ll use the example of a smell receptor, which is a protein that changes shape to send a signal when some other chemical hits it. If it has the right shape to match the chemical it detects, your brain will get a signal that it will interpret as “strawberry” or whatever.
Now we run three scenarios to explain how this affects a trait (here, it’s ‘smelling strawberries’):
– You get two ‘good’ copies of the strawberry smell gene from mom and dad. your nose has the receptors, you can smell strawberries, life is groovy.
– You get one ‘good’ copy and one ‘bad copy’. Maybe mom’s egg got hit with some radiation and messed up that little bit of DNA. That smell receptor won’t react to strawberries anymore, but hey, you’ve got dad’s good copy, so you can still smell strawberries. Maybe it’s not quite the same but you’ve still got the trait.
– You get two bad copies of the allele. Neither mom’s nor dad’s gene makes the right shape for your smell receptor. You will never know the joy of strawberry smell the way the rest of us do.
**Skipping a generation:**
It is entirely possible for a mom and dad who can both smell strawberries to have a kid who can’t. For this to happen, both mom and dad must have one good and one bad copy, so they’ve got the right receptors but by chance (50% per parent, so 25% chance overall), you ended up with their bad copies instead of their good one,
The same is true for mom and dad, the trait can hide for a long time before the right two people have a kid who is unlucky enough to get two bad copies.
**This is only one type of inheritance, there are others:**
What I’ve described here is an autosomal recessive inheritance. It can skip generations. Sometimes, though, the weird version of the gene causes a problem instead of just losing a trait. In this case, just one bad copy is enough. One example of this is Huntington’s disease, where the bad protein literally starts killing brain cells. We call this a dominant inheritance pattern. Dominant traits will never skip a generation since the moment someone doesn’t pass on the ‘bad’ copy, all the next generations automatically don’t have the problematic allele anymore.
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