When we’re told a condition we have is “genetic”, it’s tempting to think it means our parents had it, and their parents before them, all the way up to some mysterious Eve-bites-the-apple event. But if you’re told you have muscular dystrophy and your parents don’t have it, what’s going on? A trip to IKEA may help answer that.
Imagine receiving nearly identical instructions manuals, one from your mom and one from your dad. They’ll help you assemble “his and her” bathroom faucets. The problem is, one of the manuals has a mistake in it, which means the “his” faucet can’t be turned off. It doesn’t matter that next to it is a perfectly functional bathroom fixture for her; your room is now flooded. What I’ve just described is how Huntington’s disease is inherited. That perfectly healthy copy of the gene you received from parent #1 is insufficient if the copy you got from parent #2 is defective. This is known as a dominant mode of inheritance. Conditions that spread in this way will affect multiple generations and both men and women equally.
But if, instead, you’re receiving two sets of instructions for nightstands, you may be able to function perfectly well with just the one bedside table if the other table’s instructions manual is missing a page. Both sets need to be defective to find yourself up a particular creek without a paddle. (And no, you can’t use the good manual to assemble the other table in this scenario!) We call this mode of inheritance “recessive”. Unlike with dominant conditions, the parents of a child with a recessive disease will not show signs of the disease. They are carriers. They are each able to assemble one nightstand and survive without consequences. It’s the child that ends up with a stark bedroom. Tay-Sachs disease is an example of a fatal disorder that is transmitted in this way. It causes a degeneration of the central nervous system, but only if you have two mutated copies of the gene. The parents are merely carriers.
And now we complicate things a bit. What if women were to receive two sets of instructions for the same piece of furniture but men, only one? You may remember from basic biology that men and women have different sets of sex chromosomes: women have two X chromosomes, whereas men have an X and a Y. What if a mutation affects a gene on the X chromosome? This is the case of Duchenne muscular dystrophy. In these circumstances, a woman can usually get by with the instructions found on her other X chromosome, but a man is stuck with only one copy. So once again, the parents are fine, but some of the sons develop the condition. (It is extremely rare to find mutations on the Y chromosome, in large part because it is so itty-bitty.)
However, if you’re a young man and you suddenly lose your sight in one eye and then the other, what you may wish to point the finger at an entirely different set of instructions, one that usually comes solely from your mom. There’s more than one collection of handbooks in a cell; mitochondria, which generate energy, have their own set of how-to guides, and it too can be mutated and passed down the generations. Mitochondrial DNA is inherited by all children from their mother, and the sudden loss of vision I described above (called Leber hereditary optic neuropathy) is caused by a faulty gene in the mitochondria.
These different modes of transmission, while already demonstrating how complex genetic inheritance can be, are all rather simple: one mutation causes one disease. However, when we think of very common conditions, like asthma, there’s an added twist. Multiple mutations in many genes end up causing the disease, and these mutations interact with our environment. It’s like receiving a slew of instructions manuals with tiny errors in them, resulting in a furnished room that is just “off”. And I haven’t even mentioned penetrance, which is that, if you’ll allow me to continue with this IKEA analogy, some skilled handymen will receive a defective manual but still know how to build the furniture right. There’s a mutation… but there’s no disease.
Medical geneticists and genetic counsellors will look at a pedigree (a family tree with squares and circles) to help figure out how a particular condition is being transmitted and to advise would-be parents of any risk. At the end of the day, genetics is more complicated than it may seem. It’s a lot like assembling flat pack furniture: it looks easy from the website but when you’ve got three screws and two knobs leftover, you start to wonder if it’s time to call in an expert.
Take-home message:
- If your health condition is said to be genetic, it does not necessarily imply that your parents have it as well
- There are many different ways in which diseases can be inherited, including situations where parents are merely carriers and conditions that are caused by many mutations in multiple genes interacting with the environment
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