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Biologists please help!(19 Posts)
My DS is revising GCSE science and we are both confused by the info on polydactyly so I need someone to explain to me in very very simple terms.
The book and the videos both suggest that polydactyly is genetically inherited and form a dominant allele. This to me means , if your parents, or one of them has polydactyly, you will too, but I suspect I've not got that bit? It also said there is no such thing as carriers. It differentiates this from CH, for example.
Why do they particularly teach polydactyly ? In my day , it was eye colour!
Anyway, this si why I am both intrigued and confused : I was born with an extra toe (contain yourselves with your peering at my feet, people!) but there is absolutely no history of extra digits in my family.
I learnt at school, in a random accidentally viewed science video that I was not some sort of genetic defect but , in fact, I mutated in the womb in a failure of differentiation. When I had my DSs I told midwives to count fingers and toes, and each time they treated me like I was bonkers.
Neither of my children have extra anything .
I am very very confused. The internet is confusing me more...
If it wasn't an inheritable genetic mutation then you wont pass it on.
It sounds like the doctors decided that there was a failure in development in the womb and two sets of stem cells got the message grow a toe here.
This to me means , if your parents, or one of them has polydactyly, you will too, but I suspect I've not got that bit?
Most people with polydactyly will be heterozygous for the allele. Pp. to be homozygous dominant, you’d need two parents with polydactyly.
People without have to be homozygous recessive. pp so it’s right that there are no carriers.
Drawing a punnet square, you can see that children will have a 50% chance of inheriting it.
Inheritance is always a bit more complicated that GCSE textbooks though!
The way inheritance is taught at GCSE level is a massive oversimplification. The most common type of polydactyly is caused by a single, dominant gene, but there are other kinds.
However, your polydactyly could have been caused by random mutation- but it would just have been one of your alleles. If your children all inherited the other allele, they wouldn't have polydactyly. So your example can fit in with the GCSE explanation too.
The reason eye colour isn't taught anymore is because it's a lot more complicated than was taught at GCSE, and it is possible to get a child with e.g. brown eyes from blue eyed parents. I've heard stories, probably/possibly urban legends, of children questioning their paternity after learning about over simplified eye colour genetics.
It's actually quite hard to find things in humans that are controlled by one gene with two alleles, one dominant and one recessive- so examples can be a bit niche! Most people won't know someone with polydactyly, so won't question this example
Ha, purple is that meant to be layman's terms!!??
I think I learnt online that polydactyly if inherited might be linked to another condition ( I was once linked to syringomyelia, which is v obscure but apparently connected) and also a thing called Arnold Chiari Malformation. Those are heritable.
I don't think my doctors decided anything, tbh. They just lobbed my toe off at 13 months! However, I have learnt that my particular extra 'thumb' toe is very rare!
Am I right in saying that , if it is inherited form a couple of generations back, my own two children do stand a chance of having children with polydactyly? I guess we'' know then if it was inherited?
It is interesting what you say about it being hard to teach. I found it really fascinating at school but the diagrams muddled me (poor spatial awareness!) and it is interesting that my own DC is now muddled because the 'helpful' real life example doesn't fit his family experience!
No, your children haven't inherited the polydactyly, so, unless there's another random genetic mutation, their children won't inherit the condition either.
Ha, purple is that meant to be layman's terms!!?
I assumed your son would know those terms because they’re on the gcse syllabus.
He does and I sort of do, but I just wanted it simplified if poss : it's a bit jargon this bit of the spec.
I think I sort of get it now. Essentially, they should teach : sometimes it is not inherited. But they don't!
We do! Not both alleles from each parent for each gene are inherited. Your dominant polydactyly allele wasn't inherited by either of your children.
and it is possible to get a child with e.g. brown eyes from blue eyed parents
I think we were taught that this is possible. We were taught that it is impossble for two brown eyed parents to have a blue eyed child. Not sure that's correct either!
As a PP said, GCSE (and A level) genetics is massively simplified and the model doesn't explain a lot of reality.
I always caution my students not to jump to the conclusion that they're adopted if their particular characteristics don't follow the examples given.
I also run a support group for people with a genetic condition and have to repeat often that their GCSE understanding of genetics doesn't adequately explain the intricate technicalities of this real-life condition.
As for the 'jargon', it's appropriate technical language that students need to understand and use correctly at this level. Much of science education involves learning a whole new language, and applying commonly used words in a very specific way. This is increasingly important in the new specifications.
Simplification: For most genes in your DNA, you have two copies. You inherit one of these from your mum and one from your dad. Different versions of genes are called alleles. Some alleles are recessive and some are dominant. In order to display the recessive trait i.e cystic fibrosis, both copies of the gene that a person has must be recessive.
If just one copy of the gene that the person has is dominant, it means they will display that dominant characteristic.
Polydactyly is caused by a dominant allele. A person only needs one copy of the gene to display polydactyly.
This is a gross oversimplification, but is perfectly adequate for GCSE
Sorry just read the rest of your post! Yes it is vastly more complicated than the textbooks suggest. They don't use eye colour anymore because it is a polygenic trait. So it was just plain wrong.
Just to be clear, I am not ant-jargon. My DS will learn it. I w as just being , myself, the confused layperson!
Thanks for the explanations!
I think I was confused because the word recessive and dominant were on the same page : One for CF and the other for the fingers and toes and it just all got in a bit of a muddle.
I blame CGP!
The answer to your other question is that exam boards need an example of both a recessive and a dominant disorder ( on instruction from OFQUAL) we all used to use Huntingtons for the dominant example ( dominant disorders are relatively rare in comparison to recessive conditions) but it was thought that for teenagers one unpleasant and life limiting condition ( cf, recessive example) was sufficient since polydactyly is relatively common and not particularly upsetting to study, due to not having an effect on survival of the person.
Ah, I see.
I have heard of quite a lot of people , but never met any IRL, with extra digits and not one of them seems to have inherited it. Still a bit flummoxed by that.
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