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  • So today, I want to talk to you about the effects of inbreeding

  • and how it's not really the best for a population.

  • So before we do that, let's review

  • the concept of natural selection.

  • And natural selection is the idea

  • that a member of a population that

  • has a special genetic trait that's advantageous

  • is more likely to live to an age where they can reproduce

  • and pass on that special trait to their offspring.

  • And you should also remember that a population can

  • get a lot out of having a big gene pool.

  • And the bigger the gene pool, the more genetic diversity

  • the population has, which allows the group

  • to adapt to many different environmental changes.

  • So what is inbreeding, exactly?

  • Well, inbreeding is when people in a population

  • will selectively have offspring with a certain smaller

  • group within that larger population.

  • And this can be for a bunch of reasons,

  • like religion or culture, or maybe just because

  • of preference.

  • And when inbreeding occurs with non-human populations,

  • it's almost always due to geographical barriers, where

  • the greater population simply isn't accessible.

  • Now, when people usually think of inbreeding,

  • words like "incest" come to mind.

  • But inbreeding really isn't limited to members

  • of the same family having offspring together.

  • Lots of small religious and cultural groups in the world

  • have some people with common ancestors

  • and are only distantly related.

  • So you can see that the effects of inbreeding

  • can exist without close relatives actually having

  • children together.

  • So why is inbreeding a problem in the first place?

  • Well, let's look at an example.

  • So Tay-Sachs disease is an autosomal recessive disorder.

  • And what that means is that people

  • with no copies of the genes are unaffected by the disease.

  • And I've drawn these people in blue.

  • People with just one copy of the gene

  • are not affected by the disease, but are carriers for the gene.

  • And I've drawn these people in red.

  • And people with two copies of the gene

  • are affected by the disease.

  • And I've drawn these guys in purple.

  • So let's say we have someone who's a carrier for Tay-Sachs.

  • So he has just one copy of the gene.

  • If we're looking at the general population,

  • we can see that the odds of the person choosing a mate that's

  • also a carrier for the disease are pretty low.

  • And if he eventually has some kids, none of them

  • will be affected by the disease, and only a few

  • will even be carriers.

  • It's likely that the copies of the gene

  • will be so spread out among the population

  • that it would be quite rare for two carriers

  • to actually end up mating together.

  • Now if we look at an inbred population

  • where a bunch more people could be carriers for the disease,

  • the chances of our guy choosing a mate that's also a carrier

  • are a little higher.

  • So more of his children will be carriers for the disease.

  • But there's also a chance that some of his offspring

  • may get two copies of the gene and actually

  • be affected by the disease.

  • Now, we just talked about an example

  • with an autosomal recessive disorder.

  • But maybe you're wondering how inbreeding

  • affects autosomal dominant disorders.

  • Well, let's look at Huntington's disease, which

  • is autosomal dominant.

  • And since this disease is autosomal dominant,

  • if a person has no copies of the gene,

  • they'll be unaffected by the gene.

  • And I've drawn these people in blue once again.

  • However, if a person has either one or two copies of the gene,

  • then that person will be affected

  • by the disease either way.

  • And I've drawn both of these people in red.

  • The key difference in this case is that

  • no matter who the guy has children with,

  • even if that guy just has one copy of the Huntington's gene,

  • there's still a chance that there

  • will be children affected by the disease.

  • Now, of course, if our guy has children with someone

  • who was also affected by the disease,

  • then more of his children would be affected.

  • But there's still a chance either way.

  • Now, one of the other reasons why we're less concerned

  • about inbreeding affecting autosomal dominant diseases

  • is that carriers for dominant disorders

  • are generally aware that they're affected and are well

  • aware of the risks of them having diseased children.

  • With recessive disorders, carriers usually

  • don't have any symptoms at all.

  • And they may not even know that they're

  • carriers until they've had a diseased child.

  • And this makes it much more important

  • for people in inbred populations to seek genetic counseling so

  • that they are aware of the risks of them

  • having diseased children.

  • So what did we learn?

  • Well, first we learned that certain inbred populations

  • can have many more individuals that

  • may carry a diseased chromosome than the general population.

  • But we also learned that this is mostly

  • a concern with autosomal recessive diseases,

  • since those generally go more unnoticed

  • than dominant ones do.

So today, I want to talk to you about the effects of inbreeding

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B1 中級

近親繁殖 (Inbreeding)

  • 90 9
    Morris Du 發佈於 2021 年 01 月 14 日
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