If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Inbreeding

Learn about inbreeding and how it can hurt a population's genetic diversity. By Ross Firestone. Created by Ross Firestone.

Want to join the conversation?

  • blobby green style avatar for user nathan
    At , the author describes the different allele combinations and their phenotypic expression for an autosomal dominant disorder. For Huntington's disease, the AA genotype is reported to have an unaffected phenotype. Is this true? Wouldn't the individual so afflicted have two defective copies of the gene and therefore manifest the dominant phenotype? The Wikipedia page on the inheritance pattern of Huntington's disease seems to confirm this result. Am I missing something? Thank you.
    (13 votes)
    Default Khan Academy avatar avatar for user
    • piceratops sapling style avatar for user Marie Anderson
      Many professors and textbooks use capital letters to denote a dominant allele. However, this speaker uses a capital A to represent the normal (undamaged) allele, regardless of whether it is dominant or recessive. Therefore, genotype AA in an autosomal dominant disorder has an unaffected phenotype (the person is not diseased) because capital A stands for the normal allele, rather than the dominant (damaged) one. It boils down to a difference in notation from what you (and I, actually) are used to from our previous biology education.
      (18 votes)
  • blobby green style avatar for user Brian Park
    Shouldn't the AA and Aa be diseased and aa not disease in the case of the presenter's example for Huntington disease, because it is autosomal dominant disease? aprox
    (4 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Abid Ali
      In this example, the speaker is using lower-cased letters to represent the dominant allele, while using capital-letters to represent the recessive allele. You are right though, it confused me at first as well.
      (8 votes)
  • duskpin tree style avatar for user Mitochondria
    People with Huntington's disease are usually not aware of carrying this disease, because it has late onset. Therefore people with this disease are not aware of it until their 30's after they have offspring. Huntington's disease is a bad example.
    (4 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user Levi Arnold
    What does autosomal mean?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • mr pants teal style avatar for user Allison Turbitt
    Is it inbreeding if two humans who are related by adoption reproduce?
    (0 votes)
    Default Khan Academy avatar avatar for user
  • male robot johnny style avatar for user Dododeda
    Just to clarify, for autosomal dominant disorders, AA and Aa are BOTH indicators of having the disease (not "aa"). Because textbooks and the mainstream scientific community represent a dominant allele with a capital letter, this is the correct notation to use for autosomal dominant disorders.
    (3 votes)
    Default Khan Academy avatar avatar for user
  • female robot grace style avatar for user Eda Fenercioglu
    Just wanted to make a note that although people with Huntington's Disease may be aware of their disease, the symptoms begin to show up after the age of 50 - aka after their period of reproductive fitness. Thus, they would have likely passed on their genes for HD before they're aware that they carry the dominant allele.
    (2 votes)
    Default Khan Academy avatar avatar for user
  • mr pants teal style avatar for user Aiza Zia
    So just to clarify, a person can't be a "carrier" for an autosomal dominant disease, correct? Because if that person has even one gene for the disease, he or she will display it
    (2 votes)
    Default Khan Academy avatar avatar for user
  • male robot hal style avatar for user Brett Kramer
    But won't more breeding with the general population create more carriers in the general population for autosomal recessive diseases and we'll be back to square one when the general population would have the same prevalence for carrier of the disease as the inbreed population?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • blobby green style avatar for user BfreeXI
      The "general population" is much larger in terms of number of people and genetic variability. Yes, breeding with the general population COULD create more carriers, but draw out the punnet square. Someone who is a carrier that breeds with someone who is completely unaffected by a disease (as in more likely in the general population) only has a 50% chance of producing a child who is a carrier. There are 7 billion people in the general population--the chance of running into someone who is a carrier is very slim. When talking about inbred populations, we are talking about very small populations--relative to the 7 billion people on this planet. Someone in an inbred population is much more likely to breed with someone who is also a carrier--and they have a 25% chance of producing someone affected with the disease, and a 50% chance of producing a carrier.
      (3 votes)
  • male robot hal style avatar for user Abid Ali
    Why is he using lower-cased letters to represent a dominant disorder?
    (2 votes)
    Default Khan Academy avatar avatar for user

Video transcript

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.