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An Introduction to Mendelian Genetics

Let's explore the kinds of inheritance patterns seen in Mendelian Genetics! Uncover the secrets of alleles, and explore the difference between homozygous and heterozygous traits. Learn about dominant and recessive traits, and how they influence genotypes and phenotypes. Finally, discover how a Punnett Square can predict gene inheritance. By Ross Firestone.


Created by Ross Firestone.

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  • old spice man green style avatar for user Zachary Meade
    Why are the practice problems always at the beginning of the section?
    (27 votes)
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  • ohnoes default style avatar for user Cyan Wind
    As far as I know, the 'AA' type and the 'AO' type is the same blood type: A. Are these two types (AA & AO) different from each other, especially in blood transfusion?
    (9 votes)
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    • winston default style avatar for user Kenan
      Since the A allele is dominant, both AA and AO are considered the A blood type. The categorization of the blood types refers to the antigens (or small proteins) that are found on the red blood cells and recognized by the body as "self". A would have A antigens, B would have B antigens while O has no antigen. Thus, if you are type A then your body will attack B antigens if blood type B is transfused .Taking this into account, there should be no problem with blood transfusion from AA to AO or vice versa.
      (21 votes)
  • orange juice squid orange style avatar for user Jake Northrup
    Is there a reason Mendel was not mentioned in the video?
    (7 votes)
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  • hopper jumping style avatar for user Ishan Khare
    Why is the A allele dominant and the O allele recessive? Is it because of the order that they are listed in, or is this known knowledge from studying blood types?
    (2 votes)
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    • piceratops seed style avatar for user jon
      It's easier to look at it as both antigen A and antigen B are dominant and O is neither. B can't cancel out A and A can't cancel out B, so you can assume they are both dominant. Now the reason why O is neither is because it doesn't have either antigen A or B therefore you end up with nothing where they call it O type. This has no relation to order, but only on the presence of antigen A or antigen B or both A&B.
      (4 votes)
  • blobby green style avatar for user Kajal
    so we have three alleles, this is exception to mendelian genetics ?
    (3 votes)
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    • starky tree style avatar for user Madison Ackroyd
      This is not an exception to Mendelian genetics because one diploid individual can still only have two alleles. This is just showing that there are three possible alleles in the population (there are actually more than that but that is beyond the scope of this question (e.g., Bombay phenotype)). This is called polymorphism: multiple wild-type alleles for a single gene in the population.
      (1 vote)
  • duskpin seedling style avatar for user Alreem
    Hi guys
    I’m Lisa
    I have a question
    What is law of segregation ?? I’m confused T-T
    (3 votes)
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  • old spice man green style avatar for user kelvin jones
    At , why is A(the dominant blood type) put next to O(the recessive)?
    (1 vote)
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    • blobby green style avatar for user Billy Jabbar
      When we refer to the different blood types (ie. AB, A, B, or O), we are classifying them according to the presence of a series of sugar molecules on their surface. These sugar molecules are selectively cleaved and modified based on what enzymes an individual has present in their system. On a genetic level, individuals who are blood type O lack the ability to produce the enzymes necessary for the attachment of these sugar residues. By default, they are dubbed the recessive phenotype. Other blood types have some variant of enzymes functioning and these enzymes are expressed independently of the other. So if by some chance you are lucky enough to be blood type AB, you would have the ability to produce both of these enzymes. We refer to this as co-dominance. (In the classical case of 'dominance' one gene masks the other 'recessive' gene; notice this isn't the case here).
      (4 votes)
  • leafers ultimate style avatar for user ff142
    How can humans have > 2 alleles per gene?
    (1 vote)
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    • piceratops seed style avatar for user Irma Arciga
      An individual can only have two alleles at most for a given gene. Multiple alleles (as in the case of A/B/O for blood type) are a phenomena for a population. So one individual can have AB and another can have AO, and another can have OO, therefore accounting for the difference in blood types amongst humans.
      (3 votes)
  • female robot ada style avatar for user MCATchristina
    Can both of the parents give their offspring the same allele? @0.54
    (1 vote)
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    • leaf red style avatar for user cmbunch
      Yes! An allele, as discussed in the video, is a form of a gene. We typically think of human genes as having two alleles: one from the father and one from the mother. What alleles are passed onto the offspring compose the genotype of the offspring (eg. AA, Aa, or aa), hence where terms such as dominance, recessive, homozygotes, and heterzygotes come into play. Therefore, any homozygote (genotype AA or aa) MUST have received the same allele from each parent!
      (3 votes)
  • duskpin ultimate style avatar for user SUKANNYA
    What if both the parents have blood type AB?
    (0 votes)
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    • blobby green style avatar for user Jada Bean
      If both parents had blood type AB, that would mean that the offspring would have:
      25% chance of having the genotype AA (phenotype would be blood type A)
      50% chance of having the genotype AB (phenotype would be blood type AB)
      25% chance of having the genotype BB (phenotype would be blood type B)
      (4 votes)

Video transcript

-[Voiceover] An introduction to Mendelian Genetics. Now before we start, let's review the idea that human cells contain 46 chromosomes, which contain the DNA that makes each cell unique. 23 of these chromosomes were inherited from a person's father and 23 were inherited from the mother. We can say that each person's made up of a combination of genetic code from both of their parents. Now sometimes we like to say that we have 23 pairs of chromosomes. Instead of saying that we have 46 total because that way we remind ourselves that for each chromosome we have a maternal and paternal copy. Now the first thing I want to introduce is the term allele. If we have a chromosome here and then an allele is one small section on that chromosome that codes for a specific gene that makes you, you. Since humans have at least two copies of each chromosome, we can say that humans usually have at least two alleles for every specific gene. One allele from their mother and one from their father. Let's look at an example and we'll start by talking about blood type. I'm sure that you've heard that blood types are usually named with letters like A, B, and O. What does that actually mean? Well there's a specific allele that codes for blood type. Let's say that we have this guy here and his alleles both code for blood type A. I'll use the letter A for that. Let's say we have this girl here who has one allele coding for A and another allele coding for blood type O. Now for the guy, he has both alleles coding for blood type A then it's pretty clear that when we check his actual blood type it will be A. For the girl, we're not so sure since she has one of each. Now, I'm going to introduce a couple new terms to you. The first is that since the guy has two alleles that code with the same thing both code for blood type A then we say that this guy is homozygous. Homo means the two alleles are the same, homo the same and zygous refers to mixture of DNA that he got from his parents. Someone who is homozygous got the same allele from both parents. In the case of the girl, is she going to have blood type A or blood type O? Well it turns out that she's going to have blood type A and that's because the A allele is the dominant allele. While the O allele is the recessive allele. When an allele is dominant that means if someone has two different alleles it will be the dominant one that wins. In this case since A is dominant over O which is recessive, A will win and she'll have blood type A. Since this girl has two different alleles we call her heterozygous since hetero means different and zygous refers to the same thing, a mixture of DNA that she got from her parents. Now I want to introduce two more terms. We can describe a person's genes in two different ways. We can look at the person's individual alleles and we call this the genotype. For this guy his genotype is AA referring to his two alleles which both code for blood type A. We can also look at a person's physical traits which we call the phenotype. For this guy and girl the phenotype would be blood type A. You can see that genotype and phenotype are different but it is possible for two different genotypes to make the same phenotype. Since some alleles are dominant over others. Let's talk about gene inheritance for a bit. Let's say that our guy and girl from before have offspring together. We can use something called a Punnett Square to determine what different genotypes their kids could have. Each of the parents two alleles are on separate chromosomes, so each parent will contribute one of their two alleles to the child. The Punnett Square allows you to determine all possible combinations. If we take the father's alleles and line them up vertically and then take the mother's alleles and line them up horizontally, we can fill in the chart to find the possible genotypes for our offspring. In this case, two of our boxes will have the AA in them and two will have AO in them. That means half of the children will have the genotype AA and half of the children will have genotype AO. Since both of these genotypes code for the same phenotype all of the children will have the blood type A phenotype. Let's see what happens if we change our father's genotype to match our mother's genotype. Now only one-quarter of the children will have the AA genotype, half will have the AO genotype since the order of the two alleles doesn't matter OA and AO are the same. One quarter will have the OO genotype. This means that 75% of the children will have blood type A in their phenotype. Since AA and AO make blood type A but 25% of the children will have the blood type O phenotype, since OO makes blood type O. What did we learn? Well first we learned what an allele is and the difference between homozygous and heterozygous, as well as the difference between dominant and recessive traits in relation to alleles. Second, we learned about the difference between genotype and phenotype and how the genotype refers to a persons DNA while a phenotype refers to the physical traits that the DNA codes for. Finally we learned about how we can use a Punnett Square to determine how different alleles will be inherited from two parents.