An Introduction to Mendelian Genetics | Biomolecules | MCAT | Khan Academy
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An Introduction to Mendelian Genetics | Biomolecules | MCAT | Khan Academy

-[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.

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35 thoughts on “An Introduction to Mendelian Genetics | Biomolecules | MCAT | Khan Academy

  1. idk if its just me, but i'm trying to watch this and can't pay attention bc I keep thinking this guy sounds exactly like the main guy in the Netflix series "Love"

  2. This was so helpful. I was reading and become totally overloaded with a lot of information. This was straight to the point.

  3. allele – small section of gene that codes for a trait
    homozygous – same allele for both parent, mixture
    dominant – wins
    recessive – nope
    genotype – individual alleles

  4. As far as my knowledge.
    Its not 75% of children have blood A and 25% children have blood O
    Its the probability in a same child ( a child born will have 75% probalility to have blood group A and 25% probability to have blood group O , and its same to every child born ).

  5. When describing the Punnett Square, your explanation is partially wrong. Although the combination of alleles shows 2 x AA genotypes and 2 x AO genotypes, this does not mean that half of the children will have AO and half will have AA.

    Consider what would happen if this couple only ever had one child? There is no order of dominance for which aspect of the allele will be expressed in the zygote from each parent (that we know of). It is not like the father will give the A allele to the first child and the mother will also give their A to the child to make AA, this is random as far as we know and influenced by MANY things but not by dominance of inheritance. So it would come down to probability as opposed to actual allocation of each allele.

    Therefore, there is a 50% probability that the child will have genotype AA, 50% probability the child will have type AO but a 100% probability the child will have phenotype blood group A

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