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  • -[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.

-[Voiceover] An introduction to Mendelian Genetics.

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B1 中級

メンデル遺伝学入門 (An Introduction to Mendelian Genetics)

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    TeacherJennifer Bryne に公開 2021 年 01 月 14 日
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