An Introduction to Mendelian Genetics | Biomolecules | MCAT | Khan Academy
Summary
TLDRThis video script offers an introduction to Mendelian Genetics, explaining the significance of chromosomes and alleles in determining an individual's traits. It distinguishes between homozygous and heterozygous genotypes and dominant and recessive alleles, using blood type as an example. It also clarifies the difference between genotype and phenotype and demonstrates how a Punnett Square can predict offspring's genetic outcomes, illustrating inheritance patterns with hypothetical parents.
Takeaways
- 𧬠Human cells contain 46 chromosomes, with 23 inherited from each parent.
- π©βπ¬ Chromosomes come in pairs, one from the mother and one from the father.
- π’ An allele is a section of a chromosome that codes for a specific gene.
- π§βπ€βπ§ Humans usually have two alleles for each gene: one from the mother and one from the father.
- π ° Blood type is determined by specific alleles, like A, B, and O.
- βοΈ The terms homozygous and heterozygous refer to having identical or different alleles for a gene.
- π₯ Dominant alleles, like A, overpower recessive alleles, like O.
- 𧬠Genotype refers to a person's genetic makeup, while phenotype refers to their physical traits.
- π A Punnett Square helps predict possible genotypes and phenotypes of offspring.
- π¨βπ©βπ¦ Allele combinations from parents determine the likelihood of certain traits, such as blood type.
Q & A
How many chromosomes do human cells typically contain?
-Human cells typically contain 46 chromosomes.
How are the 46 chromosomes in human cells inherited?
-23 chromosomes are inherited from a person's father and 23 from the mother.
What is the term used to describe a small section on a chromosome that codes for a specific gene?
-The term used is 'allele'.
Why do we sometimes say humans have 23 pairs of chromosomes instead of 46?
-It serves as a reminder that for each chromosome, we have a maternal and paternal copy.
What is the difference between homozygous and heterozygous individuals in terms of their alleles?
-A homozygous individual has two identical alleles for a gene, while a heterozygous individual has two different alleles.
What does it mean for an allele to be dominant?
-A dominant allele will be expressed in the phenotype if an individual has two different alleles.
What is the term for an allele that is not expressed when another, dominant allele is present?
-The term is 'recessive allele'.
What is the difference between genotype and phenotype?
-Genotype refers to an individual's genetic makeup, including the specific alleles they carry, while phenotype refers to the observable physical or biochemical traits expressed by those genes.
How can a Punnett Square be used to predict the genotypes of offspring?
-A Punnett Square can be used to determine all possible combinations of alleles that offspring can inherit from their parents by aligning the alleles of each parent.
What is the expected blood type phenotype for a child with the genotype AO?
-The child with the genotype AO will have blood type A as the A allele is dominant over the O allele.
If both parents have the genotype AO, what are the possible genotypes and phenotypes of their children?
-The possible genotypes are AA, AO, and OO. The phenotypes will be blood type A for AA and AO, and blood type O for OO.
Outlines
𧬠Introduction to Mendelian Genetics
This paragraph introduces the basics of Mendelian genetics, explaining that human cells contain 46 chromosomes, with 23 inherited from each parent. It discusses the concept of alleles, which are sections of chromosomes that code for specific genes. The paragraph uses blood type as an example, illustrating how alleles from both parents determine an individual's blood type. It introduces the terms 'homozygous' for individuals with two identical alleles and 'heterozygous' for those with different alleles. The dominant and recessive nature of alleles is also explained, with blood type A being dominant over type O. The paragraph concludes with the definitions of genotype, which refers to an individual's genetic makeup, and phenotype, which refers to the physical traits expressed by the genotype.
π Understanding Gene Inheritance with Punnett Squares
The second paragraph delves into the concept of gene inheritance, using a Punnett Square to demonstrate how alleles are passed from parents to offspring. It explains that each parent contributes one allele to their child, resulting in various possible genotypes. The paragraph uses the example of a couple with different blood type genotypes and shows how their children could inherit either AA, AO, or OO genotypes, leading to different blood type phenotypes. The Punnett Square is a tool for predicting these genetic outcomes, and the paragraph highlights how it can be used to understand the probabilities of different genotypes and phenotypes in offspring.
Mindmap
Keywords
π‘Chromosomes
π‘Alleles
π‘Homozygous
π‘Heterozygous
π‘Dominant Allele
π‘Recessive Allele
π‘Genotype
π‘Phenotype
π‘Punnett Square
π‘Inheritance
π‘DNA
Highlights
Human cells contain 46 chromosomes, with 23 inherited from each parent.
Each person is made up of a combination of genetic code from both parents.
Alleles are small sections on chromosomes that code for specific genes.
Humans usually have at least two alleles for every specific gene, one from each parent.
Blood type is determined by specific alleles, with A, B, and O being common examples.
An individual with two identical alleles for a trait is homozygous.
The dominant allele will determine the phenotype when two different alleles are present.
An individual with two different alleles is heterozygous.
Genotype refers to an individual's alleles, while phenotype refers to the physical traits expressed.
Different genotypes can result in the same phenotype due to dominant and recessive alleles.
Punnett Square is a tool used to predict the genotypes of offspring from two parents.
Children can inherit one allele from each parent, resulting in various genotype combinations.
If both parents are heterozygous, three-quarters of their children will have blood type A.
If one parent is homozygous and the other heterozygous, one-quarter of children will have blood type O.
Understanding alleles, genotypes, and phenotypes is crucial for predicting genetic inheritance.
Dominant and recessive traits play a significant role in determining an individual's phenotype.
The Punnett Square illustrates how genetic traits can be passed down from parents to offspring.
Transcripts
-[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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