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

Summary

This 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

• 🧬 Human cells contain 46 chromosomes, with 23 inherited from each parent.
• 👩‍🔬 Chromosomes come in pairs, one from the mother and one from the father.
• 🔢 An allele is a section of a chromosome that codes for a specific gene.
• 🧑‍🤝‍🧑 Humans usually have two alleles for each gene: one from the mother and one from the father.
• 🅰 Blood type is determined by specific alleles, like A, B, and O.
• ⚖️ The terms homozygous and heterozygous refer to having identical or different alleles for a gene.
• 🥇 Dominant alleles, like A, overpower recessive alleles, like O.
• 🧬 Genotype refers to a person's genetic makeup, while phenotype refers to their physical traits.
• 📊 A Punnett Square helps predict possible genotypes and phenotypes of offspring.
• 👨‍👩‍👦 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

00:00

🧬 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.

05:00

🌟 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.

Keywords

💡Chromosomes

Chromosomes are thread-like structures found in the nucleus of human cells, composed of DNA and proteins, and carry genetic information. In the context of the video, it is mentioned that human cells contain 46 chromosomes, with 23 inherited from each parent, which contribute to making each individual genetically unique. This concept is foundational to understanding Mendelian genetics as it sets the stage for how genetic traits are inherited.

💡Alleles

Alleles are different forms of a gene that can exist at the same place on a chromosome. The video explains that each chromosome has small sections called alleles that code for specific genes, and since humans have two copies of each chromosome, they typically have two alleles for every gene, one from each parent. The example of blood type is used to illustrate how alleles determine physical traits.

💡Homozygous

Homozygous refers to an individual having two identical alleles for a particular gene. The video uses the term to describe a person with two alleles that both code for blood type A, indicating that this individual inherited the same allele from both parents. This term is crucial for understanding how genetic traits can be consistently expressed in an individual.

💡Heterozygous

Heterozygous is the term used to describe an individual who has two different alleles for a specific gene. In the video, the girl with one allele for blood type A and another for blood type O is described as heterozygous. This concept is important for understanding how traits can be mixed and expressed differently in offspring.

💡Dominant Allele

A dominant allele is one that will be expressed in the phenotype if it is present alongside a recessive allele. The video explains that the A allele for blood type is dominant over the O allele, which is recessive. This means that if an individual has one A and one O allele, their blood type will be A. Understanding dominance is key to predicting how traits will appear in individuals.

💡Recessive Allele

A recessive allele is only expressed in the phenotype if two copies of the recessive allele are present. The video uses blood type O as an example of a recessive allele, which is only expressed when an individual has two O alleles. This concept is essential for understanding how certain traits can skip generations and appear in offspring.

💡Genotype

Genotype refers to the specific set of genes, or alleles, that an individual carries for a particular trait. The video explains that the genotype can be represented by the alleles an individual has, such as AA for the man with two A alleles for blood type. Genotype is a fundamental concept in genetics as it represents the genetic makeup of an individual.

💡Phenotype

Phenotype is the observable physical or biochemical characteristics of an individual, which result from the interaction of their genotype with the environment. In the video, the phenotype is exemplified by blood type, which is the expression of the genotype. Understanding phenotype is important as it relates to the physical expression of genetic traits.

💡Punnett Square

A Punnett Square is a diagram used to predict the genotypes of offspring based on the genotypes of the parents. The video demonstrates how to use a Punnett Square to determine the possible genotypes of children from a couple, one with genotype AA and the other with genotype AO for blood type. This tool is fundamental in understanding how genetic traits are inherited from one generation to the next.

💡Inheritance

Inheritance in genetics refers to the passing of genetic information from parents to offspring. The video discusses how alleles are inherited, using blood type as an example to show how different combinations of alleles can result in different blood types in children. Understanding inheritance is crucial for predicting genetic traits in offspring.

💡DNA

DNA, or deoxyribonucleic acid, is the molecule that contains the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms. While not explicitly detailed in the video, DNA is the underlying substance that chromosomes and alleles are made of, and it is central to the study of genetics and heredity.

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.