Heredity: Crash Course Biology #9

Summary

This script explores the fascinating world of genetics through the lens of siblings John and Hank, who share similar traits due to their common DNA. It delves into the history of heredity studies, from Aristotle's misconceptions to Gregor Mendel's groundbreaking pea plant experiments. The script simplifies complex genetic concepts like chromosomes, genes, dominance, and polygenic traits, using the example of earwax consistency to explain Mendelian inheritance. It also touches on sex-linked traits, humorously linking baldness to maternal lineage, and ends with a reminder of the inevitability of genetic inheritance.

Takeaways

• 👬 The script discusses the concept of heredity, focusing on the genetic similarities between brothers John and Hank, who share the same parents.
• 🧬 It explains that heredity involves the passing on of genetic traits from parents to offspring, with the source of DNA being the same for siblings.
• 🕰️ The understanding of heredity has evolved over time, with early ideas being incorrect, such as Aristotle's theory of blending traits and the concept of 'bloodlines'.
• 🌱 Gregor Mendel, an Austrian monk, is credited with the first modern studies of heredity through his experiments with pea plants, which led to the foundation of classical genetics.
• 🧬🧬 Humans have 23 pairs of chromosomes, with genes being sections of DNA that determine traits, often influenced by multiple genes in a process called polygenic inheritance.
• 🔍 Traits determined by a single gene are referred to as Mendelian traits, such as the wetness or dryness of earwax, which is controlled by a gene on chromosome 16.
• 🧬 Alleles are different versions of a gene, with one potentially being dominant and masking the expression of another recessive allele.
• 👶 Inheritance is random, with offspring receiving one allele for each gene from each parent, resulting in a 50/50 chance for certain traits like earwax type.
• 🧬👨‍👩‍👧‍👦 Genotypes refer to an individual's genetic makeup for a trait, which can be homozygous (same alleles) or heterozygous (different alleles).
• 📊 Punnett Squares are used to predict the possible genetic outcomes of offspring based on the genotypes of the parents.
• 🧬👦 Sex-linked inheritance involves traits linked to the sex chromosomes, with men being more likely to express recessive traits due to having only one X chromosome.
• 🧔 The script humorously connects genetics to everyday life, such as the correlation between earwax type and body odor, and the genetic predisposition to balding in men.

Q & A

What is the relationship between John and Hank mentioned in the script?

John and Hank are brothers who share the same parents, which is why they have similar physical characteristics due to inheriting the same DNA.

Why did people believe that brothers tend to look alike before the understanding of DNA?

People noticed similarities between siblings without understanding the science behind it, and this observation has been made since humans started paying attention to such details.

What was Aristotle's contribution to the early concept of heredity?

Aristotle suggested that a new human is a mixture of parents' traits, with the father supplying the life force and the mother providing the building blocks. He also thought semen was like highly-purified menstrual blood, which influenced the term 'bloodlines' in discussions of heredity.

Who was the first person to study heredity in a modern way?

Gregor Mendel, an Austrian monk, was the first to study heredity in a modern way by conducting experiments with pea plants and observing the patterns of inheritance.

What is the basic unit of heredity according to the script?

The basic unit of heredity is a gene, which is a section of DNA on a chromosome that contains information determining a trait.

What is a polygenic trait and how does it differ from a Mendelian trait?

A polygenic trait is determined by the interaction of many different genes, making it more complex. In contrast, a Mendelian trait is decided by a single gene, such as the color of pea flowers studied by Mendel.

What is the significance of the earwax gene and its alleles?

The earwax gene determines the consistency of earwax, with one allele for wet earwax being dominant and the other for dry earwax being recessive. The difference between the two alleles lies in a single amino acid in the gene sequence.

What is the difference between diploid and haploid cells in terms of chromosomes?

Diploid cells, like somatic cells, contain two sets of chromosomes, one from each parent. Haploid cells, like gametes (sperm and egg), contain only one set of chromosomes.

What is a Punnett Square and how is it used in genetics?

A Punnett Square is a diagram invented by Reginald C. Punnett to predict the outcome of genetic crosses. It helps visualize the possible combinations of alleles in offspring based on the genotypes of the parents.

What is the connection between earwax and body odor mentioned in the script?

The script mentions a correlation between wet earwax and body odor because both are produced by the same type of gland, and the gene responsible for earwax consistency can also affect body odor levels.

What is sex-linked inheritance and how does it relate to balding in men?

Sex-linked inheritance involves genetic traits linked to a person's sex and passed on through the sex chromosomes. Balding in men is an example of a sex-linked trait, caused by a recessive allele on the X chromosome inherited from the mother.

Outlines

00:00

👬 The Similarities in Siblings' Physical Traits

This paragraph introduces the concept of heredity, explaining how siblings like John and Hank share similar physical traits due to having the same parents and, consequently, similar DNA. It delves into the historical misunderstandings of heredity, such as Aristotle's theories, before discussing Gregor Mendel's pivotal role in establishing patterns of inheritance through his experiments with pea plants. The paragraph also touches on the basics of genetics, including chromosomes, genes, and the distinction between polygenic and pleiotropic traits, with a specific focus on Mendelian traits like earwax consistency.

05:01

🧬 Understanding Genetic Inheritance and Alleles

The second paragraph delves deeper into the mechanisms of genetic inheritance, discussing the concepts of dominance and recessiveness in alleles, which determine the expression of traits. It uses the example of earwax wetness to illustrate how a dominant allele (wet earwax) can mask the expression of a recessive allele (dry earwax). The paragraph explains the terms 'heterozygous' and 'homozygous' in the context of genetic makeup and how these influence the physical expression of traits, or phenotype. It also introduces the Punnett Square as a tool for predicting genetic outcomes and discusses the correlation between earwax wetness and body odor due to pleiotropic genes. Lastly, it touches on sex-linked inheritance, explaining how traits like baldness can be influenced by recessive alleles on the X chromosome.

Keywords

💡Heredity

Heredity refers to the passing on of genetic traits from parents to offspring. It is the central theme of the video, which explores how physical characteristics are inherited. The script discusses the historical misunderstandings of heredity and introduces Gregor Mendel's work as the foundation of modern genetic understanding.

💡DNA

DNA, or deoxyribonucleic acid, is the molecule that contains the genetic instructions for the development and function of all living organisms. In the script, DNA is mentioned as the source of genetic information that determines physical traits, highlighting its importance in heredity.

💡Chromosomes

Chromosomes are thread-like structures consisting of DNA and protein and are found in the nucleus of cells. The script explains that human cells have 23 pairs of chromosomes, which play a crucial role in the inheritance of traits from parents to offspring.

💡Gene

A gene is a segment of DNA that carries the instructions for making a specific protein, which in turn determines a particular trait. The video uses the example of earwax consistency to illustrate how a single gene can influence a specific characteristic.

💡Polygenic Trait

A polygenic trait is a characteristic determined by multiple genes working together. The script points out that most physical traits are polygenic, making the genetic influence on these traits more complex.

💡Pleiotropic

Pleiotropic refers to a single gene influencing multiple traits. The script mentions this concept in relation to the gene that affects both earwax consistency and body odor, demonstrating the pleiotropic nature of some genes.

💡Mendelian Trait

A Mendelian trait is a characteristic that is decided by a single gene, following patterns discovered by Gregor Mendel. The script uses the color of pea flowers as an example of a Mendelian trait and discusses how this concept applies to human earwax consistency.

💡Allele

An allele is a variant form of a gene, with different alleles representing the different versions that a gene can take. The video explains how alleles determine the expression of traits, such as wet or dry earwax, based on their dominance or recessiveness.

💡Dominance

Dominance is the relationship between alleles where one allele masks or suppresses the expression of another allele. In the script, the dominance of the 'wet earwax' allele over the 'dry earwax' allele is discussed to explain the inheritance of this trait.

💡Heterozygous

Heterozygous describes an organism that has two different alleles for a particular gene. The script uses the term to explain the genetic makeup of an individual who has inherited different versions of the earwax gene from each parent.

💡Homozygous

Homozygous refers to an organism that has two identical alleles for a particular gene. The video explains how a homozygous recessive individual, like the father in the script, would express the recessive trait, such as having dry earwax.

💡Punnett Square

A Punnett Square is a diagram used to predict the genotypes of offspring from a particular cross. The script uses the Punnett Square to illustrate the possible outcomes of the earwax gene inheritance from parents to their children.

💡Sex-Linked Inheritance

Sex-linked inheritance involves genes located on the sex chromosomes, which can result in traits being expressed differently in males and females. The script discusses how traits like baldness can be inherited in a sex-linked manner, with males being more likely to express certain recessive traits due to the structure of their sex chromosomes.

Highlights

Brothers John and Hank share the same parents and DNA source, leading to similar physical characteristics.

Heredity has been studied since ancient times, but early theories like Aristotle's were incorrect.

Gregor Mendel's experiments with pea plants established a foundation for understanding inheritance patterns.

Classical Genetics simplifies genetics by explaining how traits are passed from parents to offspring.

Chromosomes are the carriers of DNA, with humans having 23 pairs in their cells.

A gene is a specific section of DNA on a chromosome that determines a trait.

Polygenic traits are determined by multiple genes working together, complicating genetic inheritance.

Pleiotropic genes influence the expression of multiple traits from a single gene.

Mendelian traits are decided by a single gene, such as the color of pea flowers.

Human earwax consistency is an example of a Mendelian trait determined by a gene on Chromosome 16.

The difference between wet and dry earwax is determined by a single amino acid in the gene sequence.

Inheritance involves receiving one allele for each gene from both parents, resulting in a 50/50 chance for certain traits.

Dominance in alleles means one can mask the expression of another in determining a trait.

Genotypes can be homozygous (same alleles) or heterozygous (different alleles) for a given trait.

A Punnett Square is a tool for predicting the outcome of genetic crosses and understanding inheritance.

The correlation between wet earwax and body odor is due to a pleiotropic gene affecting multiple traits.

Sex-linked inheritance involves traits passed through sex chromosomes, affecting males and females differently.

Baldness in men is an example of a sex-linked trait, often inherited from the maternal X chromosome.

Genetics demonstrates the inevitability of inherited traits and their impact on physical characteristics.

Transcripts

So, I have this brother, John. You may have heard of him.

JOHN: Hi there!

HANK: As it happens, John and I have the exact same parents.

JOHN: Yes, Mom and Dad Green.

HANK: And since we have the same parents, it's to be expected

that John and I would have similar physical characteristics

because the source of our DNA is exactly the same.

JOHN: Hank and I share some genes, but nobody knew anything about chromosomes or DNA until

the middle of the 20th century. And people have been noticing that brothers tend to look

alike since like, people started noticing stuff or whatever.

HANK: That was very scientific, John.

JOHN: I will remind you that I am doing you a favor.

Heredity: it's basically just the passing on of genetic traits from parents to offspring.

Like John said, the study of heredity is ancient, although the first ideas about how the goods

are passed on from parents to kids were really really really really really really

wrong.

For instance, the concept that people were working with for nearly 2,000 years came from

Aristotle, who suggested that: We're each a mixture of our parents' traits,

with the father kind of supplying the life force to the new human and the mother

supplying the building blocks to put it all together.

Aristotle also thought that semen was like highly-purified

menstrual blood, which is why we still refer to "bloodlines" when

we're talking about heredity.

Anyway, since nobody had a better idea, and since nobody

really wanted to tangle with Aristotle, for hundreds of years

everybody just assumed that our parents' traits just sort of

blended together in us:

like if a black squirrel and a white squirrel fell in love and decided to start a family

together, their offspring would be gray.

The first person to really start studying and thinking about

heredity in a modern way was this Austrian monk named Gregor Mendel

and Mendel demonstrated that inheritance followed particular patterns.

In the mid-1800s, Mendel spent sort of an unhealthy amount of time grubbing around

in his garden with a bunch of pea plants, and through a series of experiments, crossing

the pea plants and seeing which traits got passed on and which didn't--he came up with

a framework for understanding how traits actually get passed from one generation to another.

So, to talk about Classical Genetics, which includes Mendel's

ideas about how traits get passed along from parents to children,

we kind of have to simplify the crap out of genetics. I hope you don't mind.

So we've all got chromosomes, which are the form that our DNA

takes in order to get passed on from parent to child.

Human cells have 23 pairs of chromosomes. Now a gene is a

section of DNA in a specific location on a chromosome that

contains information that determines a trait.

Of course, the vast majority of the time, a physical trait is a

reflection of a bunch of different genes working together, which makes this all very confusing,

and when this happens it's called a polygenic trait.

Polygenic: many genes.

And then again, sometimes a single gene can influence how

multiple traits are going to be expressed; these genes are called pleiotropic.

However, some

very few, but some

single traits are decided by a single gene. Like the

color of pea flowers for example, which is what Mendel studied when he discovered all

of this stuff, and when that happens, in Mendel's honor, we call it a

Mendelian trait.

There are a couple of examples of Mendelian traits in humans, one of them being the relative

wetness or dryness of your ear wax.

So, there is just one gene that determines the consistency of your

earwax, and that gene is located at the very same spot on each

person's chromosome.

Right here! Chromosome 16.

However, there's one version of this gene, or allele, that says the

wax is going to be wet, and there's another allele that says the wax is going to be dry.

You may be asking yourself what the difference is between these two things and I'm glad you

asked because we actually know the answer to that question.

Among the many amino acids that make up this particular

gene sequence, there is one exact slot where they're different. If

the amino acid is glycine in that slot, you're gonna have wet ear wax. But if it's arginine,

it's dry.

Now comes the question of how you get what you get from your

parents. In most animals, basically any cell in the body that isn't a sperm or an egg -- these

are called somatic cells -- are diploid, meaning there are two sets of chromosomes,

one inherited from each of your parents. So you get one earwax-determining

allele from your mom and one from your dad.

I should mention that the reason for this is that gametes, or sex cells--Senor Sperm

and Madame Egg--are haploid cells, meaning they only have one set of chromosomes.

Again, for emphasis, non-sex cells are called somatic cells and they are diploid. Sex cells

are gametes and they are haploid.

This makes a lot of sense because a sperm or an egg has a very specific motivation:

they're seriously hoping to score, and if they do, they plan to join with a complementary

haploid cell that has the other pair of chromosomes they're going to need to make a new human,

or buffalo or squid or whatever.

Also, just so you know, some plants have polyploid cells, which means they have more than two

sets of chromosomes in each cell, which isn't better or anything--it's

just how they do. But anyway, the point of all that is that we inherit

one version of the earwax gene from each of our parents.

So, back to earwax!

So, let's just say your mom gives you a wet earwax allele and your

dad gives you a dry earwax allele.

Good Lord, your dad has horribly ugly ears!

Anyway, since your parents have two alleles, each for one gene inherited from each of their

parents, the one passed along to you is entirely random.

So, a lot of what Mendel discovered is that when there are two

alleles that decide the outcome of a specific trait, one of these

alleles could be dominant and the other one recessive.

Dominance is the relationship between alleles in which one allele

masks or totally suppresses the expression of another allele.

So, back to earwax, because I know we all love talking about it so much.

It turns out that Mom's wet earwax allele is dominant, which is why she gets a BIG W,

and Dad's dry earwax allele is recessive, which is why he has to be a little w.

JOHN: Go, Mom!

HANK: Oh, you're back!

JOHN: Yeah! You sound surprised.

HANK: Anyway, Mom's allele is dominant, and that settles it, right--

we're gonna have wet earwax?

JOHN: Uh, something about the way that you said that tells

me it's not that easy.

HANK: Aw, you are so much smarter than you look. It is indeed not that easy.

So, just because an allele is recessive doesn't mean it's

less common in all your genetic material than the dominant allele.

Which leads us to the assumption, the CORRECT assumption, that there's something else going on here.

JOHN: I'm definitely getting that vibe from you.

HANK: So, it has to do with Mom and Dad's parents. Because

everybody inherits two alleles from their parents. Mom got one from Nanny and one from

Paw Paw. And let's just say Mom got a little w from Nanny and a big W allele from Paw Paw.

That means Mom's genotype, or genetic makeup when it comes to that single trait, is heterozygous,

which means she inherited two different versions of the same gene from each of her parents.

Dad, on the other hand is a homozygote.

JOHN: Let me guess, that means that he had two of the

same allele, either a little w or a Big W allele inherited from both Grandma and Grandpa.

HANK: Right! And in order for this to all work out the way that I want it to, let's

just say that both Grandma and Grandpa would have passed little w's down to Dad, making

his genotype homozygous recessive for this gene.

JOHN: Okay, so I'm keeping score in my head right now. And

according to my brain, Mom is a Big W, little w and Dad is a little w, little w.

HANK: And now we're going to figure out what our earwax phenotype is. And phenotype

is what's expressed physically, or in this case, what

you'd see if you looked into our ears.

JOHN: Alright, so are we gonna do a Punnett Square or

anything? This is why I do history, if we're going to do Punnett Squares, I'm leaving!

HANK: But I was just going to start to talk about people again. So Reginald C. Punnett,

who was a total Gregor Mendel fanboy, invented the Punnett Square as a way

to diagram the outcome of a particular cross breeding experiment.

A really simple one looks like this:

So, let's put Mom on the side here and give her a Big W and a

little w. And let's put Dad on the top, and he gets two little w's.

So if you fill this in, it looks like there's a 50/50 chance that any child of this mating

will be homozygous or heterozygous.

And as for our phenotype, it shakes out the same way: John and I both have a 50% chance

of having wet ear wax and a 50% chance of having dry ear wax.

So I just had to go and call John, because now he's not participating

because he doesn't like Punnett Sauares, and it turns out, that he has wet ear wax. I also

have wet ear wax. Which, you know, is not that unlikely, considering that our parents

were homozygous and heterozygous.

This may explain the odor of our bathroom growing up because it turns out there's a

correlation between wet ear wax and body odor, because ear wax and armpit sweat are produced

by the same type of gland.

Because this one gene has an effect on multiple traits or phenotypes, it's an example of a

pleiotropic gene, because the gene affects how wet your ear wax is, and how much you stink.

One more thing you might find interesting: sex-linked inheritance.

So we've got 23 chromosomes: 22 pairs are

autosomes, or non-sex chromosomes, and 1 pair the 23rd pair,

to be exact--is a sex chromosome. At that 23rd pair, women have two full length chromosomes,

or "XX," and men have one X chromosome (that they inherited from their Mom) and this

one little, short, puny, shriveled chromosome that we call "Y," which is why men are "XY."

So, certain genetic traits are linked to a person's sex and are

passed on through the sex chromosomes. Since dudes don't have

two full chromosomes on pair 23, there may be recessive alleles

on the X that they inherited from their mom that will get expressed,

since there's not any information on the Y chromosome to provide

the possibility for a dominant allele counteracting that specific trait.

Take, for instance, balding. Women rarely go bald in their youth

like some men do because it is caused by a recessive allele

located in a gene on the X chromosome. So it's rare that women

get 2 recessive alleles. But men need just one recessive allele

and, Doh! Baldy bald!

And that allele is on their X chromosome, which they got from

Mom. But was Mom bald? Probably not. And where did Mom get

that allele on her X chromosome? Either from her Dad or her Mom.

So if you're bald, you can go ahead and blame it on your

maternal grandmother, or your maternal-maternal great-grandfather

or your maternal-maternal-maternal great-great grandfather

who probably went bald before he was 30.

So, Genetics, you guys. Resistance is futile.

Thanks to my brother John for sharing his personal genetic

information with us, and also his face and voice and all that stuff. That was very nice.

Think of us next time you swab out your ears! Actually they say that you really shouldn't

do that because we have earwax for a reason, and you might poke your brain or something.

Okay, that's the last time I'm mentioning earwax.

Review! Click on any of these things to go back to that section of the video. If you

have any questions, please ask them in the comments.