Thomas Hunt Morgan and fruit flies

Summary

In this educational video, the focus is on Thomas Hunt Morgan's groundbreaking work with fruit flies that began in 1908. Morgan's research led to the discovery of a mutant white-eyed male fly, which he used to demonstrate a clear genetic pattern of inheritance. His observations of the 3:1 ratio of red to white eyes in the offspring, and the sex-linked nature of the white-eye trait, provided the first direct evidence that chromosomes carry inheritable factors, as proposed by Mendelian genetics. Morgan's findings, published around 1910-1911, were pivotal in establishing the chromosome theory and earned him a Nobel Prize.

Takeaways

• 🧬 The script discusses the rediscovery of Mendelian genetics and the chromosome theory of inheritance, which proposed that chromosomes are the carriers of inheritable factors.
• 🔬 Thomas Hunt Morgan's work with fruit flies in 1908 provided empirical evidence for the chromosome theory, showing a direct link between chromosomes and inheritable traits.
• 🍎 Fruit flies were chosen as a model organism for genetic studies due to their small size, low cost, short life cycle, and high reproductive rate.
• 👁️ Morgan discovered a mutant white-eyed male fruit fly, which was an unusual trait compared to the common red-eyed wild type.
• 🌟 The F1 generation of Morgan's crosses showed all red-eyed fruit flies, indicating that the white-eye trait was recessive.
• 🔍 In the F2 generation, a 3:1 ratio of red-eyed to white-eyed flies was observed, with the white-eyed trait appearing only in males, suggesting sex-linkage.
• 🧬 The white-eye trait was hypothesized to be carried on the X chromosome, with males being hemizygous for the trait, having only one X chromosome.
• 👩‍🔬 The genotypes of red-eyed females and white-eyed males were described, with females being XX and males being XY in their sex chromosome composition.
• 📊 The inheritance pattern observed by Morgan was consistent with Mendelian genetics, specifically the expected ratios for a recessive sex-linked trait.
• 🏆 Thomas Hunt Morgan's work led to a deeper understanding of genetics and earned him a Nobel Prize for his contributions to the field.
• 🔗 Morgan's findings substantiated the connection between chromosomes and inheritable factors, specifically showing that the X chromosome carries the gene for eye color in fruit flies.

Q & A

What was the significance of Mendelian genetics in the early 20th century?

Mendelian genetics, rediscovered at the turn of the century, laid the foundation for understanding inheritance patterns. It was instrumental in the development of the chromosome theory, which proposed that chromosomes were the carriers of inheritable factors as described by Mendel.

What was the chromosome theory proposed by Boveri and Sutton?

The chromosome theory, independently proposed by Boveri and Sutton, suggested that chromosomes were the location where inheritable factors, as first discussed by Mendel, were located. This was a significant step towards understanding the genetic basis of inheritance.

Why did Thomas Hunt Morgan choose fruit flies for his genetic studies?

Morgan chose fruit flies because they were small, cheap, and had short life cycles with high reproductive rates. These characteristics allowed him to quickly produce many generations and study the inheritance of traits.

What was the 'wild type' in the context of Morgan's fruit fly experiments?

The 'wild type' refers to the common or typical form of a trait in a species. In Morgan's experiments, the wild type was the red-eyed fruit flies, as opposed to the mutant white-eyed trait that he discovered.

What was the significance of the white-eyed male fruit fly discovered by Morgan?

The discovery of the white-eyed male fruit fly was significant because it represented a mutant trait. This allowed Morgan to study the inheritance pattern of this trait, leading to insights about the role of chromosomes in inheritance.

What was the inheritance pattern observed by Morgan in the F1 generation of fruit flies?

In the F1 generation, all females and males were red-eyed. This initial observation did not immediately reveal any unusual inheritance patterns, but it set the stage for further investigation in subsequent generations.

What was the ratio of red eyes to white eyes observed by Morgan in the F2 generation?

In the F2 generation, Morgan observed a three to one ratio of red eyes to white eyes. This ratio was consistent with Mendelian genetics and indicated a clear genetic inheritance pattern.

Why was it significant that white eyes were only observed in males in the F2 generation?

The observation that white eyes were only seen in males in the F2 generation was significant because it suggested a link between the eye color trait and the sex chromosomes. This was a key finding in establishing the chromosome theory.

What is the term 'hemizygous' and why is it relevant to Morgan's white-eyed male fruit flies?

The term 'hemizygous' refers to having only one allele of a gene, typically on the sex chromosome. In the case of Morgan's white-eyed male fruit flies, they were hemizygous for the white eye allele on their X chromosome, with no corresponding allele on the Y chromosome.

What did Morgan's findings contribute to the understanding of genetics?

Morgan's findings provided direct evidence linking sex chromosomes to heritable traits, specifically showing that the X chromosome carried the gene for eye color in fruit flies. This was a significant step in validating the chromosome theory and understanding the genetic basis of inheritance.

Why did Morgan and his students receive the Nobel Prize for their work on fruit flies?

Morgan and his students were awarded the Nobel Prize for their work on fruit flies because they demonstrated a direct linkage between chromosomes and heritable traits, specifically the gene for eye color. This was a groundbreaking discovery in genetics.

Outlines

00:00

🔬 Rediscovery of Mendelian Genetics and Chromosome Theory

This paragraph discusses the rediscovery of Mendelian genetics around 1902-1903 and the subsequent proposal by Boveri and Sutton of the chromosome theory, which suggests that chromosomes are the carriers of inheritable factors as described by Mendel. However, the theory lacked direct cellular evidence. The narrative then shifts to Thomas Hunt Morgan's work with fruit flies starting in 1908, highlighting the practical reasons for choosing fruit flies as a model organism due to their small size, low cost, short life cycle, and high reproductive rate. Morgan's discovery of a white-eyed male fruit fly after two years of breeding marked a significant finding, as it presented a mutant trait that could be studied for inheritance patterns.

05:00

🧬 Thomas Hunt Morgan's Fruit Fly Experiments and Sex-Linked Inheritance

The second paragraph delves into Morgan's experiments with the white-eyed male fruit fly, detailing the process of crossbreeding and the observation of inheritance patterns. It explains the concept of wild type and mutant traits, and how Morgan's observations led to the discovery of a three to one ratio of red-eyed to white-eyed offspring in the F2 generation. This ratio is significant as it aligns with Mendelian genetics. The paragraph further explores the sex-linked nature of the white-eye trait, noting its exclusive appearance in male offspring and suggesting a connection with the sex chromosomes. Morgan hypothesized that the white-eye allele is carried on the X chromosome, leading to the concept of hemizygosity in males, and the use of specific genetic notation to describe the genotypes of red-eyed and white-eyed fruit flies.

10:01

🏅 Thomas Hunt Morgan's Nobel Prize-winning Work on Sex Chromosomes

The final paragraph summarizes Morgan's continued work with fruit flies and his students, which ultimately led to the Nobel Prize. It emphasizes the groundbreaking nature of his research, which established a direct link between sex chromosomes and heritable factors, specifically showing that the X chromosome carries the gene for eye color in fruit flies. Morgan's work substantiated the chromosome theory and provided concrete evidence for the connection between chromosomes and Mendelian inheritable factors, marking a significant advancement in the field of genetics.

Keywords

💡Mendelian genetics

Mendelian genetics refers to the principles of inheritance proposed by Gregor Mendel, which describe how traits are passed from parents to offspring. In the video, Mendelian genetics is mentioned as having been rediscovered at the turn of the century and forms the foundation for understanding how inheritable factors are located on chromosomes. The script discusses how Mendel's work laid the groundwork for the chromosome theory.

💡Chromosome theory

The chromosome theory is the idea that chromosomes are the carriers of inheritable traits, as proposed by Boveri and Sutton independently. The script explains that this theory was initially based on observations of meiosis and the behavior of chromosomes, suggesting a connection between chromosomes and Mendel's inheritable factors, although it lacked direct cellular proof.

💡Thomas Hunt Morgan

Thomas Hunt Morgan is a key figure in genetics whose work is central to the video. He is known for his experiments with fruit flies, which provided evidence for the chromosome theory. Morgan's work is highlighted in the script as he discovered a mutant trait in fruit flies, leading to significant insights into how traits are inherited.

💡Fruit flies

Fruit flies, or Drosophila melanogaster, are a model organism used extensively in genetic research. The script mentions Morgan's choice to study fruit flies due to their small size, low cost, short life cycle, and high reproductive rate, which allowed him to quickly observe many generations and traits.

💡Wild type

The term 'wild type' in genetics refers to the most common form of a species or trait, as opposed to a mutant form. In the script, the wild type is described as the typical trait seen in a species, while the mutant trait is unusual and of interest in genetic studies, such as the white-eyed male fruit fly discovered by Morgan.

💡Mutant trait

A mutant trait is a variation in an organism that is different from the typical or wild type. The script describes Morgan's discovery of a white-eyed male fruit fly as a mutant trait, which was unusual and led to significant genetic studies on inheritance patterns.

💡F1 generation

The F1 generation refers to the first generation of offspring resulting from a cross between two organisms. In the script, Morgan's initial crossing of the white-eyed male with red-eyed females resulted in an F1 generation where all offspring had red eyes, indicating that the white-eyed trait was recessive.

💡Three to one ratio

The three to one ratio mentioned in the script refers to the inheritance pattern observed by Morgan in the F2 generation of fruit flies. For every four fruit flies, three had red eyes and one had white eyes, suggesting a clear genetic pattern consistent with Mendelian inheritance.

💡Sex chromosomes

Sex chromosomes are the chromosomes that determine the sex of an organism. In the script, Morgan hypothesized that the white-eyed trait was linked to the X chromosome, which is one of the sex chromosomes. His observations supported the idea that the gene for eye color in fruit flies is carried on the X chromosome.

💡Hemizygous

Hemizygous refers to a condition in which an individual has only one copy of a particular gene, typically on the sex chromosome. In the script, the white-eyed male fruit fly is described as hemizygous for the white eye allele, as it is present on the X chromosome but not on the Y chromosome.

💡Heterozygous

Heterozygous is a term used in genetics to describe an individual that has two different alleles of a gene, one from each parent. In the script, the red-eyed females in Morgan's experiments are described as heterozygous for the eye color gene, carrying both the red and white eye alleles.

Highlights

The rediscovery of Mendelian genetics at the turn of the century and the chromosome theory proposed by Boveri and Sutton.

The lack of cellular proof that chromosomes were the carriers of inheritable factors until Thomas Hunt Morgan's work.

Morgan's choice of fruit flies for genetic studies due to their practical advantages such as small size, low cost, short life cycle, and high reproduction rate.

The discovery of a mutant white-eyed male fruit fly by Morgan after two years of breeding.

The observation of a three to one ratio of red eyes to white eyes in the F2 generation, indicative of Mendelian inheritance patterns.

The exclusive appearance of the white-eyed trait in male fruit flies, suggesting a link to the sex chromosomes.

Morgan's hypothesis that the mutant eye color gene is carried on the X chromosome, leading to the concept of sex-linked traits.

The use of genotype notation to describe the genetic makeup of red-eyed and white-eyed fruit flies.

The explanation of hemizygous, homozygous, and heterozygous in the context of the white-eyed male fruit flies.

The genetic crossbreeding process that resulted in the reappearance of the white-eyed trait, demonstrating genetic transmission.

Morgan's publication of his findings in Nature in 1910 and 1911, providing empirical evidence for the chromosome theory.

The significance of Morgan's work in establishing a direct link between sex chromosomes and inheritable factors.

The long-term impact of Morgan's research, leading to further studies by him and his students, and ultimately earning a Nobel Prize.

The practical implications of Morgan's findings for understanding the role of chromosomes in heredity and the genetic basis of traits.

The historical context of Morgan's work as a bridge between Mendel's genetic theories and modern genetics.

The educational value of Morgan's fruit fly experiments in illustrating fundamental principles of genetics to students and researchers.

Transcripts

- [Voiceover] Where we left off in the last video

we were in 1902, 1903, and Mendelian genetics

had been rediscovered at the turn of the century

and Boveri and Sutton, independently, had proposed

the chromosome theory, that the chromosomes

were the location for where these inheritable factors

that Mendel first talked about, where they actually

were located.

But we talked about in that video

that that was just a theory.

This was based on some observations of meiosis

and seeing how chromosomes behaved,

and they seemed to behave in analogous ways

to some of these inheritable factors

but they really didn't have good cellular proof

that chromosomes indeed were the location

for these inheritable factors.

And we don't really start to get that

until we start looking at the work of Thomas Hunt Morgan.

Now 1908, he decides to study fruit flies.

So why does he wanna study fruit flies?

If you've ever seen a fruit fly,

they're very very very small.

So you could actually put a ton of fruit flies in one jar.

So, that's convenient.

You oftentimes don't think about the practical logistics

of science, but you could put a lot in one jar.

They were actually cheap, and that's another practical

concern of science, is you don't always have a lot

of resources to do your work.

And they had short lives, and they reproduced a lot

so you could very quickly get many many offspring

and many many generations if you wanted to study

how the different traits were passed on

or not passed on.

And so he spent some time, he started this in 1908

working with the fruit flies.

And he kept breeding them, in search for some type

of a mutant trait.

In general when you look at traits in a species,

the wild type, let me write this down,

the wild type is the one that's typically seen,

while the mutant trait is something that seems unusual.

And after two years he finally discovers a mutant trait

in his fruit flies.

He finds a white-eyed male.

So this is the white-eyed male right over here.

He says oh, okay, now this is interesting.

Let me take this white-eyed male and begin to cross it

with other, well, with the females.

And you say well how does this actually occur?

Well, what you do is you take a jar full of females

and you put the white-eyed male in there

and then the crossing happens.

And what was interesting was the inheritance pattern

that he saw for this white-eyed trait.

Because you have the parent generation here,

but then in the F1 generation

all of the females were red-eyed

and all of the males were red-eyed.

And so just off of that first generation it wasn't clear

that anything interesting was going on.

But then, when he crossed these to each other,

and I know what some of you all are thinking,

wait, aren't they all brothers and sisters being crossed

to each other?

Ah, well, yeah, they were probably half brothers and sisters

if they came from different mothers,

but some of them could have been brothers and sisters,

but yes, that's what people are talking about

when they're crossing the F1 generation.

But when they crossed these with each other,

he saw a pretty interesting pattern.

He saw a three to one ratio of red eyes to white eyes,

so for every four fruit flies he would see,

let me underline these, he would see three red-eyed

and he would see one white-eyed.

So the white-eyed, the white-eyed trait

makes a reappearance, which in and of itself is interesting.

It shows that this can be passed on genetically.

And that's interesting because this was a mutant

that just showed up after he did many many many many many

generations of observations.

But what was even more interesting about this

three to one ratio, and that three to one was something

that popped up a lot in Mendelian genetics,

but what was even more interesting was that

he only observed, he only observed the white eyes

in the males in this F2 generation,

in this second generation of the crosses right over there.

And so you're thinking, well, why is that a big deal?

Well, he was a pretty astute guy,

and he says well look, if I'm only seeing it in the males,

and it's not like he only got four offspring here

in the ratio, he may have had hundreds of them,

but it was in the ratio of two red-eyed females

for every one red-eyed male, for every one white-eyed male.

And so across these hundreds of, in this generation,

he only observed the white eyes on the males.

And he said, hm, maybe this is in some way related

to the chromosome that determines sex.

And so what he was able to do is say,

well, let's just assume that it is.

Let's assume that that trait, that mutant allele,

that mutant variation of the gene for eye color,

let's assume it's carried on the x chromosome.

And so the genotype for that first mutant fly,

that white-eyed male that he found,

we could call it, and this is the notation

that people typically use, because this is a gene

that we're assuming sits on a, it's sex-linked,

it sits on a sex chromosome, in this case the x chromosome,

the way that you would specify the genotype

of that white-eyed male is, well on his x chromosome,

he had the white variation, he had the white allele,

the white variation of that gene,

and then on his y chromosome he had no variation

for that gene.

So we assume that it's only contained on the,

only on the x chromosome.

You've probably heard of heterozygous or homozygous,

well this is a case where you're hemizygous,

you only have a version of the allele

on one of your two chromosomes, one of the two

that you've gotten from each of your parents.

So this would be the genotype right here

of the white-eyed male.

The genotype for the red-eyed female

is specified by, so it's on the x chromosome,

and the females have two x chromosomes,

just like in the situation for humans,

so on each of the x chromosomes,

we assume that the females start off with the red allele,

and the red allele, the notation is the w +, w +.

And you might say, well why don't we just use the letter r?

Well, we could have, but the general convention in genetics

is to use to letter of the first mutant type discovered

for that gene, and then to use this little plus type

for the wild type.

So the wild type is the red eyes, and then w,

which is the mutant discovered for this gene,

is the first mutant allele,

that we do so we name it after that white,

so this is the white, the white allele,

and these right here, these represent the red alleles.

So these are the genotype of the red-eyed female.

And so, when you cross that first generation

well, the white-eyed male, he can either,

he'll either produce sperm that have the x chromosome

in it, which is going to contain the allele,

or sperm which have the y chromosome in it,

which is not going to contain the allele,

and the red-eyed female, well they produce eggs either way,

either which of these x chromosomes they contribute,

they're both going to have the wild type allele.

And we can see how this crosses.

You could get an x from both parents.

If you get an x from both parents

you're going to be female, because you're going to be xx,

and each of these females, since you've got one wild type

and one mutant type, and the wild type turns out

to be dominant, they still show, their phenotype

is still red eyes, they still have red eyes.

But now they are heterozygotes, they are carrying

the white allele.

Now the male offspring right over here,

well, in order to be male, they got the y chromosome

from their dad, so they're not able to get

that white allele, and they get the red, the wild type,

from their mom.

And you could see it here, and this is why all of the males

in that first generation were red.

They only got one copy of the allele

from their wild type mother.

But then what was interesting is the crosses that you see

in that next generation.

If you took these red-eyed females

that we already established, that these are all

going to be heterozygotes, and so you can see

they have the red allele and they have the white allele.

And you cross that with red-eyed males.

You cross it with red-eyed males,

what is going to happen?

Well, the females in this generation,

in order to be female you have to get an x from your mom

and your dad, and so they get an x from their dad

which has the wild type there,

the dominant red allele,

and so regardless of which one they got from their mom,

they're still going to be red-eyed females.

Some of them might be homozygotes,

some of them might be heterozygotes.

But now we see something interesting happening

in the males.

You could have heterozygote male, male flies here,

where they got the x from, where they got the red x

from their mom.

Or you could get the hemizygous white-eyed males,

where they got the white allele, the white x,

from their mom.

And this is the exact observation

that Morgan made.

So it was a very interesting thing that he was able to see.

He started breeding these in 1908, he started breeding

these flies in 1908.

It wasn't until a couple of years that he finally found

that first mutant white-eyed male

and it was in 1910 and 1911 that he publishes

these discoveries in Nature.

And the reason why this is a big deal

is he says, look, this is, my observations

are completely consistent with this eye trait, this gene,

being on the x chromosome.

So he was able to show a direct linkage between,

in this case, sex chromosomes and these heritable factors

that Mendel first talked about.

And he would go on and his students that he worked with

would go on to study this for many many many many years

and he actually ends up getting a nobel prize for this work.

And this is a big deal, because he's finally able to draw

pretty substantive connections between these heritable

factors Mendel, this theory of Boveri and Sutton

that maybe chromosomes have something to do

with these inheritable factors,

and he's showing that this is actually the case,

these sex chromosomes seem to carry the trait,

or in this particular case, the x sex chromosome

seems to carry the gene for eye color

in these fruit flies.