Mendelian inheritance and Punnett squares | High school biology | Khan Academy

Summary

Gregor Mendel, the father of genetics, revolutionized our understanding of inheritance through his meticulous pea plant experiments conducted between 1856 and 1863. By breeding over 28,000 plants, Mendel observed the dominant and recessive traits, notably the height, and debunked the blending inheritance theory. His discovery of the 3:1 ratio in the second generation led to the formulation of the law of segregation, introducing the concept of inheritable factors, now known as genes and alleles. Mendel's insights laid the groundwork for modern genetics, despite being achieved without knowledge of chromosomes and meiosis.

Takeaways

• 👨‍🔬 Gregor Mendel is recognized as the father of genetics and conducted pivotal experiments that offered insights into how traits are inherited.
• 🏰 Mendel was an Abbot in a monastery in Moravia, which is now part of the Czech Republic.
• 🌱 His groundbreaking work involved breeding approximately 28,000 pea plants between 1856 and 1863 to study the inheritance of traits.
• 📊 Mendel observed that the offspring of tall and short pea plants were all tall initially, contradicting the then mainstream blending theory.
• 🌿 When self-fertilizing the first generation, he discovered a 3:1 ratio of tall to short plants in the second generation, indicating the reappearance of the recessive trait.
• 🧬 Mendel hypothesized the existence of inheritable factors, now known as genes, which determine specific traits and can come in different versions, or alleles.
• 🌾 Mendel's Law of Segregation states that organisms contribute one of their two allele versions to their offspring during gamete formation.
• 📚 The Punnett square, although not invented by Mendel, was conceptually aligned with his thinking and is used to predict genetic outcomes in offspring.
• 🔄 Mendel's experiments showed that dominant traits mask the presence of recessive ones, with the phenotype reflecting the dominant allele.
• 👶 The first generation of offspring from a cross is known as the F1 generation, and subsequent generations are denoted as F2, F3, etc.
• 🧬 Modern genetics confirms Mendel's findings, explaining inheritance patterns through the understanding of chromosomes and meiosis.

Q & A

Who is Gregor Mendel and why is he significant in the field of genetics?

Gregor Mendel is often known as the father of genetics. He was an Abbot of a monastery in Moravia, which is in modern-day Czech Republic. His significance lies in his groundbreaking work with pea plants, where he conducted experiments that provided insights into how traits are inherited, laying the foundation for modern genetics.

What was the time period during which Mendel conducted his pea plant experiments?

Mendel conducted his pea plant experiments from 1856 to 1863.

How many pea plants did Mendel breed during his experiments?

Mendel bred roughly 28,000 pea plants during his experiments.

What traits did Mendel study in his pea plant experiments?

Mendel studied traits such as properties of the seeds, properties of the pea pods, and the height of the plant.

What was the mainstream theory before Mendel's work on inheritance?

Before Mendel's work, the mainstream theory was that if you bred a tall parent with a short parent, you would get a medium offspring.

What did Mendel observe when he bred tall pea plants with short pea plants?

When Mendel bred tall pea plants with short pea plants, all of the offspring were tall initially. However, when he self-fertilized those plants, he observed a ratio of approximately three to one for tall to short plants in the next generation.

What hypothesis did Mendel develop to explain his experimental results?

Mendel hypothesized that there are inheritable factors, which he later called 'genes', that are inherited from an organism's parents and are related to specific traits. He also hypothesized that these factors could come in different versions, which we now call 'alleles'.

What is Mendel's law of segregation?

Mendel's law of segregation states that organisms will generally contribute one of their two versions of a gene to their offspring when they produce their gametes (sperm for males and eggs for females).

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

A Punnett square is a diagram used to depict the probabilities of various combinations of alleles based on what each parent could contribute. It was invented by Reginald Punnett in 1905 and is useful for predicting the outcomes of genetic crosses.

What is the significance of the dominant and recessive alleles in Mendel's experiments?

In Mendel's experiments, the dominant allele (e.g., capital T for tall) would mask the presence of the recessive allele (e.g., lowercase t for short). This means that even if an organism has one of each, it will exhibit the dominant trait, which is the phenotype.

How did Mendel's findings in the F2 generation support his hypothesis?

In the F2 generation, Mendel observed that there was a one in four chance of getting plants with two recessive alleles (tt), which would be short. This supported his hypothesis that the short trait was recessive and would only be expressed when both alleles were recessive.

Outlines

00:00

🌱 Gregor Mendel's Pea Plant Experiments

Gregor Mendel, often referred to as the father of genetics, conducted groundbreaking experiments with pea plants from 1856 to 1863. As an Abbot in a monastery in Moravia, now part of the Czech Republic, Mendel aimed to understand how traits are inherited. By breeding approximately 28,000 pea plants, he studied various traits such as seed properties, pea pod characteristics, and plant height. Contrary to the prevailing belief that offspring would be a blend of parental traits (e.g., a medium height from tall and short parents), Mendel observed that the first generation of offspring (F1) all exhibited the dominant trait, in this case, tallness. When these F1 plants were self-fertilized, the second generation (F2) showed a roughly three-to-one ratio of tall to short plants. Mendel hypothesized that inheritable factors, now known as genes, determine specific traits and can come in different versions, or alleles. He introduced the concept of dominant (e.g., tall) and recessive (e.g., short) traits and explained these findings using the law of segregation, which states that each parent contributes one allele to their offspring. Mendel's work laid the foundation for modern genetics, even though he did not have the knowledge of chromosomes and meiosis that we have today.

05:02

🧬 Mendel's Law of Segregation and Its Implications

Building on Mendel's foundational work, this paragraph delves deeper into the implications of his law of segregation. Mendel's hypothesis that traits are determined by inheritable factors, which he later called genes, and that these factors can be dominant or recessive, was revolutionary. He demonstrated that organisms typically have two versions of each gene (alleles), one from each parent, and that these alleles segregate during gamete formation. This segregation can result in offspring with different combinations of alleles, leading to the expression of dominant or recessive traits. The Punnett square, a tool invented by Reginald Punnett in 1905, was used to illustrate the possible genetic combinations in offspring, showing that a dominant allele (e.g., T for tall) would mask the presence of a recessive allele (e.g., t for short). Mendel's observations in the F2 generation, where he saw a 3:1 ratio of tall to short plants, supported his hypothesis. His work, though conducted without knowledge of chromosomes, foreshadowed the modern understanding of genetics, including the role of chromosomes in inheritance and the process of meiosis.

Keywords

💡Gregor Mendel

Gregor Mendel is known as the father of genetics, a title he earned through his groundbreaking work with pea plants that laid the foundation for the science of inheritance. In the script, Mendel's experiments are highlighted as pivotal in understanding how traits are passed from one generation to the next, setting the stage for modern genetics.

💡Pea Plant Experiments

Mendel's pea plant experiments, conducted from 1856 to 1863, were a series of breeding studies that allowed him to observe and quantify the inheritance of various traits. The script mentions that he bred approximately 28,000 pea plants to study traits like seed properties and plant height, which is central to the video's theme of genetic inheritance.

💡Traits

Traits refer to the characteristics or qualities of an organism that can be inherited. In the context of the video, traits are the physical and genetic properties of pea plants, such as seed shape or plant height, which Mendel studied to understand their patterns of inheritance.

💡Inheritable Factors

Inheritable factors, as hypothesized by Mendel, are elements passed down from an organism's parents that determine specific traits. The script explains that Mendel believed these factors, which we now call genes, come in different versions and are responsible for the expression of traits in offspring.

💡Alleles

Alleles are different versions of a gene that determine a particular trait. The script uses the terms 'capital T' for tall and 'lowercase t' for short to represent different alleles of the height gene in pea plants. Alleles are a key concept in understanding the genetic basis of traits.

💡Mendel's Law of Segregation

Mendel's law of segregation states that an organism's sex cells (gametes) receive only one of the two alleles for each trait. The script illustrates this law by explaining that when pea plants produce gametes, they contribute one allele for each trait, leading to the segregation of alleles in the offspring.

💡Punnett Square

A Punnett square is a diagram used to predict the genotypes of offspring based on the genotypes of the parents. Although not invented by Mendel, as the script clarifies, it is named after Reginald Punnett and is used to depict the probabilities of various genetic combinations, as shown in the script with the example of tall and short pea plants.

💡Dominant and Recessive

In genetics, dominant refers to an allele that masks the effect of another allele when present. Recessive alleles only express their trait when no dominant allele is present. The script uses the terms 'capital T' for the dominant tall allele and 'lowercase t' for the recessive short allele, illustrating how these concepts explain the observed ratios in Mendel's experiments.

💡Phenotype

Phenotype is the observable expression of an organism's traits, as determined by its genotype and environmental factors. The script explains that even when an organism has one dominant and one recessive allele for height, the phenotype will be tall due to the dominance of the tall allele.

💡Meiosis

Meiosis is the process of cell division that results in four sex cells, each with half the number of chromosomes of the parent cell. The script mentions meiosis in the context of gamete formation, where the segregation of alleles occurs, ensuring that each gamete receives one allele for each trait.

💡F1 and F2 Generations

The F1 generation refers to the first generation of offspring, while the F2 generation refers to the offspring of the F1 generation. The script uses these terms to describe the progression of Mendel's experiments, where the F1 generation exhibited a uniform trait (all tall plants), and the F2 generation showed a 3:1 ratio of tall to short plants.

Highlights

Gregor Mendel is known as the father of genetics.

Mendel was an Abbot of a monastery in Moravia, modern day Czech Republic.

Mendel conducted pea plant experiments from 1856 to 1863.

He bred approximately 28,000 pea plants to study trait inheritance.

Mendel studied seed properties, pea pod properties, and plant height.

Contrary to the mainstream theory, Mendel found that offspring from tall and short parents were all tall.

Mendel observed a three to one ratio of tall to short plants in the second generation.

Mendel hypothesized inheritable factors related to specific traits.

These inheritable factors are now known as genes.

Mendel proposed that these factors could have different versions, now called alleles.

Organisms typically have two versions of their genes, one from each parent.

Mendel's law of segregation describes the contribution of one allele to the offspring.

Punnett square was invented by Reginald Punnett in 1905 to depict genetic probabilities.

The tall trait is dominant, and the short trait is recessive.

Mendel's hypothesis explains the phenotypic expression in the F1 and F2 generations.

Mendel's findings were groundbreaking despite lacking knowledge of chromosomes.

Today, we understand gene inheritance through the segregation of chromosomes during meiosis.

Transcripts

- [Narrator] This is a photo of Gregor Mendel,

who is often known as the father of genetics.

And we'll see in a few seconds why,

and he was an Abbot of a monastery in Moravia,

which is in modern day Czech Republic.

And many people had bred plants for agricultural purposes

for hundreds, if not thousands of years before Mendel,

but he really gave us a glimpse, gave us insights

in how traits are really passed.

And he did this through his pea plant experiments

that he conducted from 1856 to 1863.

Over that time period, he bred roughly 28,000 pea plants

in order to get a better understanding

of how they passed down different traits.

And he studied things like properties of the seeds,

properties of the pea prods,

and things like the height of the plant.

And in his time, the mainstream theory

was that if you bred a tall parent with a short parent,

you would get a medium offspring,

but that's not what Mendel saw.

When he bred tall pea plants with short pea plants,

all of the offspring were tall.

But then when he self fertilized those plants

and plants have the interesting property

that they can fertilize themselves.

So the same plant can contribute both the female gamete

and the male gametes.

In other words the same plant can be both

the female parent and the male parent.

Well, then he saw that roughly there was a ratio

of three to one, tall to short.

Those weren't the exact numbers,

but pretty close to three to one.

And so there's a lot of really interesting things here.

First of all, at least for this trait,

he didn't see any blending occur.

And then the other thing is, this short trait reappeared

in this second generation.

In order to explain these results, he hypothesized

that there are inheritable factors

that are inherited from an organism's parents

and they're related to a specific trait.

So in this case it would be around height.

Now we know what he called these inheritable factors.

We now call genes, although he did not use the term,

and he also hypothesized that these factors

could come in different versions.

Now, today we know that the different versions of a gene,

we call alleles, although he did not use that term,

but in this case, the versions that we have at our disposal,

you could have a tall height,

which we can shorthand say, capital T, capital T for tall,

or you could have a short height,

which I will use lowercase t for, for a short height.

Now generally speaking, organisms will have two versions

of their genes like this.

For example, an organism could have two tall alleles,

or two short, or one of each,

but when it produces its gametes, the sex cells,

so the sperm for a male and the egg for a female,

it will generally contribute one of its two versions

to its offspring.

And this contribution of one allele or the other

is known as Mendel's law of segregation.

And we can draw, what's known as a Punnett square

to depict this.

So let me draw a little bit of a grid here.

And Mendel actually did not invent the Punnett square,

although he was thinking in these terms.

It was actually invented by Reginald Punnett in 1905.

And this is useful to think about the probabilities

of various combinations based on what

each parent could contribute.

So let's say we're talking about the tall plant,

and let's say it has two tall versions.

So it can contribute a capital T or a capital T.

And let's say that this short plant over here

has two of the short versions for now.

So it could contribute either a lowercase t,

or a lowercase t.

And so what are all the possible combinations

for its offspring?

Well, in one scenario, you could get this capital T

from the male parent, and this lowercase t,

from the female parent.

In another scenario, you could get this capital T

from the male parent, and a lowercase t

from the female parent.

In this scenario, and I know these look very similar,

a capital T from the male parent and this lowercase t

from the female parent, and then last but not least,

I know this looks repetitive, you could get this capital T

from the male parent and this lowercase t

from the female parent.

And the reason why in all of these cases,

you see a tall plant, is because the tall version,

and he coined this term, is dominant.

And once again, this was all his hypothesis

to explain his results.

So this is dominant and the short is recessive.

So even if you have one of each,

you're actually going to show the dominant trait.

We now call that your phenotype,

what you show is going to be tall.

Now, what's interesting about this hypothesis

is it seems to explain what happens in the next generation.

Just so a little bit of notation.

The first generation is usually called

the P generation for parental,

and then the first generation of offspring

is known as the F1, F for filial.

And that comes filials, which means sun in Greek,

and then the generation after that would be F2,

that's just a little bit of notation there,

but let's think about what would happen

at the F2 generation, if you self fertilized,

some of these characters right over here.

Well, in that situation, let me draw another Punnett square,

on the male parents side, you could contribute

either your capital T version,

which we now call your dominant allele,

or you could contribute the lowercase t version,

and on the female parents side and once again for a plant,

you can have the same plant that has both the male

and the female parent.

You could contribute the dominant version, the capital T,

or the recessive version, the lowercase t.

Now we see something interesting happen in the offspring.

There is a one in four chance you get both capital Ts.

So capital T, capital T.

There's also a one in four chance that you get a lowercase t

from the male parent up here, and then you get a capital T

from the female parent.

There is another one in four chance you get a capital T

from the male parent, and a lowercase t

from the female parent.

And then there's a one in four chance

that you get two lowercase ts,

one each from the male and female parent.

Now, if we accept the dominant and recessive hypothesis,

we would expect that plants that got both capital Ts

would be tall, but we would also expect that these over here

would be tall as well, because the capital T is dominant.

They would exhibit the tall phenotype.

And then you would expect probabilistically

that one fourth of your plants over time, would be short

because they only have the recessive alleles,

the recessive traits in this situation.

And that's actually what Mendel saw.

Now what's amazing is that Mendel was able

to figure this out without knowing about chromosomes,

without knowing all that we know today.

Today we know this works because we have

23 pair of chromosomes, and each of those pairs have copies,

have different versions of usually the same gene,

and that when meiosis occurs and you have gamete formation,

one member of each pair will segregate randomly

into the newly formed sex cell, into the sperm or the egg.

That's why this occur and we go into some detail on that

in other videos, but it's pretty cool,

that Mendel was able to figure this out in the 19th century.