Sources of genetic variation | Inheritance and variation | High school biology | Khan Academy

Summary

This video script delves into the critical role of genetic variation in the process of evolution and natural selection. It explains that while organisms with diverse genotypes can exhibit a range of phenotypes, environmental pressures can favor certain traits, leading to their propagation over generations. The script identifies two primary sources of genetic variation: mutation, where errors in DNA replication can occasionally result in advantageous changes, and sexual reproduction, which introduces variation through the process of meiosis. Meiosis, illustrated through a simplified diagram, involves the independent assortment of homologous chromosomes and the exchange of genetic material through crossover, leading to a vast number of possible genetic combinations. The script concludes by emphasizing the immense genetic diversity that arises from sexual reproduction and the additional variation contributed by crossover and mutation, highlighting the potential for over 70 trillion combinations from just two parents.

Takeaways

• 🌱 **Genetic Variation is Essential**: Genetic variation is crucial for evolution and natural selection to occur.
• 🧬 **Different Phenotypes**: Organisms with different genotypes can express different phenotypes, which are influenced by environmental conditions.
• 🧬 **Natural Selection Process**: Favorable phenotypes for survival and reproduction pass on their genes, leading to changes in the gene pool over generations.
• 🧬 **Origin of Variation**: Variation arises from several sources, including mutation and sexual reproduction.
• 🛠️ **Mutation as a Source**: Errors in DNA copying can lead to mutations, which occasionally produce advantageous phenotypes.
• 🌟 **Sexual Reproduction and Variation**: Sexual reproduction contributes significantly to genetic variation through the formation of gametes.
• 🧬 **Meiosis Process**: Meiosis involves the independent assortment of homologous chromosomes and the potential for crossover, leading to a vast array of genetic combinations.
• 🧬 **Homologous Chromosomes**: Homologous chromosomes carry the same genes but can have different alleles, contributing to genetic diversity.
• 🔄 **Crossover Effect**: Crossover during meiosis mixes DNA between chromosomes, further increasing genetic variation.
• 🧬 **Human Genetic Combinations**: Humans have 23 pairs of chromosomes, leading to over 8 million possible combinations before considering crossover and mutation.
• 🌟 **Combining Gametes**: The combination of gametes from two parents can result in over 70 trillion possible genetic combinations, highlighting the immense genetic diversity sexual reproduction can produce.

Q & A

What is genetic variation and why is it important for evolution and natural selection?

Genetic variation refers to the differences in the genetic makeup (genotypes) among organisms, which can express themselves as different physical traits (phenotypes). It is crucial for evolution and natural selection because it provides the raw material for selection to act upon. When environmental conditions change, some genetic variations may confer a survival and reproductive advantage, allowing those traits to be passed on to future generations.

How do different phenotypes arise from genetic variation?

Different phenotypes arise from genetic variation as organisms with different genotypes express their traits in various forms. For example, in the script, different colored circles are used to represent various phenotypes of a single trait. These phenotypes can be more or less favorable depending on the environment, influencing the survival and reproduction of the organisms.

What is the role of mutation in genetic variation?

Mutation is a key source of genetic variation. Although cells are highly accurate when copying DNA, occasional errors occur. These mutations can introduce new genetic variations. While most mutations may be harmful or neutral, some can result in a new phenotype that may be advantageous in certain environments.

How does sexual reproduction contribute to genetic variation?

Sexual reproduction contributes to genetic variation through the process of meiosis, which forms gametes (sperm and egg cells). During meiosis, independent assortment of homologous chromosomes and crossover (genetic recombination) create new combinations of genetic material, increasing the potential for variation in offspring.

What is meiosis and why is it significant for genetic variation?

Meiosis is a type of cell division that results in four daughter cells, each with half the number of chromosomes of the parent cell. It is significant for genetic variation because it involves the independent assortment of chromosomes and crossover, which mix up the genetic material and produce new combinations of genes in the gametes.

What are homologous chromosomes and how do they relate to genetic variation?

Homologous chromosomes are a pair of chromosomes that have the same genes, but may have different versions (alleles) of those genes. They play a role in genetic variation because during meiosis, they can assort independently, and crossover can occur, exchanging genetic material between them and creating new combinations of alleles.

How does the process of independent assortment during meiosis contribute to genetic diversity?

Independent assortment occurs during meiosis I, where homologous chromosomes line up and separate independently of one another. This means that which chromosome from each pair goes into a gamete is a random event. For humans with 23 pairs of chromosomes, this results in 2^23 (over 8 million) possible combinations of chromosomes in the gametes, contributing significantly to genetic diversity.

What is crossover and how does it increase genetic variation?

Crossover, also known as genetic recombination, is the process where segments of DNA are exchanged between homologous chromosomes during meiosis. This exchange can mix alleles that were originally on different chromosomes, creating new combinations of genetic material and further increasing the genetic variation among offspring.

How many possible combinations of chromosomes are there in human gametes before considering crossover and mutation?

In human gametes, before considering crossover and mutation, there are 2^23 (approximately 8.4 million) possible combinations of chromosomes due to the independent assortment of the 23 pairs of chromosomes.

What is the combined effect of sexual reproduction, crossover, and mutation on the number of genetic combinations in offspring?

Considering sexual reproduction alone, the number of genetic combinations in human offspring is 2^23 times 2^23, which is over 70 trillion. When crossover and mutation are also taken into account, the number of possible genetic combinations increases even further, leading to an immense diversity of genetic material.

How does the process of mutation affect the accuracy of DNA replication?

Although cells are incredibly accurate when copying DNA, mutations can occur, introducing errors in the genetic code. Most of these errors may be neutral or harmful, but occasionally they can result in a beneficial change in the phenotype, which can provide an evolutionary advantage.

What is the significance of the number of base pairs in a gene and how does it relate to genetic variation?

The number of base pairs in a gene can vary widely, with genes averaging about 27,000 base pairs but some being much longer. This length and the specific sequence of base pairs determine the genetic information carried by the gene. Variations in these sequences can lead to different alleles of a gene, contributing to the overall genetic variation within a population.

Outlines

00:00

🧬 Introduction to Genetic Variation

The first paragraph introduces the concept of genetic variation, which is crucial for evolution and natural selection. It explains how organisms with different genotypes can express different phenotypes, represented by colored circles. The environment's selective pressure favors certain phenotypes, allowing their genes to be passed on to future generations, leading to changes in the gene pool over time. The paragraph also raises the question of where genetic variation originates, highlighting mutation as a key source. It notes that while cells are typically accurate in copying DNA, errors can occur, leading to mutations that may sometimes result in advantageous phenotypes. Sexual reproduction is also mentioned as another significant source of genetic variation, with a diagram of meiosis provided to illustrate the process of gamete formation.

05:01

🧵 The Role of Meiosis and Crossover in Genetic Variation

The second paragraph delves into the specifics of sexual reproduction and its role in generating genetic variation. It discusses how the process of meiosis, specifically the independent assortment of homologous chromosomes and the occurrence of crossover events, contributes to the vast diversity of genetic combinations. The paragraph explains that each human has 23 pairs of chromosomes, and the independent assortment during meiosis results in over 8 million possible combinations of these chromosomes, even before considering crossover and mutation. Crossover is described as a process where segments of homologous chromosomes are exchanged, further increasing genetic diversity. The paragraph concludes by emphasizing the immense number of combinations possible from the haploid gametes of two parents, which, when considering crossover and mutation, can exceed 70 trillion different combinations.

Keywords

💡Genetic variation

Genetic variation refers to the differences in the genetic makeup among individuals within a population. It is crucial for evolution and natural selection as it provides the raw material for these processes to act upon. In the video, genetic variation is illustrated through different colored circles representing different phenotypes of a single trait, which can be more or less favorable depending on the environment.

💡Natural selection

Natural selection is the process by which certain heritable traits become more or less common in a population due to differential reproduction of their bearers relative to others. It is a key mechanism of evolution. In the context of the video, natural selection is discussed in relation to how environmental factors can favor certain phenotypes, leading to a change in the gene pool over generations.

💡Phenotype

A phenotype is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment. The video uses the example of different colored circles to represent various phenotypes that can arise from different genotypes, emphasizing how environmental conditions can influence which phenotypes are advantageous for survival and reproduction.

💡Genotype

The genotype is the genetic constitution of an individual organism, which determines its hereditary traits. In the video, it is mentioned in the context of different organisms having different genotypes that can express as various phenotypes. The script explains how genotypes can influence an organism's success in terms of survival and reproduction.

💡Mutation

Mutation is a change in the DNA sequence that makes up a gene. It is one of the primary sources of genetic variation. The video explains that while cells are highly accurate in copying DNA, errors can occur, leading to mutations. These can sometimes result in a new phenotype that may provide a survival advantage, thus contributing to evolution.

💡Sexual reproduction

Sexual reproduction is a biological process that involves the combination of genetic material from two parent organisms to produce offspring with a mix of traits from both. The video highlights sexual reproduction as a major source of genetic variation, particularly through the process of meiosis, which includes independent assortment and crossover.

💡Meiosis

Meiosis is a type of cell division that reduces the chromosome number by half to create gametes, which are necessary for sexual reproduction. The video uses a diagram of meiosis to explain how genetic variation arises from the independent assortment of homologous chromosomes and the process of crossover, which mixes DNA between chromosomes.

💡Homologous chromosomes

Homologous chromosomes are a pair of chromosomes that have the same structure and gene sequence, but may carry different alleles of those genes. In the video, the concept is used to explain how genetic variation can occur during meiosis, as different versions of the same genes are assorted independently into gametes.

💡Independent assortment

Independent assortment is a process in meiosis where each chromosome from a pair aligns independently of any other pair of chromosomes. This leads to numerous possible combinations of maternal and paternal chromosomes in the gametes, contributing to genetic variation. The video illustrates how this process can result in a vast number of genetic combinations in offspring.

💡Crossover

Crossover, also known as genetic recombination, occurs during meiosis when homologous chromosomes exchange segments of DNA. This process further increases genetic variation by mixing alleles from different chromosomes. The video emphasizes the role of crossover in creating new combinations of genetic material, adding to the diversity of potential offspring.

💡Gametes

Gametes are the reproductive cells (sperm and egg) that fuse during fertilization to form a new individual. The video discusses how the genetic makeup of gametes, which are produced through meiosis, contributes to genetic variation. The combination of gametes from two parents can result in a vast array of genetic combinations in the offspring.

Highlights

Sources of genetic variation are crucial for evolution and natural selection.

Different organisms with diverse genetics can express various phenotypes.

Environmental factors can favor certain phenotypes for survival and reproduction.

Genetic variation can arise from mutations, which are errors in DNA copying.

Most mutations are harmful or neutral, but occasionally they can produce advantageous phenotypes.

Sexual reproduction is another major source of genetic variation.

Meiosis is the process that forms gametes, contributing to genetic variation.

Homologous chromosomes have the same genes but can have different alleles.

Human chromosomes can contain up to 100 million base pairs.

Independent assortment of homologous chromosomes during meiosis I creates genetic diversity.

There are over 8 million possible combinations from a single set of homologous chromosomes in humans.

Crossover during meiosis introduces additional genetic variation by mixing DNA between chromosomes.

Crossover is common and significantly increases the potential combinations of genetic material.

The final phase of meiosis produces gametes with a unique combination of genetic material.

Combining gametes from two parents can result in over 70 trillion possible genetic combinations.

Mutation and crossover increase the number of genetic combinations beyond those resulting from sexual reproduction alone.

The process of meiosis ensures that offspring inherit a mix of genetic traits from both parents.

Understanding the sources of genetic variation is key to studying evolution and the diversity of life.

Transcripts

- [Instructor] In this video, we're going to talk about

sources of genetic variation,

which is key for evolution and natural selection to happen.

Just as a little bit of a primer, natural selection,

you can have a bunch of different organisms

with different genetics, different genotypes,

and they can express themselves as different phenotypes.

And I'll just do this as different colored circles

right over here.

So there are all of these different phenotypes,

and I'm just expressing different phenotypes of one trait.

And then depending on what's going on in the environment,

some of these phenotypes might be more favorable

for survival and reproducing

and therefore passing on those genes to the next generation.

And if you do that over many, many, many, many generations,

you can have a change in your gene pool

because the genes that provide the variants of phenotypes

that are more successful will exist more.

But an interesting question is,

where does this variation come from?

And there's several sources of it.

So one of the key and probably the most primitive version

of genetic variation is mutation.

Cells are incredibly accurate when they are copying DNA,

but there are going to be some errors.

Now, most of these errors can oftentimes

break the organism in some way or might not matter at all,

but every now and then, some of these errors,

either as an individual base pair change,

or maybe cumulatively can produce a different phenotype

and potentially a phenotype that has an advantage.

And so this has always been the case.

Now, another major source of genetic variation

is sexual reproduction.

And to remind ourselves of sexual reproduction,

I will show you this diagram of meiosis.

Now sexual reproduction is the process

by which we form gametes.

So for a male organism,

that would be producing the sperm cells,

or for a female organism,

that would be producing the egg cells.

This meiosis diagram is for an organism

that has two pairs of chromosomes,

while we know that human beings actually have 23 pairs.

But if we saw a diagram with 23 pairs,

it would get very complicated, very fast,

so the two pairs help us understand what's going on

and help us understand where some of this genetic variation

is going to come from.

So I've already pre-labeled the homologous chromosomes here.

And just as a reminder,

homologous chromosomes are ones

that have the same genes on them.

Now they could have different versions of the genes on them,

but they're fundamentally coding for the same genes.

You can view chromosomes as really long stretches of DNA

that has all been rolled in and bunched in together,

something like this.

A human chromosome can have on the order of

100 million base pairs in it.

Now, if you were to straighten that string of DNA,

if you were to unwind it,

you would see different section's code for different genes.

So that might be one gene there,

that might be another gene there.

You might have one long gene right over there.

On average, the genes are about 27,000 base pairs in length

but some of them could be millions of base pairs.

So on one of these chromosomes,

you can actually have thousands of genes being coded.

And so if you were to pick this chromosome

and this chromosome right over here,

they would be coding for the same genes

because they're homologous.

But once again, they could have different alleles,

different versions of those genes on them.

And similarly, this chromosome and this chromosome

are also homologous.

They're also coding for the same genes.

Now, as we go into meiosis,

the first step is that the chromosomes

are essentially going to copy themselves

into two sister chromatids.

So, for example, this one right over here

has now copied itself and it has that telltale X shape.

But this side of this now chromosome,

which we would call a chromatid,

and this sister chromatid should be identical.

Now there might be some errors

that got introduced through mutation.

But if we don't assume mutation, they would be identical.

Likewise, this side and this side,

assuming no mutations, they would be identical.

Now what's interesting about this

is what happens in the next phase.

In the next phase, you have the independent assortment

of homologous chromosomes.

So as we said, this and this might be coding

for the same genes, it might just have different versions.

But as we go into this phase,

as we do meiosis I, as it's often known right over here,

this blue chromosome could go here,

while the homologous red chromosome would go there.

The purple chromosome is going here

and the light blue chromosome is going there.

And this is really interesting

because there's a lot of different ways this could happen.

In this situation, you have two pairs.

Each of these intermediary steps in meiosis

could randomly have one from each pair.

So just in this example, you have two to the number

of pairs combinations at this stage right over here.

Now this was only when we have two pairs.

If we're talking about a human being,

we're talking about two to the 23rd different combinations

of which of the two homologous chromosomes you get.

So there's a lot of variation here.

Now on top of that,

some of y'all might have noticed something interesting.

If you just follow the colors here,

it looks like a little chunk of this chromosome got swapped

with a little chunk of this chromosome.

You could see it here.

The red is now on the big blue X

and the blue is now on the big red X.

This is another source of genetic variation

and it is known as crossover.

And what it does is,

it can actually mix DNA between chromosomes.

Once again, these are homologous chromosomes,

they are encoding the same genes,

but now alleles that were sitting on the blue one

could now sit with the rest of the red one

and the alleles that was sitting with the red one

can now sit with the rest of the blue ones.

And crossover is actually reasonably common during meiosis.

So once again, it's mixing things up even more

than this two to the 23rd combinations.

So a lot of variation that you can produce

through sexual reproduction.

And then as we go into this last phase into meiosis II,

and we're actually producing the gametes,

if this meiosis is going on in the gonads of a male,

this would be the chromosomal makeup of the sperm cells.

If this is going on within the female,

then this would be the DNA makeup of the egg cells.

And what you see, and just to make it clear

what's happened here is that your sister chromatids

have now spread apart, although they're no longer identical,

especially if you have the crossover.

So for example, this one went over here

and this one went over here as well.

And then you have another scenario where you have this one

and this one ended up in this gamete,

and we can go on and on.

So actually you can have,

especially if you consider crossover,

more than two to the 23rd possible combinations.

Now two to the 23rd power is approximately

a little bit more than eight million combinations.

And if do you want a little math trick

for estimating powers of two,

you can just recognize that two to the 10th power

is a little bit more than 1,000.

So this is going to be two to the 20th,

which is about a million, and then two to the third,

which is eight, so a little bit more than 8 million.

And once again,

that's before considering crossover and mutation,

which is going to make the combinations even larger.

And I'll also point out these are the combinations

for the gametes, and the gametes are haploid.

They have half the DNA of a full organism.

And so when the gametes combine,

you're actually going to have two to the 23rd

times two to the 23rd combinations,

just from sexual reproduction,

and you'll have even more from mutation and crossover.

And so that's going to lead you

to more than 70 trillion combinations

just from these two parents.