Steve JH Lee

How Variability is Produced in a Population

• Genetic Variability within a species measures how much variety of the genotype of organisms exists in a population

This means how many types of a specific gene/trait ex-ists in a population

• DO NOT CONFUSE THIS WITH GENETIC DIVERSITY

What is Variability?

Genetic Diversity Genetic Variability

Tendency of individual geno-types to vary from one an-

other

The amount of variation seen (phenotype from genotype) in

a particular population

Cause Effect

• Lets consider a hypothetical case: 2 men are trapped in a savannah. Each brought with him 5

survival items. Joe brings a knife, compass, water purifier, a pistol,

and a sunglass Kevin brings 5 hunting rifles and unlimited ammuni-

tion to last his stay

Who do you think has a better chance of surviving? Of course at

first it may seem as if Kevin has the advantage because he

will not have trouble feeding himself and Joe has brought a lot

of useless stuff

Why is Genetic Variability significant?

• But what if there was a severe sandstorm and both were

swept away and lost all sense of direction? Who would be

more prepared?

Of course Joe with his compass

Why is Genetic Variability significant? (2)

• Having a variety of genes in a popula-

tion is important because it gives BIO-

DIVERSITY

• Without variability, it becomes difficult

for a population to adapt to unpre-

dictable environmental changes

Remember Kevin’s mistake!!

• Genetic Variability Genetic Diver-

sity

Better Fitness

Why is Genetic Variability significant?(3)

• For example, lets say a deadly virus/bacteria kills off organ-

isms that have Gene A1

If all organisms were uniform and had only 1 type of

the gene, A1, all organisms in the population would

DIE

• However if a diverse population has multiple versions of

gene, or a VARIETY of genes: A1, A2, A3, A4, .. A99 then

only A1 would die and the rest would survive.

Why is Genetic Variability significant?(4)

• That means less Variability =

more prone to extinction

• Genetic variability is also the key

factor in evolution, natural selection.

• This is because it affects how much

of a population are handle environ-

mental stress and survive

Why is Genetic Variability significant?(4)

Now we will look at the individual mechanisms that produce Genetic Variability

• In biology there are 3 main sources of Genetic Variability:

1. Meiosis

Crossing Over (Homologous Recombination)

Random Orientation

Independent Assortment

Random Fertilization

2. Mutations

Point Mutations

Insertions/Deletions

Amplifications

3. Random Mating

How is Variability Produced?

• Homologous Recombination is one major

source of genetic variety in animals

• Remember in Interphase homologous chro-

mosomes duplicate and associate during

Prophase I

• This formation is known as a Tetrad or a Bi-

valent

• You generate variety when you guarantee in-

finite # of possible combinations

Meiosis: Homologous Recombination

• Crossing Over takes place where

alleles between non-sister chro-

matids are exchanged between

homologous chromosomes.

In a sense, the genes are mixed

up together

Knowing that there are millions of

alleles on a single chromosome

think of the number of the possibili-

ties!!

Meiosis: Homologous Recombination

Meiosis: Homologous Recombination

The Steps of Crossing Over/Homolo-

gous Recombination

• Synapsis: the homologous chromo-

somes associate and join

• Chiasma: alleles in neighboring non-

sister chromatids are exchanged via help

of cutting enzymes.

• Recombination: after the exchange

the alleles are swapped.

10.1.2 Outline the formation of chiasmata during cross-over

Step 1. Interphase, homologous pair repli-

cates. 2 copies of each pair held together by

centromere and each copy is called sister

chromatid

Step 2. Molecule cohesion occurs and homol-

ogous pairs are held together. Parallel “arms”

between non-sister chromatids exchange al-

leles.

Chiasmata= allele exchange sites

Step 3. cohesion breaks apart. Homologous

pair still held at the chiasmata.

Step 4. Anaphase finally breaks apart the

homologous pair with different gene combina-

tions

Recall: More Variability = More Combination Possibilities

• The next major source of variation is Metaphase I and Metaphase II when

the homologous chromosomes (I)/sister chromatids (II) separate

• Random orientation: there is no fixed designation of how homologous

chromosomes separate

Essentially this means homologous chromosomes are free to go which

side they want

Assuming that there are 2 possibilities for each chromosome (2 poles)

that means there are 223 possible types of cells with different combinations

of genes!!

Meiosis: Random Orientation

Meiosis: Random Orientation

• The 2nd Random Orientation occurs during Metaphase II

Unlike Metaphase I, it is much less significant in terms of con-

tributing to genetic diversity

Meiosis: Random Orientation

Metaphase I Metaphase II

Entire homologous chromosomes separate, both significantly different from each other in terms of type and

orientation of alleles

Sister chromatids separate which are not as dramatically different except ar-eas where crossing-over has taken

place

223 223

Lots of variation in gametes Some variation in gametes

Random Orientation Independent Assortment in that the way

chromosomes/chromatids randomly segregate form different

combinations of chromosomes in each final haploid gamete

Meiosis: Independent Assortment

Very helpful animation regarding Random Orientation/Independent AssortmentLINK: http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

10.1.4 State Mendel’s Law of Independent Assortment

Mendel’s law of Independent Assortment states that :

• Allele pairs separate independently during the forma-

tion of gametes (Random Orientation)

• Traits are transmitted to offspring independent of one an-

other

EXCEPTION: Linked Genes

Linked genes are genes that are inherited together because they are

relatively close to each other on the same chromosome

Because of their distance, it is unlikely for them to get recombi-

nated

10.1.3 Explain how meiosis results in infinite genetic variety in gametes

Cross-Over/Homologous Gene Recombination

Increases the variety in genes through random allele re-

arrangement

Even new combinations of linked genes

Increases genetic diversity of pop

Random Orientation/Independent Assortment

2n possible gametes

For humans that is 8,388,608 possible gametes for one meio-

sis

Is that all for Meiosis?

NO!!!

Random Fertilization means that a

random male gamete sperm will

fertilize a random female game

egg

Random Fertilization

8.4 million possibilities x 8.4 million pos-sibilities = 70,036,874,418,000 possibilities

…. And this is NOT counting Crossing Over

1. Crossing Over: homologous gene recombination leads to re-arrangement of alleles

2. Random Orientation: homologous chromosomes/sister chromatids randomly segregate and assort with each other creating 2^n possi-bilities

3. Random Fertilization: a randomly produced male gamete fertil-izes a random female gamete, squaring the number of variable pos-sibilities.

Assessment Statements with Meiosis Variability: 10.1.1 10.1.2 10.1.3 10.1.4

SUMMARY: Meiosis

Whereas meiosis is a more direct and short term mechanism

for genetic variability, mutations are long-term and evolu-

tionary mechanism for population diversity

By long-term, takes multiple generation for the

changes to get integrated

4.1.3 Define Genetic Mutations

Genetic mutations are changes in a cell’s genome/DNA se-

quence with multiple causes including radiation, viruses, and er-

rors in meiosis and DNA replication

A mutation is passed on to the offspring stably who will ei-

ther resemble that mutation or carry the allele.

Acquired Mutations such as cancer however cannot be

passed on to offspring

Point Mutations or base-substitu-

tions occur when a single nucleo-

tide is replaced with another

Often a result of chemical dam-

age or malfunction in DNA

replication

Result?: the specific codon in the spe-

cific allele is misread, protein struc-

ture is altered

In another words that protein

is defunct

Mutations: Point Mutations

In insertions and deletions occur when one

or more nucleotides are added or deleted

from the DNA sequence.

Usually caused by transposable ele-

ments or errors during replication (such

as AT repeats)

More significant than point mutations

this is because the entire DNA sequence

is shifted/moved and all genes in that

DNA sequence are misread

ALL GENES in that sequence cannot

be read

Mutations: Insertions and Deletions

Mutations can be achieved in 2 ways

1. Somatic Mutations (acquired mutations) NOT passed

down

Ex: UV light, cancer

2. Germ-line Mutations (mutation in a person’s DNA that is

passed down through gamete formation)

When a mutation is beneficial, such as pesticide-resistance, or

harmful according to the situation, it fuels natural selection

Significance of Genetic Mutations

List of all Assessment Statements

4.1.3 Define Gene Mutation

4.1.4 Explain the consequences of base-substitution in relation to

transcription/translation using Sickle-cell anemia as an example

10.1.1 Describe the behavior of chromosomes in phases of meiosis

10.1.2 Outline the formation of chiasmata during cross-over

10.1.3 Explain how meiosis results in effective infinite variety in ga-

metes through crossing-over in Prophase I and random orientation

in Metaphase I and II

10.1.4 State Mendel’s Law of Independent Assortment

1. Click 4 Biology: topics 4 and 10

- Topic 4: http://click4biology.info/c4b/4/gene4.htm

- Topic 10: http://click4biology.info/c4b/10/gene10.htm

2. Intranet Biology: topic 8- genetics

- Link: http://intranet.canacad.ac.jp:3445/BiologyIBHL1/5326

3. Wikipedia articles

- Mutations: http://en.wikipedia.org/wiki/Mutation

- Homologous Recombination: http://en.wikipedia.org/wiki/Homologous_recombination

4. I-biology Presentations

- 4.1

http://i-biology.net/ibdpbio/04-genetics-and-genetic-engineering/chromosomes-genes-alleles-an

d-mutations

/

- 10.1 http://i-biology.net/ahl/10-genetics-ahl/10-1-meiosis/

5. Interesting Independent Assortment Resource

- Link:

http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

List of all External Resources

List of all External Resources (2)

6. Clegg Textbook (helps clarify some details)

- pg 97 – 100; pg 328 – 333

7. Senior Biology Questions

- pg 181-182

- pg 185-190

- pg 194-199

8. Study Guide (brief summary)

- pg 56