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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