Genetic variability within a population measures the variety of genotypes that exist. It is important for biodiversity and a population's ability to adapt to environmental changes. Genetic variability is produced through three main sources: meiosis, mutations, and random mating. Meiosis involves crossing over during prophase I, which exchanges alleles between homologous chromosomes, producing new combinations. It also involves random orientation of homologous chromosomes during metaphase I and sister chromatids during metaphase II. This, along with random fertilization, results in an effectively infinite number of genetically unique gametes. Mutations, such as point mutations and insertions/deletions, introduce new variants over long periods of time and act as an evolutionary mechanism of diversity.

1. Steve JH Lee

2. • 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 exists in a
population
• DO NOT CONFUSE THIS WITH GENETIC DIVERSITY
Genetic Diversity Genetic Variability
Tendency of individual genotypes The amount of variation seen
to vary from one another (phenotype from genotype) in a
particular population
Cause Effect

3. • 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 ammunition 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

4. • 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

5. • Having a variety of genes in a population
is important because it gives
BIODIVERSITY
• Without variability, it becomes difficult for
a population to adapt to unpredictable
environmental changes
 Remember Kevin’s mistake!!
• Genetic Variability  Genetic Diversity
 Better Fitness

6. • For example, lets say a deadly virus/bacteria kills off organisms 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.

7. • 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 environmental
stress and survive
Now we will look at the individual mechanisms that produce Genetic Variability

8. • 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

9. • Homologous Recombination is one major source
of genetic variety in animals
• Remember in Interphase homologous
chromosomes duplicate and associate during
Prophase I
• This formation is known as a Tetrad or a Bivalent
• You generate variety when you guarantee infinite #
of possible combinations

10. • Crossing Over takes place where
alleles between non-sister chromatids
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 possibilities!!

11. The Steps of Crossing Over/Homologous
Recombination
• Synapsis: the homologous chromosomes
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.

12. Step 1. Interphase, homologous pair replicates. 2
copies of each pair held together by centromere and
each copy is called sister chromatid
Step 2. Molecule cohesion occurs and homologous
pairs are held together. Parallel “arms” between non-
sister chromatids exchange alleles.
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 combinations

13. 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
!!

15. • The 2nd Random Orientation occurs during Metaphase II
 Unlike Metaphase I, it is much less significant in terms of contributing to
genetic diversity
Metaphase I Metaphase II
Entire homologous chromosomes separate, Sister chromatids separate which are not as
both significantly different from each other in dramatically different except areas where
terms of type and orientation of alleles crossing-over has taken place
223 223
Lots of variation in gametes Some variation in gametes

16. Random Orientation  Independent Assortment in that the way
chromosomes/chromatids randomly segregate form different
combinations of chromosomes in each final haploid gamete
Very helpful animation regarding Random Orientation/Independent Assortment
LINK: http://www.sumanasinc.com/webcontent/animations/content/independentassortment.html

17. Mendel’s law of Independent Assortment states that :
• Allele pairs separate independently during the formation of
gametes (Random Orientation)
• Traits are transmitted to offspring independent of one another
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 recombinated

18.  Cross-Over/Homologous Gene Recombination
 Increases the variety in genes through random allele rearrangement
 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 meiosis

19. Is that all for Meiosis?

20. NO!!!

21. Random Fertilization means that a
random male gamete sperm will fertilize
a random female game egg
8.4 million possibilities x 8.4 million possibilities
= 70,036,874,418,000 possibilities
…. And this is NOT counting Crossing Over

22. 1. Crossing Over: homologous gene recombination leads to rearrangement
of alleles
2. Random Orientation: homologous chromosomes/sister chromatids
randomly segregate and assort with each other creating 2^n possibilities
3. Random Fertilization: a randomly produced male gamete fertilizes a
random female gamete, squaring the number of variable possibilities.
Assessment Statements with Meiosis Variability:
10.1.1
10.1.2
10.1.3
10.1.4

23. Whereas meiosis is a more direct and short term mechanism for
genetic variability, mutations are long-term and evolutionary
mechanism for population diversity
By long-term, takes multiple generation for the changes to get
integrated

24. Genetic mutations are changes in a cell’s genome/DNA sequence with
multiple causes including radiation, viruses, and errors in meiosis and DNA
replication
 A mutation is passed on to the offspring stably who will either
resemble that mutation or carry the allele.
 Acquired Mutations such as cancer however cannot be passed on
to offspring

25. Point Mutations or base-substitutions
occur when a single nucleotide is
replaced with another
 Often a result of chemical damage or
malfunction in DNA replication
Result?: the specific codon in the specific
allele is misread, protein structure is
altered
 In another words that protein is
defunct

26. In insertions and deletions occur when one or
more nucleotides are added or deleted from the
DNA sequence.
 Usually caused by transposable elements 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

27. 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

28. 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 gametes
through crossing-over in Prophase I and random orientation in Metaphase I
and II
10.1.4 State Mendel’s Law of Independent Assortment

29. List of all External Resources
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-and-mut
ations/
- 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

30. 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