1. Mendelian Genetics, Meiosis and
Evolution
•
Mendel‟s Laws of Heredity (10.1)
• Meiosis (10.2)
• The Theory of Evolution (15)

2. Answer these questions…

What is heredity?


What is genetics?



A seed
What is pollination?


Fertilization
In a plant, what does a zygote develop into?


Male sex cell + Female sex cell  Fertilized cell
Male gamete + Female gameteZygote
What is the above process called?


The branch of biology that studies heredity
What is sexual reproduction?


The passing of traits from parents to offspring
The transfer of pollen grains from a male reproductive organ to a
female reproductive organ in a plant
What is a hybrid?

The offspring of parents that have different forms of a trait

3. Gregor Mendel

Old thinking: Blending


Traits from both parents get
blended and the offspring is a
mixture of both
Cross-pollinated pea plants
to study how traits are
passed from parents to
offspring

6. Mendel’s 3 Conclusions

The rule of unit factors



Each organism has two factors that control each of its
traits
Factors = genes (located on chromosomes)
Genes exist in alternative forms



Alleles
Example: A pea plant could have 2 alleles for purple, 2 alleles
for white or 1 allele for purple and 1 allele for white
An organism‟s two alleles are located on different copies of a
chromosome (one from the female parent, one from the male)

7. Mendel’s 3 Conclusions

The rule of dominance




Purple flower + White flower = Purple flower
Only the purple trait is observed, therefore it is dominant
The white trait „disappeared‟, therefore it is recessive
The law of segregation



Every individual has two alleles of each gene
When gametes (sex cells) are produced each gamete
receives one of these alleles
During fertilization, these gametes randomly pair to
produce four combinations of alleles

9. Answer these questions…

What are the alleles of
the purple-flowered
plant in the parental
generation?


The white-flowered?


2 for purple
2 for white
The first gen. offspring?

1 for purple, 1 for white

10. Phenotypes and Genotypes

The purple parent has 2 purple alleles and the
purple offspring has 1 purple and 1 white allele

Therefore, two organisms can look alike, but have
different underling allele combinations

The way an organism looks and behaves is called its
phenotype

The allele combination an organism contains is
called its genotype

11. Phenotypes and Genotypes

If an organisms alleles are the same for a trait (think
purple parent) then the organism is homozygous for that
trait

If an organisms alleles are different for a trait (think first
gen. purple offspring) then the organism is
heterozygous for that trait

Mendel‟s conclusions allow us to…


Predict the probability of the genotype of an offspring when
given the parents genotype
Determine a phenotype based on genotype

14. Answer this question…

If two plants are crossed that have two different
traits, will the two traits stay together or will they be
inherited independently of each other?

16. The Law of Independent Assortment

Genes for different traits are inherited independently
of each other

19. Answer these questions…

Where are genes located?


How many chromosomes do humans have?


23 from mom, 23 from dad
Which cells combine to form a zygote (and then offspring)?


46 (23 pairs)
Where do these chromosomes come from?


46 (23 pairs)
How many chromosomes in each cell?


Chromosomes
Gametes (sex cells)
Therefore, how many chromosomes in a gamete?

23

20. Diploid and Haploid Cells

A cell with pairs of each chromosome is called a
diploid cell (2n)

A cell with one of each chromosome is called a
haploid cell (n)

The two chromosomes of each pair in a diploid cell
are called homologouschromosomes



Not identical
Contain information for the same traits
Can have different alleles

21. Typical Animal Life Cycle

22. Homologous Chromosomes (Homologs)




Chromosomes 1 & 2 are
homologous
chromosomes
Chromosomes 3 & 4 are
homologous
chromosomes
Chromosomes 1 & 3
came from the mother
Chromosomes 2 & 4
came from the father

23. Meiosis – Overview

Meiosis is a special type of cell division that
occurs in sexually reproducing organisms

Chromosome number reduced by half, enabling sexual
recombination to occur.


Meiosis of diploid cells haploid daughter cells (which may
function as gametes)
Gametes undergo fertilization, restoring the diploid number of
chromosomes in the zygote

25. Meiosis – Overview

Meiosis and fertilization introduce genetic variation in
three ways:


Crossing over between homologous chromosomes at
prophase I
Independent assortment of homologous pairs at
metaphase I


Each homologous pair can orient in either of two ways at the
plane of cell division
Random chance fertilization between any one female
gamete with any other male gamete

28. Meiosis - Overview

Sexual reproduction in a population should decline in
frequency relative to asexual reproduction



Asexual – No males are needed, all individuals can
produce offspring
Sexual – Only females can produce offspring, therefore
fewer are produced
Sexual reproduction may exist because it provides
genetic variability that reduces susceptibility of a
population to pathogen attack

This is the role of sexual reproduction in evolution

29. Meiosis

2 main stages

Meiosis I


Prophase I, Metaphase I,
Anaphase I, Telophase I
Meiosis II

Prophase II, Metaphase II,
Anaphase II, Telophase II

30. Meiosis I – Prophase I




The chromosomes condense and
become visible
The centrioles form and move
toward the poles
The nuclear membrane begins to
dissolve
The homologs pair up, forming a
tetrad


Each tetrad is comprised of four
chromotids - the two homologs, each
with their sister chromatid
Homologous chromosomes will swap
genetic material in a process known
as crossing over

Crossing over serves to increase
genetic diversity by creating four
unique chromatids

32. Meiosis I – Metaphase I

Microtubules grow from
the centrioles and
attach to the
centromeres

The tetrads line up
along the cell equator

33. Meiosis I – Anaphase I

Homologous
chromosomes
separate (note that the
sister chromatids are
still attached)

Cytokinesis begins

34. Meiosis I – Telophase I

The chromosomes may
decondense (depends
on species)

Cytokinesis reaches
completion, creating
two haploid daughter
cells

35. Meiosis II – Prophase II

Centrioles form and
move toward the poles

The nuclear membrane
dissolves

36. Meiosis II – Metaphase II

Microtubules grow from
the centrioles and
attach to the
centromeres

The sister chromatids
line up along the cell
equator

37. Meiosis II – Anaphase II

The centromeres break
and sister chromatids
separate

Cytokinesis begins

38. Meiosis II – Telophase II

The chromosomes may
decondense (depends
on species)

Cytokinesis reaches
completion, creating
four haploid daughter
cells

https://www.youtube.co
m/watch?v=D1_mQS_FZ0

41. Answer these questions…

How does the number of daughter cells produced
from mitosis and meiosis differ?


How does the ploidy of the daughter cells produced
from mitosis and meiosis differ?


Mitosis produces diploid (2n) cells. Meiosis produces haploid
(n) cells.
Do the daughter cells produced from mitosis contain
identical genetic complements?


When mitosis is complete, there are two daughter cells. When
meiosis is complete, there are four.
Yes, the purpose of mitosis is to produce two identical cells
Do any of the daughter cells produced from meiosis
contain identical genetic complements?

No, the genetic information swapped between homologous
chromosomes during crossing over insures that each daughter
cell produced during meiosis will be unique

42. Answer these questions…

When do the homologous chromosomes separate
during mitosis?


When do the homologous chromosomes separate
during meiosis?


Homologs separate during Anaphase I, when the tetrads break
When do sister chromatids separate during mitosis?


Never, they are never joined during mitosis (no tetrads are
formed)
Sister chromatids separate during Anaphase.
When do sister chromatids separate during meiosis?

Sister chromatids separate during Anaphase II.

43. The Consequences of Meiotic Mistakes

Nondisjunctions occur when homologous
chromosomes fail to separate at meiosis I or when
chromatids fail to separate at meiosis II.

45. The Consequences of Meiotic Mistakes

Nondisjunctions occur when homologous
chromosomes fail to separate at meiosis I or when
chromatids fail to separate at meiosis II



Fertilization can result in embryos that are 2n + 1 (a
"trisomy")
Abnormal copy numbers of one or more chromosomes is
usually, but not always, fatal (Example: Down syndrome)
Polyploidy can occur when whole sets of
chromosomes fail to separate at meiosis I or II


The resulting 2n gametes, if fertilized by normal sperm,
create 3n zygotes (triploid)
Organisms with an odd number of chromosome sets
cannot produce viable gametes (Example: seedless fruits)

46. Answer these questions…

How do mutations drive evolution?


Mutations  change traits of and organism  if change
helps the organism survive the greater the chance of that
organism living long enough to reproduce  trait gets
passed down through generations
How does sexual reproduction drive evolution?

Sexual reproduction may exist because it provides
genetic variability that reduces susceptibility of a
population to pathogen attack

47. Evolution

All of the similarities and
dissimilarities among
groups of organisms
that are the result of the
branching process
creating the great tree
of life, were viewed by
early 19th century
philosophers and
scientists as a
consequence of
omnipotent design.

48. Evolution

In 1859, Charles Darwin
published his famous
On the Origin of
Species

Patterns in the
distribution and
similarity of organisms
had an important
influence of Darwin's
thinking

51. Darwin’s Theory of Evolution

Species (populations of interbreeding organisms)
change over time and space. The representatives of
species living today differ from those that lived in the
recent past, and populations in different geographic
regions today differ slightly in form or
behavior. These differences extend into the fossil
record, which provides ample support for this claim.

52. Darwin’s Theory of Evolution

All organisms share common ancestors with other
organisms. Over time, populations may divide into
different species, which share a common ancestral
population. Far enough back in time, any pair of
organisms shares a common ancestor. For
example, humans shared a common ancestor with
chimpanzees about eight million years ago, with
whales about 60 million years ago, and with
kangaroos over 100 million years ago. Shared
ancestry explains the similarities of organisms that
are classified together: their similarities reflect the
inheritance of traits from a common ancestor.

53. Darwin’s Theory of Evolution

Evolutionary change is gradual and slow in Darwin‟s
view. This claim was supported by the long episodes of
gradual change in organisms in the fossil record and the
fact that no naturalist had observed the sudden
appearance of a new species in Darwin‟s time.

Since then, biologists and paleontologists have
documented a broad spectrum of slow to rapid rates of
evolutionary change within lineages.

The primary mechanism of change over time is natural
selection

54. The Process of Natural Selection

Variation

Organisms (within populations) exhibit individual variation
in appearance and behavior


Some traits show little to no variation among individuals


Body size, hair color, facial markings, voice properties etc.
Number of eyes in vertebrates
Inheritance


Some traits are consistently passed on from parent to
offspring
Other traits are strongly influenced by environmental
conditions and show weak heritability

55. The Process of Natural Selection

High rate of population growth



Most populations have more offspring each year than
local resources can support leading to a struggle for
resources.
Each generation experiences substantial mortality.
Differential survival and reproduction

Individuals possessing traits well suited for the struggle
for local resources will contribute more offspring to the
next generation.

56. Final Thoughts on Evolution

In order for natural selection to operate on a trait, the
trait must possess heritable variation and must
confer an advantage in the competition for
resources. If one of these requirements does not
occur, then the trait does not experience natural
selection

“…as natural selection acts by competition for
resources, it adapts the inhabitants of each country
only in relation to the degree of perfection of their
associates” (Charles Darwin, On the Origin of
Species, 1859).

57. Final Thoughts on Evolution

Variations arise by mutation

Mutations arise by chance and without foresight for
the potential advantage or disadvantage of the
mutation.

In other words, variations do not arise because they
are needed.