Chapter 13 –
An Introduction
to Heredity:
Meiosis
Heredity: The transmission of traits from one generation to the
next
Genes are DNA segments
A locus is a gene’s specific
location along the length
of a chromosome
Offspring acquire genes
from parents by
inheriting chromosomes
Characteristics of Asexual Reproduction
Asexual reproduction = form of cell
division; A system of cloning and
creating exact genetic replicas so there
exists no genetic variation
Single-celled organisms can
reproduce to create new
organisms
Budding is a form of
asexual reproduction in
multi-cellular organisms
Characteristics of Sexual Reproduction
2 parents must give rise to
offspring; each parent giving
a haploid set of
chromosomes
Each cell of a diploid
organism must have 2
sets of chromosomes.
One set donated from
the mom, the other set
donated from dad.
Two haploid gametes
(reproductive cells) must
combine to create a new
diploid organism.
Offspring has greater
genetic variation
Homologous
Human
cells have a
Chromosomes:
diploid
number of
a pair of
46
chromosomes
2N
= 46 the
containing
same
lineareach cell
This
means
gene
has
23sequences,
homologous
each derived
pairs
from one
parent.
There
is one(such
exception…
Karyotypes
as
these)
pictures of an
The sexare
chromosomes
individual’s
(pair # 23), is homologous
chromosomes.
in females (XX)
Autosomes
are non-sex in
but is only hemizygous
chromosomes
(pairs 1males (XY)
22 in humans)
Let’s review mitosis = a form of nuclear division,
whereby the chromosome number and genetic
content in the organism remains the same in both
parent and daughter cells.
Meiosis = A form of nuclear division that results in the
reduction of chromosome number by half (from
diploid cell to a haploid cell)
How does mitosis and meiosis compare?
How does meiosis increase genetic
diversity?
Crossing over during prophase I
Independent Assortment or random alignment of
homologous chromosomes along metaphase plate
Genetic
diversity Iin a species increases the
during
metaphase
species’ chances of survival and prevents
Segregation
extinction. (separation) of homologous
chromosomes in Anaphase I and segregation of
In a chromatids
diverse geneinpool,
there IIis a higher
sister
Anaphase
probability of a gene that increases fitness
Random
Fertilization
during times
of instability or stress.
Varieties in Sexual Life Cycles
Type 1: Dominant Stage (stage it spends most of its life in) = Diploid Multicellular
(Ex. Humans)
2N
N
Mitosis/Cytokinesis
Meiosis/cytokinesis
2N
N
Fertilization
Diploid
Multicellular
Haploid unicellular
Varieties in Sexual Life Cycles
Type 2: Dominant Stage = Haploid Multicellular (Ex. Fungi and Algae)
N
Diploid Single Cell
Mitosis/cytokinesis Haploid multicellular
2N
N
fertilization
One cell from multicellular haploid
cluster is designated a haploid
gamete
Meiosis/Cytokinesis
N
N
N
N
Sordaria (fungi) – lab #3
Example of Type 2 Life Cycle
Gene that codes for ascospore color
Black (+) = wildtype black spore 
Tan (tn) = mutant brown spore 
Sordaria is a haploid organism, so fungi’s
phenotype for spore color is dependent on
the single gene that it inherits
Example of Type 2 Life Cycle
Areas where both haploid strands
canTan
fuse,
become diploid, and
Black
spores
spores
undergo immediate meiosis/cross
over.
Both haploid strands grown on an agar plate
Example of Type 2 Life Cycle
Two haploid genes
fuses to become
diploid
Ascus containing
8 ascospores
Cross over
might occur
Completion of meiosis
mitosis
Ascopore pattern w/o crossover
Gene
expression
tn
tn
tn
DNA replication
+
+
tn
tn
Meiosis I
+
+
Meiosis II
tn
+
Meiosis II
+
tn
tn
Mitosis
tn
tn
+
+
tn
tn
tn
tn
+
+
Mitosis
+
+
+
+
Ascospore
with 4X4
pattern
Ascopore pattern #1 w/ crossover
tn
tn
+
tn
tn
+
tn
DNA replication
+
tn
++
Meiosis II
+
Meiosis I
tn
tn
+
+
Meiosis II
+
Cross-over
+
tn
tn
Mitosis
+
+
+
tn
tn
Mitosis
+
+
Ascospore
with
2X2x2x2
pattern
Ascopore pattern #2 w/ crossover
+
tn
tn
+
tn
DNA replication
+
tn
++
Meiosis II
tn
+
tn
Meiosis I
tn
tn
+
+
Meiosis II
+
Cross-over
+
+
+
Mitosis
tn
tn
+
tn
tn
Mitosis
+
+
Ascospore
with 2x4x2
pattern
Varieties in Sexual Life Cycles
Type 3: Alternation of Generation = Dominant haploid AND diploid multicellular stages
2N
Meiosis/cyt
One cell from multicellular haplo
cluster is designated a haploid
gamete
Mitosis/cyt
N
N
Diploid multicellular stage =
Sporophyte
2N
4 Haploid spores
Haploid multicellular stage =
Gametophyte
Mitosis/cyt
Meiosis/cyt
fertilization
N
N
Mitosis/Cytokinesis
2N
2N
No
What can go wrong in meiosis?
Non-disjunction Disorders
Meiosis I of
- Failure
to
Definition: When members
homologous
separate
chromosomes fail to separate during
Meiosis I – or – when sister chromatids fail
Meiosis II Failure
to separate to
during
Meiosis II.
separate
Examples: Down Syndrome, Turner’s syndrome,
Klinefelter’s syndrome
Abnormal
Gametes
Normal
Gametes
Polyploidy
Polyploidy is a term that describes the
inheritance of more that two sets of
chromosomes due to non-disjunction
during meiosis
Interpret these karyotypes
Klinefelter’s syndrome
Interpret these karyotypes
Down Syndrome
Try these on-line activities
• http://www.biology.arizona.edu/human_bio
/activities/karyotyping/karyotyping2.html
X-inactivation (in mammals)
In female mammals,
one of the Xchromosomes turns
“off” and condenses
into a compact barr
body.
The barr body is
reactivated in the
ovary cells only
during meiosis for
reproduction.
Two X-chromosomes
(autosomes not shown)
Barr body due to Xinactivation
Other non-disjuntion disorders…
- XO individual (missing 1-X chromosome) –
physically female (same phenotype as
normal female who has barr body), but
sterile since there is no barr body to
reactivate during puberty
- XXX female – sterile
- XYY male – abnormally tall
- XXY male – one X becomes a barr body,
so phenotypically male, but is sterile when
barr body is reativated. He has abnormally
small testes
What is a mutation?
Mutations are changes in the
DNA. During meiosis, there
are 4 different types of
chromosomal mutations that
can occur.
1. Deletion: A segment of the chromosome is removed (not just
one nuclotide)
A B C D E
F G H
A B C E
F G H
2. Duplication: A segment of the chromosome is repeated
A B C D E
F G H
A B C B C D E
F G H
A B C D E
F G H
A D C B E
A B C D E
F G H
M N O C D E
M N O P Q
R
3. Inversion: A segment within a chromosome is reversed
F G H
4. Translocation: A segment from one chromosome moves to
another, non-homologous one
A B P Q
R
F G H