Meiosis
Vocabulary
SOMATIC:
Any cell except a sperm or egg
 AUTOSOME
Any chromosome other than a sex
chromosome (X or Y)
 GAMETE
An egg or sperm

Mitosis allows asexual
reproduction
Meiosis
allows sexual
reproduction.
Parents are diploid
Homologous
chromosomes are
separated into single
sets.
Humans have
23 pairs of
chromosomes
.
FEMALE
 One
pair (the
sex
chromosomes)
are not
identical in
males.
MALE
Sex ratios: 106 boys: 100
girls (at birth)
Karyotypes are pictures
of chromosomes.
Some (More)
Vocabulary
Alleles
Eye Color
Eye Color
Brown,
green
Meiosis separates
chromosome pairs to form
haploid gametes.
Meiosis I: Homologous
pairs are separated (as
sister chromatids) into 2
cells
Meiosis II: Similar to
mitosis: sister chromatids
are separated = 4 haploid
cells.
MEIOSIS I
Interphase I
• Interphase is very
similar to interphase
in mitosis
MEIOSIS I
Prophase I
• Double stranded
chromosomes and
spindle fibers appear;
• Nuclear membrane and
nucleolus fade (synapsis
aligns homologous
chromosomes)
MEIOSIS I
Metaphase I
• Chromosome pairs
(chromatids) line up
• Spindle fibers attach to
centromeres and
centrioles
MEIOSIS I
Anaphase I
• Chromatids separate
from matching pair
(independent
assortment occurs
here)
MEIOSIS I
Telophase I
• Cytoplasm divides
and 2 cells form
• Each cell still has a
diploid (2n) number
MEIOSIS II
Prophase
II
Metaphase
II
Anaphase
II
Telophase/
cytokinesis
II
Meiosis II follows the same steps as
mitosis
 Key differences:
◦ In interphase II, there is no replication
of genetic material
◦ When the sister chromatids separate
during anaphase II, they are now
haploid (1n)
◦ After telophase II, you have 4 haploid
(1n) cells with genetic variation (they
are no longer identical)

SUMMARY OF MEIOSIS II
Genetic Variation
Independent Assortment: Homologous pairs can separate
in different parental combinations.
2n possible gametes created
In humans:
 223 = 8 million possible gametes


This is without crossing over, which
increases the number of distinct
chromatids (exact number of chiasma
determines how many different
chromatids will exist)
Possible Gametes
Genetic Variation
Crossing Over: During
synapsis (Prophase I)
portions of non-sister
chromatids are
exchanged.
Genetic Variation
8 million possible
gametes for each
parent:
8,000,0002 =
64 trillion possible
zygotes
Random Fertilization:
Each sperm will
fertilize and egg
randomly
Without Crossing
over. With it, there are
even more!
 These
are a pair
of homologous
chromosomes
with genes A E.
 These
homologous
chromosomes
are made of 2
sister
chromatids
each.
These are called
TETRADS
Synaptonemal Complex!
 The
tetrads line
up side by side
(synapsis) in
Prophase I
 Segments cross
over (form
chiasma) and
the pieces are
traded.
 The
sister
chromatids
end up with
different
copies of the
genes.
Notice that genes that are located close
together will not separate during crossing
over as much as a pair of genes that are
further apart.
Frequency of Crossing Over

Measuring the
frequency of crossing
over tells you the
distance between
two genes (in map
units)
 With
all of the variation
produced in meiosis, how can
parents have IDENTICAL
TWINS?
 How does the production of
sperm differ from production of
egg?
What do you think?

Errors in meiosis are passed down to
offspring (because they occur in sperm or
eggs)

These errors can create sperm and eggs
that have the wrong number or types of
chromosomes
Errors in Meiosis
Deletion: part of a chromosome is deleted
(removed)
 Duplication: part of a chromosome is
repeated
 Translocation: part of a chromosome is
moved to another chromosome
 Inversion: part of a chromosome is
removed, then flipped around and
reattached

Types of errors
Inversion
Non-disjunction
Failure of
chromosomes to
separate properly
in meiosis
 Meiosis I:
homologues don’t
separate
 Meiosis II: Sister
chromatids don’t
separate


Monosomy: one parent does not give a
copy of one chromosome
◦ There is only 1 total in the child
 45 instead of 46 total chromosomes

The only monosomy that we see in
humans (that is nonfatal) is Turner
syndrome
Types of errors
Turner syndrome

Females inherit only one X chromosome
(XO), instead of XX


1/2500 girls
98% of fetuses with Turner Syndrome
miscarry

Short stature, broad chest, webbed
neck
Turner syndrome
Trisomies

When one parent
gives two copies
of a chromosome
◦ The child has 3
total of that
chromosome (47
instead of 46 total
chromosomes)
Down’s syndrome
 3 copies of chromosome 21
 Impairment of cognitive ability (avg. IQ
50, vs. 100 in rest of population)
 Almond-shaped eyes
 Health problems: heart failure, GERD, ear
infections, sleep apnea
 1/733 births
◦ More common with older mothers, but 80% are
born to women under 35
Trisomy 21
Trisomy 18 May be a partial trisomy (less severe)

Edwards
syndrome
◦ 2nd most common
trisomy
95% miscarry
Of those born,
50% die within 2
months; 5-10%
survive first year
 Apnea, heart and
kidney defects
 1/3000 live births


Trisomy 13
May be a partial trisomy (less severe)

Patau
syndrome
◦ Mental
impairment
◦ Polydactyly
(extra fingers
or toes)
◦ Microcephaly
(small brain)
◦ 85% do not
survive 1 year

Trisomy 16
◦ Most common trisomy, but results in
miscarriage

Trisomy 9
◦ Mental retardation, deformities of skull,
nervous system

Trisomies may be partial (due to
translocation) or mosaic
Trisomies

Mosaicism: During very early development,
non-disjunction occurs in mitosis and as a
result some percentage of an individual’s cells
(but not all) have the trisomy/monosomy
46
46
46
46
47
47
Mosaicism
47
XXX: No difference between this and a
normal woman (XX)
◦ (X inactivation -> Barr bodies)
 XXY: Klinefelter
◦ Small testicles, reduced fertility
◦ Most common sex trisomy (1/500
males)
 XYY: Not named because normally
asymptomatic (normal phenotype)

Trisomies in sex chromosomes

Many other possibilities, many don’t have
any symptoms
Trisomies in sex chromosomes
Meiosis Interphase I
Meiosis Early prophase I
Meiosis Mid-prophase I
Meiosis Late prophase I
Meiosis Metaphase I
Meiosis Anaphase I
Meiosis Telophase I
Meiosis Cytokinesis complete
Meiosis I complete;
cytokinesis by cell plate
formation.
Meiosis Prophase II
Meosis Metaphase II
Meiosis Anaphase II
Meiosis Telophase II and
cytokinesis beginning.
Meiosis Cytokinesis complete.