Meiosis Notes
November 14, 2012
1st…. A Review of Chromosomes
• http://learn.genetics.utah.edu/content/begin/
scale/
Chromosome Numbers
• In a human, a body cell contains 46
chromosomes.
Chromosome Numbers
• Chromosomes come in homologous pairs, thus genes
come in pairs.
Homologous pairs – matching genes – one from female
parent and one from male parent
• When homologous chromosomes are paired together,
this is referred to as a tetrad.
• Example: Humans have 46 chromosomes or 23 pairs.
One set from dad
23 in sperm
One set from mom
23 in egg
Chromosome Numbers
• Cells with both sets of homologous chromosomes
are called diploid. These cells are represented by
2N. In humans, the diploid number is 46, so
2N=46.
• In sexually reproducing organisms, the gametes
(sex cells – sperm and egg) contain one set of
chromosomes. They are called haploid, and are
represented by the symbol N. In humans, the
haploid is 23, so N=23.
Match the Numbers!
23 CHROMOSOMES
HAPLOID
GAMETES
DIPLOID
N
SEX CELLS
2N
SOMATIC CELLS
BODY CELLS
ASEXUAL
REPRODUCTION
46 CHROMOSOMES
SEXUAL
REPRODUCTION
MEIOSIS
PHASES OF MEIOSIS
• Meiosis is a process of reduction division
where the number of chromosomes per cell is
cut in half through the separation of
homologous chromosomes in a diploid cell to
produce 4 haploid cells.
• Meiosis is divided into two process
– Meiosis I
– Meiosis II
Meiosis makes gametes (haploid cells)
*illustrate in notes
MEIOSIS
MITOSIS
2N
2N
2N
?
?
?
?
?
?
?
?
?
PHASES OF MEIOSIS I
MEIOSIS I
• The end result is two
haploid cells, each with
half the number of
chromosomes as the
original cell. The
chromosome exists as
sister chromatids at this
point.
PHASES OF MEIOSIS I
PRE-MEIOSIS - INTERPHASE
• During interphase, DNA and
organelles are copied.
• 46 chromosomes  92
chromosomes (diploid)
• Don’t forget….centrioles are
also copied!
PHASES OF MEIOSIS I
PROPHASE I
• Spindle fibers appear; nuclear
membrane disappears.
• Homologous chromosomes are
paired, forming a tetrad
– Each tetrad contains 4 sister
chromatids
• Crossing over occurs. Portions of
chromatids are exchanged,
contributing to genetic variation
in offspring
CROSSING OVER
• As homologous chromosomes pair up and form
tetrads in Meiosis I, they actually exchange portions
of their chromatids.
• Crossing over is the reason for genetic diversity! It
produces new combinations of genes.
PHASES OF MEIOSIS I
METAPHASE I
• Tetrads/homologous
chromosomes line up in the
middle of the cell
• Spindle fibers attach
PHASES OF MEIOSIS I
ANAPHASE I
Spindle fibers pull the
homologous
chromosomes toward
opposite ends of the
cell
PHASES OF MEIOSIS I
TELOPHASE I
• Cytokinesis occurs; two new
nuclear envelopes form
• End result is two daughter
cells, each with 46
chromosomes
PHASES OF MEIOSIS II
PROPHASE II
• Spindle fibers appear;
nuclear envelope
disappears.
• Question of the day: why is
there not another
interphase?
PHASES OF MEIOSIS II
METAPHASE II
• Chromosomes
line up in the
middle of the cell
PHASES OF MEIOSIS II
ANAPHASE II
• Sister chromatids separate
and move toward opposite
ends of the cell
PHASES OF MEIOSIS II
TELOPHASE II
• Cytokinesis occurs; four
new nuclear envelopes form
• End result is four daughter
cells, each with 23 haploid
chromosomes
PHASES OF MEIOSIS II - CYTOKINESIS
MALES
• In males, cytokinesis makes four
equal cells that develop heads and
tails – sperm – called
spermatogenesis
FEMALES
• In females, cytokinesis is unequal;
one cell gets most of the
cytoplasm to become the mature
egg, while the other three usually
die and are known as polar bodies.
Called oogenesis.
• Egg and Sperm are called
gametes.
FEMALES
http:www.cellsalive.com/meiosis.htm
Chromosome Problems
A. Failure of the homologous chromosomes to
separate normally during meiosis -- nondisjunction
1. missing a chromosome – monosomy (45
chromosomes)
2. having an extra chromosome – trisomy (47
chromosomes)
3. most embryos fail to survive, but some do
a. short, round face, upper eyelids cover the inner
part of the eye, mental retardation – trisomy 21 -three #21 chromosomes
(47 chromosomes) – Down Syndrome
b. male with longer-than-average limbs, sterile – XXY
Klinefelter’s Syndrome
c. female with short stature, webbed neck, sterile – XO
Turner’s Syndrome
B. Parts of chromosomes break off and reattach in
different ways, occurs in meiosis:
deletion / translocation / duplication / inversion
Deletion
Duplication
Inversion
Translocation
C. Detecting chromosome mutations:
1. Picture of individual’s chromosomes – karyotype
2. Amniotic fluid surrounding an embryo is removed for
analysis (done 3½ to 4 months of pregnancy) –
amniocentesis
3. Analysis of chorionic villi which grows between
mother’s uterus and placenta (done 2
months of pregnancy – chorionic villi sampling (CVS)
COMPARISONS!
MITOSIS
• Occurs in ALL body cells
• Chromosomes are duplicated
• Chromosomes are pulled
apart by spindle fibers
• An identical, complete set of
chromosomes moves towards
each side of cell
• Cytokinesis
• End product are two
IDENTICAL cells
• Daughter cells are diploid
– 2N = 46
MEIOSIS
• Occurs in sex cells ONLY
• During first meiotic division,
crossing over occurs
• During second meiotic division,
chromosome duplication does
not occur
• Produces four cells, each with
half a set of complete
chromosomes
• Cells are haploid (N = 23)
• Cells are GENETICALLY
DIFFERENT from each other
GENETICS
THE PROCESS OF GETTING US HOW
WE ARE.
Genetics Notes
Who is Gregor Mendel?
“Father of Genetics”
A. Mendel stated that
physical traits are
inherited as “particles”
B. Mendel did not know the
“particles” were actually
chromosomes and DNA
Traits
• Genetics – study of how traits are passed from parent
to offspring
• Traits are determined by the genes on the
chromosomes. A gene is a segment of DNA that
determines a trait.
• One pair of Homologous Chromosomes:
Gene for eye color
(blue eyes)
Homologous pair
of chromosomes
Gene for eye color
(brown eyes)
Alleles – different genes (possibilities) for the same trait –
ex: blue eyes or brown eyes
Gene Linkage
•Genes located on same chromosome –
gene linkage
•Genes located on same chromosome
cannot go through-- independent
assortment
•Chromosomes are distributed to
gametes randomly or independently
Package
of genes
inherited
together
Law of Independent Assortment
1. Principle of Independent Assortment – genes
for different traits can segregate independently
during the formation of gametes. Therefore, the
inheritance of one trait has no effect on the
inheritance of another.
• Example: Hair color and Eye color These genes
segregate independently and do not influence
each other’s inheritance (i.e. not all people with
blonde hair will have blue eyes).
NOVEMBER 15, 2013
• WHY DO SOME SIBLINGS HAVE SIMILAR
TRAITS WHILE OTHERS DO NOT?
Dominant and Recessive Genes
• Gene that prevents the other gene from “showing” –
dominant
• Gene that does NOT “show” even through it is present –
recessive
• Symbol – Dominant gene – upper case letter – T
Recessive gene – lower case letter – t
Dominant
color
Recessive
color
Example: Straight thumb is dominant to hitchhiker thumb
T = straight thumb t = hitchhikers thumb
(Always use the same letter for the same alleles—
No S = straight, h = hitchhiker’s)
Straight thumb = TT
Straight thumb = Tt
Hitchhikers thumb = tt
* Must have 2 recessive alleles
for a recessive trait to “show”
• Both genes of a pair are the same –
homozygous or purebred
TT – homozygous dominant
tt – homozygous recessive
• One dominant and one recessive gene –
heterozygous or hybrid
Tt – heterozygous
BB – Black
Bb – Black w/
white gene
bb – White
Law of Segregation
• During the formation of gametes (eggs or
sperm), the two alleles responsible for a trait
separate from each other (during MEIOSIS)
• Alleles for a trait are then “recombined” at
fertilization, producing the genotype for the
traits of the offspring.
Genotype and Phenotype
• Combination of genes an organism has (actual gene
makeup) – genotype
Ex: TT, Tt, tt
• Physical appearance resulting from gene make-up –
phenotype
Ex: hitchhiker’s thumb or straight thumb
Law of Dominance
• In a cross of parents that are homozygous
dominant and homozygous recessive, only
one form of the trait will appear in the next
generation.
• All the offspring will be heterozygous and
express only the dominant trait.
• RR x rr yields all Rr (Round Seeds)
Generation “Gap”
• Parental P1 Generation = the parental
generation in a breeding experiment.
• F1 generation = the first – generation offspring
in a breeding. (1st filial generation)
– From breeding individuals from the P1 generation
• F2 generation = the second – generation
offspring in a breeding experiment
– 2nd filial generation
– From breeding individuals in the F1 generation
Punnett Square and Probability
• Used to predict the possible gene makeup of offspring –
Punnett Square
• Cross involving one trait -- monohybrid
• Example: Black fur (B) is dominant to white fur (b) in mice
1. Cross a heterozygous male with a homozygous recessive female.
Black fur (B)
Heterozygous
male
White fur (b)
White fur (b)
Homozygous
recessive female
White fur (b)
Male = Bb X Female = bb
b
Male gametes - N
(One gene in
sperm)
B
b
b
Bb
Bb
bb
bb
Female gametes – N
(One gene in egg)
Possible offspring – 2N
Write the ratios in the following orders:
Genotypic ratio = 2 Bb : 2 bb
50% Bb : 50% bb
Genotypic ratio
homozygous : heterozygous : homozygous
dominant
recessive
Phenotypic ratio = 2 black : 2 white
50% black : 50% white Phenotypic ratio
dominant : recessive
Cross 2 hybrid mice and give the genotypic ratio and
phenotypic ratio.
B
b
B
BB
Bb
b
Bb
bb
Bb X Bb
Genotypic ratio = 1 BB : 2 Bb : 1 bb
25% BB : 50% Bb : 25% bb
Phenotypic ratio = 3 black : 1 white
75% black : 25% white
Example: A man and woman, both with brown eyes (B)
marry and have a blue eyed (b) child. What are the
genotypes of the man, woman and child? What is the
probability of them having a blue eyed child?
Bb X Bb
Man = Bb
B
b
B
BB
Bb
b
Bb
bb
Woman = Bb
3 brown : 1 blue
(2 carriers)
¼ or 25% blue
NOVEMBER 18, 2013
DIHYBRID CROSSES
• HOW TO:
AABB x CCDD
Crossing involving 2 traits – Dihybrid crosses
• Example: In rabbits black coat (B) is dominant over brown (b) and
straight hair (H) is dominant to curly (h). Cross 2 hybrid rabbits
and give the phenotypic ratio for the first generation of offspring.
Possible gametes:
BbHh X BbHh
BH
BH
Gametes
Bh
Bh
bH
bH
BH
bh
bh
Phenotypes - 9:3:3:1
9 black and straight
3 black and curly
3 brown and straight
1 brown and curly
BH
Bh
bH
bh
BBHH
BBHh
BbHH
BbHh
Bh
BBHh
BBhh
BbHh
Bbhh
bH
BbHH
BbHh
bbHH
bbHh
bh
BbHh
Bbhh
bbHh
bbhh
• Example: In rabbits black coat (B) is dominant over brown (b) and
straight hair (H) is dominant to curly (h). Cross a rabbit that is
homozygous dominant for both traits with a rabbit that is
homozygous dominant for black coat and heterozygous for straight
hair. Then give the phenotypic ratio for the first generation of
offspring.
BBHH X BBHh
Possible gametes: BH
BH
Bh
BH
Phenotypes:
100% black and straight
BH BBHH
Bh
Gametes
BBHh
Gametes
(Hint: Only design Punnett squares to suit the number of possible gametes.)
Gamete Formation Practice:
• Black fur (B) in guinea pigs is dominant to White Fur (b), and rough fur (R)
in guinea pigs is dominant to smooth (r). Cross a heterozygous parent
(BbRr) with a heterozygous parent (BbRr).
BbRr
BbRr
– Possible Gametes:
Parent 1: BR, Br, bR, br
Parent 2: BR, Br, bR, br
BR
Br
bR
br
BR
Br
bR
br
• Brown eyes (B) are dominant to blue eyes (b), and brown hair (H) is
dominant to blonde hair (h). Cross a blued eyed, homozygous brown
haired parent with a blue eyed, blonde haired parent.
– Possible Gametes:
Parent 1: bH
Parent 2: bh
bbHH
bH
bH
bH
bH
bbhh
bh
bh
bh
bh
Sex Determination
1. What is the probability of a husband and wife having a
boy or girl baby?
Chance of having female baby? 50%
male baby? 50%
X
X
X
XX
XX
Y
XY
XY
Who determines the sex of the child? father
Incomplete dominance and Codominance
• When one allele is NOT completely dominant over
another (they blend) – incomplete dominance
Example: In carnations the color red (R) is incompletely
dominant over white (r). The hybrid color is
pink. Give the genotypic and phenotypic ratio from a
cross between 2 pink flowers.
Rr X Rr
R
r
R
RR
Rr
r
Rr
rr
Genotypic = 1 RR : 2 Rr : 1 rr
Phenotypic = 1 red : 2 pink : 1 white
• When both alleles are expressed – Codominance
Example: In certain species of chickens black feathers
(FB) are codominant with white feathers (FW).
Heterozygous chickens have black and white speckled
feathers. Show the F1 from crossing 2 hybrid chickens.
Give the genotypic and phenotypic ratio.
FBFB = black
FWFW = white
FBFW = speckled
FB
FW
FB
F BF B
F BF W
FW
F BF W
FWFW
F BF W X F B F W
Genotypic = 1 FBFB : 2 FBFW : 1 FWFW
Phenotypic = 1 black : 2 speckled: 1 white
Sex – linked Traits
• Genes for these traits are
located only on the X
chromosome (NOT on the Y
chromosome)
• X linked alleles always show
up in males whether
dominant or recessive
because males have only
one X chromosome
• Examples of recessive sex-linked disorders:
1. colorblindness – inability to distinguish between
certain colors (most common red/green)
You should see 58
(upper left), 18
(upper right), E
(lower left) and 17
(lower right).
Color blindness is the inability to distinguish the differences between certain colors. The most
common type is red-green color blindness, where red and green are seen as the same color.
2. hemophilia – blood won’t clot (blood clotting
factor VIII defective)
3. Duchenne muscular dystrophy – wasting
away of skeletal muscles
• Example: A female that has normal vision but is a carrier
for colorblindness marries a male with normal vision.
Give the expected phenotypes of their children.
N = normal vision
n = colorblindness
XN Xn X XN Y
XN
Xn
XN XNXN
XNXn
XNY
XnY
Y
Phenotype: 1normal vision female
1 normal vision female
(carrier)
1 normal vision male
1 colorblind male
Take out meiosis review and notes
• HAPPY TUESDAY – NO WARM UP TODAY 
Meiosis Review
• Meiosis Animation
• Fertilized egg  Zygote  embryo
Pedigrees
• Graphic representation of how a trait is
passed from parents to offspring
• Tips for making a pedigree
1. Circles are for females
2. Squares are for males
3. Horizontal lines connecting a male and a
female represent a marriage
4. Vertical line and brackets connect parent
to offspring
5. A shaded circle or square indicates a
person has the trait
6. A circle or square NOT shaded represents
an individual who does NOT have the trait
7. Partial shade indicates a carrier –
someone who is heterozygous for the trait
• Example: Make a pedigree chart for the following
couple. Dana is color blind; her husband Jeff is not.
They have two boys and two girls.
HINT: Colorblindness is a recessive sex-linked trait.
XnXn
Has trait
XNY
Can pass trait to
offspring
Multiple Alleles
• 3 or more alleles of the same gene that code for a single trait
• In humans, blood type is determined by 3 alleles – IA, IB, and
iO
BUT genes come in pairs, so each human can only inherit 2
alleles
1. Codominant – IA, IB
Recessive – iO
2. Blood type – A = IAIA or IAiO
B = IBIB or IBiO
AB = IAIB
O = iOiO
Example: A woman homozygous for type B blood marries
a man who is heterozygous type A. What will be the
possible genotypes and phenotypes of their children?
IBIB X IA iO
IB
IB
IA
IAIB
IAIB
iO
IBiO
IBiO
Genotypic = 2 IAIB : 2 IBiO
Phenotypic = 2 AB : 2 B
Polygenic Inheritance
• Effect of 2 or more genes on a single phenotypic
characteristic
• Examples: eye color, skin color, height, body
mass
• Skin color: dark is dominant to light, genes are not linked
Ex: aabbcc = very light
AABBCC = very dark
AaBbCc = medium color – 3 dominant, 3 recessive
AABbcc = medium color – 3 dominant, 3 recessive
• Disorders that are polygenic: autism, diabetes, cancer
Gel Electrophoresis
Mutations
• Mutation – sudden genetic change (change in base pair sequence
of DNA)
• Can be :
Harmful mutations – organism less able to survive: genetic
disorders, cancer, death
5 – 8 genes in humans results in death – lethal mutation
Beneficial mutations – allows organism to better survive:
provides genetic variation
Neutral mutations – neither harmful
nor helpful to organism
• Mutations can occur in 2 ways:
chromosomal mutation or
gene/point mutation
• Only mutations in sex cells are
passed from parent to offspring
Chromosomal mutation:
• less common than a gene mutation
• more drastic – affects entire chromosome, so affects
many genes rather than just one
• caused by failure of the homologous chromosomes to
separate normally during meiosis (nondisjunction)
• chromosome pairs no longer look the same – too few or
too many genes, different shape
• Examples:
Down’s syndrome – (Trisomy 21) 47 chromosomes,
extra chromosome at pair #21
Turner’s syndrome – only 45 chromosomes, missing a
sex chromosome (X)
Klinefelter’s syndrome – 47 chromosomes, extra X
chromosomes (XXY)
Gene or Point Mutation
• most common and least drastic
• usually only one gene is altered
• Examples:
Recessive gene mutations:
Sickle cell anemia – red
blood cells are sickle
shaped instead of round
and get stuck in the blood
vessels – can cut off blood
supply to organs –
heterozygous condition
protects people from
malaria
Cystic fibrosis – mucus clogs lungs,
liver and pancreas
Tay-Sachs Disease – deterioration
of the nervous system – early
death
Phenylketonuria (PKU) – an amino
acid common in milk cannot be
broken down and as it builds up it
causes mental retardation –
newborns are tested for this
Dominant gene mutations:
Huntington’s disease – gradual
deterioration of brain tissue,
shows up in middle age and is fatal
Dwarfism – variety of skeletal
abnormalities