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Agenda
• To Turn In Today:
– Homework 3
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Phases of Mitosis Identification
Haploid gametes (n = 23)
Meiosis
n
Egg cell
ANIMATION:
Campbell Ch 8 Meiosis
Overview
n
Sperm cell
Meiosis
Fertilization
Diploid
zygote
(2n = 46)
ANIMATION:
Comparison of
Mitosis and
Meiosis
Multicellular
diploid adults
(2n = 46)
Mitosis and
development
2n
Variation in Gametes
Independent Assortment
Crossing-over
Comparison of Mitosis and Meiosis
MITOSIS
MEIOSIS
Starts with
Diploid cell (2n)
Diploid cell (2n)
Replicates DNA
Once
Once
Cell divides
Once
Twice
Pairing of
homologues?
No
Yes
Crossing over?
No
Yes
Centromeres
separate
Anaphase
Anaphase II
Daughter DNA
Identical
Different
# of daughter cells
2
4
Daughter cell is
Diploid cell (2n)
Haploid cell (1n)
Reproduction
SEXUAL
• “Two” parents
– Egg and Sperm
– Can sometimes be one organism
• Offspring’s DNA is NOT
identical to parent’s DNA
• Useful when organism needs
to adapt to its environment
Somatic Cells
• Body Cell
• DIPLOID (2n): 2 copies
of each chromosome
(why?)
• Produced by mitosis
• Humans: 46
chromosomes
Gametes
• Eggs and sperm
• HAPLOID (1n): 1 copy of
each chromosome
(why?)
• Produced by meiosis
• Humans: 23
chromosomes
Fertilization
• Fusion of two HAPLOID
gametes to produce a
DIPLOID ZYGOTE
23
23
46
Variation in Gametes
• 23 chromosomes = ?
possibilities
• 2 possibilities per
chromosome
• 223 = 8388608
Variation in Gametes
• 8388608 combinations
of gametes produced by
mother
• 8388608 combinations
of gametes produced by
father
• Any egg can be fertilized
by any sperm
What makes chromosomes different
from each other?
• Carry different genes
Homologous Chromosomes
• Each member of pair
can carry different
ALLELES of a gene at the
same LOCUS
– Allele = different
versions of a gene
– Locus (loci) = location on
a chromosome
Example: eye and coat color in mice
Coat-color
genes
Eye-color
genes
Brown
Black
C
E
C
E
C
E
c
e
c
e
Meiosis
c
e
White
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Chromosomes of
the four gametes
Example: eye and coat color in mice
Brown coat (C); black eyes (E)
White coat (c); pink eyes (e)
Patterns of Inheritance
Patterns of Inheritance
• GENETICS is about how traits are passed down
– Predicting inheritance of a trait
– Determining probability of inheriting a disease
– Purebred animals vs “mutts”
Genetics History
• Hippocrates' theory of Pangenesis: “particles”
from each part of the body travel to eggs or
sperm and are passed on
• Blending hypothesis (early 19th century): traits
from both parents mix in the offspring
Genetics History
• Gregor Mendel (1822 – 1884)
–
–
–
–
Monk
University educated
Studied variation in pea plants
Work rejected by contemporaries,
its importance was not realized
until the 20th century
Mendel’s Studies
Flower color
Purple
White
Flower position
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Stem length
• Crossed pea plants that
differed in certain
characteristics
• Could control matings
• Developed true-breeding
varieties
• Traced traits from
generation to generation
Mendel’s Studies
Removed stamens
from purple
flower
Pea flower
White
Stamens
Petal
Carpel
Parents
(P)
Purple
Transferred pollen from stamens o
white flower to carpel of purple
flower
Pollinated carpel
matured into pod
Planted seeds
from pod
Stamen
Carpel
Offspring
(F1)
Mendel’s Experiments Terminology
• Self-fertilization: fertilization of eggs by sperm-carrying
pollen of the same flower
• Cross-fertilization (cross): fertilization of one plant by
pollen from a different plant
• True-breeding: identical offspring from self-fertilizing
parents
• Hybrid: offspring of two different varieties
• P (parental) generation: true-breeding parents
• F1 generation: hybrid offspring of true-breeding
parents
• F2 generation: offspring of self-fertilizing F1 parents
Mendel’s Observations
P generation
(true-breeding
parents)

Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1 F1)
F2
When true-breeding parents are
crossed, some traits cover up
generation
other traits in the F1 generation
3 of plants
have4purple flowers
1 of plants
have4 white flowers
Purple is DOMINANT
over white
White is RECESSIVE
to purple
Mendel’s Observations
P generation
(true-breeding
parents)

Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1 F1)
F2 generation
3 of plants
have4purple flowers
1 of plants
have4 white flowers
Purple is DOMINANT
over white
White is RECESSIVE
to purple
Recessive traits
reappear in the F2
generation
Mendel’s Observations
• An organism's
appearance does not
always reveal its genetic
composition
– Phenotype: Expressed
(physical) traits
– Genotype: Genetic
makeup
Mendel’s Observations
• Characteristics are NOT blended
together like different colors of
paint
• Characteristics ARE determined
by definite, discrete particles of
inheritance (“factors” / genes)
• There are alternative forms of
these “factors” that determine
traits
• For each characteristic, an
organism inherits two “factors”,
one from each parent
Mendel’s Law of Segregation
• Describes inheritance of a single characteristic
• Of a pair of alleles, only one is carried in a
single gamete
Homologous
Chromosomes
Diploidy: Two Alleles
• HETEROZYGOUS: Two different alleles, 1 from each
parent
• HOMOZYGOUS: Two identical alleles (1 f/ ea P)
Mendel’s Law of Independent
Assortment
• Describes inheritance of more than one
characteristic
• Genes for each characteristic are inherited
independently
1
2
3
23 = 2 x 2 x 2 = 8
Monohybrid cross
• P = purple flowers
• p = white flowers
Pp x Pp
Genotype or phenotype?
Homozygous or heterozygous?
Dominant or recessive phenotype?
What is the phenotype?
Monohybrid cross
• P = purple flowers
• p = white flowers
P
p
Pp x Pp
P
p
What gametes can these parental flowers make?
Monohybrid cross
• P = purple flowers
• p = white flowers
P
P
P
p
p
P
p
p
Pp x Pp
What combinations of gametes will be in the offspring?
Punnett Square
• Shows possible
combinations of alleles
• Demonstrates Mendel’s
Laws
• Purple flower allele: P
• White flower allele: p
P
p
P PP
Pp
p Pp
pp
Punnett Square Problems
1. Heterozygous x white
2. Homozygous yellow (Y) x homozygous green
(y)
Human Inheritance
Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
More than one trait?
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P generation
rryy
RRYY
ry
Gametes RY
RrYy
F1 generation
Sperm
1
2
F2 generation
1
2
RY
1
2

ry
Will the F1 and
RrYy F2 generations
maintain the same combinations
Sperm
of traits
as
the
parental
plants?
RY
rY
Ry
ry
1
4
1
4
RY
rryy
Gametes RY
ry
1
4
1
4
1
4
RY
RRYY
RrYY
RRYy
RrYy
rY
Will
yellow
always
be round and
RrYY
rrYY
RrYy
rrYy
Eggs
Yellow
green wrinkled?
Ry
round
Eggs
1
2
RRYY
1
4
ry
9
16
1
4
Actual results
contradict hypothesis
1
4
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
support hypothesis
3
16
3
16
1
16
Green
round
Yellow
wrinkled
Green
wrinkled
More than one trait?
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P generation
rryy
RRYY
RRYY
ry
Gametes RY
rryy
ry
RrYy
F1 generation
RrYy
Sperm
Sperm
1
2
F2 generation

Gametes RY
1
2
RY
1
2
1
4
ry
RY
Eggs
1
2
ry
1
4
RY
1
4
rY
1
4
Ry
Eggs
Actual results
contradict hypothesis
1
4
RY
1
4
rY
1
4
Ry
1
4
ry
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
9
16
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
ry
Actual results
support hypothesis
3
16
3
16
1
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
Dihybrid Cross
1. Determine the gametes that can be produced
RrYy
RRYY
RY
Ry
rY
ry
RY
RY
RY
RY
Dihybrid Cross
2. Fill in the Punnett Square
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Rules of Probability
• Mendel’s Laws follow the rules of probability
1. Events that follow rules of probability are
independent events
– If you flip a coin once, the probability of getting
heads is ½.
– If you flip a coin a second time, the probability of
getting heads is ½ regardless of what you got the
first time.
– If you already have two girls, the probability of a
third baby being a girl is still ½.
Rules of Probability
• Rule of multiplication:
The probability of two
events occurring
together is the
product of the
separate probabilities
of the independent
events
F1 genotypes
Bb male
Formation of sperm
Bb female
Formation of eggs
1
2
1
2
B
1
2
b
1
2
B
B
B
b
B
1
4
F2 genotypes
b
1
4
B
1
4
b
b
b
1
4
Test Cross
• Used to determine unknown genotypes
Black is dominant over brown
What are the possible GENOTYPES for a black lab?
Test Cross
• How can we figure out if the dog is BB or Bb?

Testcross:
Genotypes
bb
B_
Two possibilities for the black dog:
BB
B
Gametes
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
When it’s not so simple
• GENE LINKAGE: genes on the same chromosome
are likely to be inherited together
Tetrad
A
B
a
b
A
B
a
b
A
b
a
B
Crossing over
Gametes
When it’s not so simple
• GENE LINKAGE: genes on the same chromosome
are likely to be inherited together UNLESS
crossing over occurs between the linked genes
Tetrad
A
B
a
b
A
B
a
b
A
b
a
B
Crossing over
Gametes
When it’s not so simple
• Genes that are further apart will recombine
more frequently
Chromosome
g
c
l
17%
9%
9.5%
Recombination
frequencies
When it’s not so simple
• INCOMPLETE DOMINANCE: heterozygotes have
a unique or intermediate phenotype
When it’s not so simple
• INCOMPLETE DOMINANCE: heterozygotes have
a unique or intermediate phenotype
Genotypes:
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Phenotypes:
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
Incomplete Dominance Problem
• The color of fruit for a certain plant is
determined by two alleles. When two plants
with orange fruits are crossed the following
phenotypic ratios are present in the offspring:
25% red fruit, 50% orange fruit, 25% yellow
fruit. What are the genotypes of the parent
orange-fruited plants?
When it’s not so simple
• CODOMINANCE: both alleles are expressed
equally in the phenotype of a heterozygote
•RR = red hair
•rr = white hair
•Rr = red hairs and
white hairs
Codominant Alleles - Bloodtype
• Three alleles for blood type
–A
–B
–O
• This allele is for a protein (blood antigen) on
the outer surface of red blood cells
Codominant Alleles - Bloodtype
ANTIGENS
• A gene  A protein
• B gene  B protein
• O gene  no protein
ANTIBODIES
• Your body produces
antibodies to any protein
that you don’t have the
gene for
Codominant Alleles - Bloodtype
Blood Type & Multiple Alleles
Blood Type Problem
• Mary wants to know who the father of her
baby is. She has blood type A and the baby
has blood type AB. The potential fathers have:
– Father #1 is type A
– Father #2 is type B
– Father #3 is type O
Pleiotropy
A single gene influences multiple characteristics
• Example: sickle cell disease
– Allele causes production of
abnormal hemoglobin in
homozygotes
– Many severe physical effects
– Heterozygotes generally
healthy
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle-cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
Polygenic
the additive effects of two or more genes on a single
phenotypic characteristic
• Example: human skin color
– controlled by at least three
genes
Epistasis
The action of one gene is modified by the expression
of another gene
• Coat color in mice is
controlled by B gene
– B = black; b = brown
• Presence or lack of pigment
is controlled by C gene
– C = presence; c = albino
• If mouse has cc and can
produce no pigment, the
color of pigment doesn’t
matter
Epistasis
The action of one gene is modified by the expression
of another gene
• Albinism also found in other
organisms
Epistasis
The action of one gene is modified by the expression
of another gene
Environmental Affects
Nature vs nurture
• Many characteristics are a
result of genetic AND
environmental factors
• Only genetic characteristics
are inherited
Sex Linkage
• Sex Chromosomes = X
and Y
• What do you notice
about the X and the Y
chromosomes?
X chromosome has many genes
Y chromosome has few
Sex Linkage
• Female = ?
• Male = ?
X
X
Y
X
Riddle me this…
• Hemophilia is a
recessive sex-linked
disease. Are boys or
girls more likely to
suffer from hemophilia?
WHY?
XH
Xh
H
X
XHXH XHXh
Y
XHY
XhY
Sex Determination in other Animals
32
16
22
+
XX
22
+
X
76
+
ZW
76
+
ZZ
Pedigrees
• Used to trace
inheritance
Dd
Joshua
Lambert
Dd
Abigail
Linnell
D?
Abigail
Lambert
D?
John
Eddy
dd
Jonathan
Lambert
Dd
Dd
dd
D?
Hepzibah
Daggett
Dd
Elizabeth
Eddy
Dd
Dd
Dd
dd
Female Male
Deaf
Hearing
Hemophilia in the Royal Families of Europe
Human Inheritance
Inbreeding
• Does not create mutant
monster offspring
• Increases the likelihood
that both parents will
have the same recessive
genes that will be
expressed in offspring
Inbreeding vs hybrid vigor
Purebred animals
Hip Dysplasia
Inbreeding vs hybrid vigor
Hybrid animals
Less likelihood of being
homozygous recessive
Inbreeding among endangered
animals
Cheetahs
• 99% same genes
• poor sperm viability
• poor survival
• entire population
susceptible to diseases