Download Mendel and inheritance

1. Gregor Mendel
Peas, please
2. Segregation of alleles
Shown by monohybrid crosses
3. Independent assortment of alleles
shown by dihybrid crosses
• Gregor Mendel - pea research done 1856-1863
Basic ideas:
a. Genetic elements come in
pairs
b. Elements do not change
over generations
a. Pairs separate when gametes
form
Figure 9.2Ax
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
White
• Pea plants:
1
Removed
stamens
from purple
flower
• Self- or cross-pollinate
Stamens
• Rapid life cycle
Carpel
PARENTS
(P)
• Variety of traits
2 Transferred
Purple
pollen from
stamens of white
flower to carpel
of purple flower
3 Pollinated carpel
matured into pod
4
• Shown here:
cross-fertilization
Figure 9.2C
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OFFSPRING
(F1)
Planted
seeds
from pod
Phenotype - appearance or function of body
Genotype - genes that determine the phenotype
Genetic terms
•
•
•
•
P = parental generation
F1= first progeny generation (filial)
F2= second progeny generation
Monohybrid cross = parents differ in one
gene
female
YY YY
YY
male
YY
yy
P generation
y y
Y Y
Y
yy
yy yy
y
Y
y
Y
Yy
Y
y
y
Yy
Yy
possible
outcomes
in fertilization
Yy
Yy Yy
Yy
Yy
Y
F1 generation:
Y
YY
y
Yy
y
Punnett square shows
parental gametes
and genotypes
of next generation
Yy
yy
F2 generation
YY
Yy
Yy
yy
three
genotypes
two
phenotypes
yellow
green
1st law - segregation of alleles
• Cells contain 2 copies of each gene (alleles)
• Alleles do not blend (dominant, recessive)
• Alleles separate during gamete formation
(meiosis)
F1 generation
Yy
self-pollination
YY
Yy
Yy
Yy
F2 generation
YY Yy
Yy
yy
yy
“pure” yellow
“pure” green
mixed
YY
YY
YY
YY YY
YY
YY
Yy Yy
yy
Yy
F3 generation
YY
Yy
yy
yy
yy
yy
yy
yy
yy
Monohybrid crosses in Mendel’s peas
What happens in dihybrid crosses?
- parents differ in genes for 2 traits
HYPOTHESIS:
DEPENDENT ASSORTMENT
RRYY
P
GENERATION
rryy
Gametes
RRYY
ry
RY
rryy
Gametes
ry
RY
RrYy
F1
GENERATION
Eggs
1/
HYPOTHESIS:
INDEPENDENT ASSORTMENT
2
1/
2
RY
1/
2
RrYy
RY
1/
ry
Sperm
2
1/
ry
1/
F2
GENERATION
1/
Eggs
1/
4
4
4
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RY
1/
4
RrYy
rY
1/
RrYY
rrYY
rrYy
Figure 9.5A
4
RRYY
RRYy
ACTUAL
RESULTS
SUPPORT
HYPOTHESIS
1/
RrYY
RrYy
Actual results
contradict
hypothesis
RY
rY
Ry
ry
4
RrYy
RrYy
RRyy
Rryy
rryy
Ry
1/
RrYy
rrYy
Rryy
4
4
ry
9/
16
3/
16
3/
16
1/
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
Law of Independent Assortment
• During gamete formation, genes for
different traits separate independently
into gametes
• Why? random alignment of homologues at
Meiosis I
• A sperm or egg carries only one allele of
each pair
• Independent assortment of two genes in the
Labrador retriever
Blind
Blind
PHENOTYPES
GENOTYPES
Black coat,
normal
vision
B_N_
MATING OF HETEROZYOTES
(black, normal vision)
PHENOTYPIC RATIO
OF OFFSPRING
9 black coat,
normal vision
Black coat,
blind (PRA)
B_nn
Chocolate coat,
normal vision
bbN_
BbNn
BbNn
3 black coat,
blind (PRA)
3 chocolate coat,
normal vision
Figure 9.5B
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Chocolate coat,
blind (PRA)
bbnn
1 chocolate coat,
blind (PRA)
Chromosome behavior accounts for Mendel’s
principles
Figure 9.17
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The offspring of a testcross can reveal the
genotype of a parent.
TESTCROSS:
GENOTYPES
B_
bb
Two possibilities for the black dog:
BB
b
OFFSPRING
Bb
B
GAMETES
Figure 9.6
or
Bb
All black
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B
b
Bb
b
bb
1 black : 1 chocolate
A
B
a
b
a
B
A B
a
b
Tetrad
A
b
Crossing over
Gametes
• Genes on the same chromosome tend to be
inherited together = linked genes
• Crossing over produces gametes with
recombinant chromosomes
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Geneticists use crossover data to map genes
• Crossing over is more likely to occur between
genes that are farther apart
– Recombination frequencies
Chromosome
g
c
l
17%
9%
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
9.5%
Figure 9.20B
VARIATIONS ON MENDEL’S PRINCIPLES
P GENERATION
White
rr
Red
RR
Incomplete dominance
Gametes
• an offspring’s
phenotype is
intermediate
between the
phenotypes of its
parents
R
r
Pink
Rr
F1 GENERATION
1/
1/
Eggs
1/
F2 GENERATION
2
2
2
R
1/
2
r
1/
R
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R
Red
RR
r
Pink
Rr
Sperm
1/
Pink
rR
White
rr
Figure 9.12A
2
2
r
• Incomplete dominance in human
hypercholesterolemia
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
Figure 9.12B
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Severe disease
Many genes have more than two alleles
in the population
The three alleles for ABO blood type in humans is
an example
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Blood contains genetically determined proteins
A foreign protein (antigen) may be attacked by the
immune system
Blood is “typed” by using antibodies that will cause
blood with certain proteins to clump (agglutination)
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• ABO blood types
Figure 9.13x
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• pleiotropy is when a single gene affects
phenotype in many ways
– Ex. Marfan syndrome - fibrillin
The gene’s effects may be dependent on
environment, and not be simultaneous.
– Ex. sickle-cell disease - hemoglobin
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Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Breakdown of red
blood cells
Physical
weakness
Anemia
Clumping of cells
and clogging of
small blood vessels
Heart
failure
Pain and
fever
Brain
damage
Pneumonia
Impaired
Paralysis
and other
mental
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Inc. publishing as Benjamin Cummings infections
Accumulation of
sickled cells in spleen
Damage to
other organs
Rheumatism
Spleen
damage
Figure
Kidney9.14
failure
A single characteristic may be influenced by many
genes
• This situation creates a continuum of
phenotypes
• Quantitative traits
– Example: skin color, height
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P GENERATION
aabbcc
AABBCC
(very light) (very dark)
F1 GENERATION
Eggs
Sperm
Fraction of population
AaBbCc AaBbCc
Skin pigmentation
F2 GENERATION
Figure 9.16
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Environmental Effects on Phenotype
• Genotype and environment can interact to
affect phenotype
– Himalayan rabbit ice pack experiment
– Transplantation of plant cuttings to different
elevations
– Human depression
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Sex-linked genes exhibit a unique pattern of
inheritance
• All genes on the sex chromosomes are said to be
sex-linked
– the X chromosome carries many genes unrelated
to sex
– Ex. Fruit fly eye
color
Figure 9.22A
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Sex-linked disorders affect mostly males
• Most sex-linked human
disorders are due to
recessive alleles
– Ex: hemophilia,
red-green color blindness
– mostly in males
Figure 9.23A
– If a male receives a single X-linked recessive
allele from his mother, he will have the disorder;
while a female has to receive the allele from both
parents to be affected
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• A high incidence of hemophilia has plagued the
royal families of Europe
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
Figure 9.23B
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• Chromosomes determine sex in many species
– The X-O system
– The Z-W system
– Chromosome number
Figure 9.21B-D
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Variations on Mendel’s Principles
• Codominance, multiple alleles
• Pleiotropy
• Polygenic traits
• Sex-linked genes
• Environmental effects
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Connection: Fetal testing can spot many inherited
disorders early in pregnancy
• Karyotyping and biochemical tests of fetal cells
and molecules can help people make
reproductive decisions
– Fetal cells can be obtained through
amniocentesis
Amniotic
fluid
Amniotic
fluid
withdrawn
Centrifugation
Fluid
Fetal
cells
Fetus
(14-20
weeks)
Biochemical
tests
Placenta
Figure 9.10A
Uterus
Cervix
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Several
weeks later
Cell culture
Karyotyping
• Chorionic villus sampling is another procedure
that obtains fetal cells for karyotyping
Fetus
(10-12
weeks)
Several hours
later
Placenta
Suction
Chorionic villi
Fetal cells
(from chorionic villi)
Karyotyping
Some
biochemical
tests
Figure 9.10B
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• Examination of the fetus with ultrasound is
another helpful technique
Figure 9.10C, D
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Genetic traits in humans can be tracked
through family pedigrees
• The inheritance of many
human traits follows
Mendel’s principles and
the rules of probability
Figure 9.8A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Family pedigrees are used to determine
patterns of inheritance and individual
genotypes
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
Figure 9.8B
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Hearing
• A few are caused by dominant alleles
– Examples: achondroplasia, Huntington’s disease
Figure 9.9B
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Table 9.9
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Connection: Many inherited disorders in humans
are controlled by a single gene
• Most such
disorders are
caused by
autosomal
recessive alleles
– Examples:
cystic fibrosis,
sickle-cell
disease
Normal
Dd
PARENTS
Normal
Dd
D
D
Eggs
Sperm
DD
Normal
d
OFFSPRING
d
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Figure 9.9A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings