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Fig. 07.01
Mendelian Genetics
Mendelian Genetics Outline
I. Mendel’s Ideas About Genetics
1. Experimental Design with garden peas
2. Monohybrid Crosses
1. Principle of Segregation
2. Principle of Dominance
3. Dihybrid cross
1. Principle of Independent Assortment
II. Extensions of Mendelian Genetics: Gene Interactions
1. Test Cross
2. Incomplete Dominance
3. Multiple Alleles
4. Epistasis
5. Polygenic Inheritance
III. Human Genetics
Fig. 07.05
Why peas?
1. Many pea varieties were available.
2. Small plants were easy to grow.
3. Peas self-fertilize.
4. Peas cross-fertilize.
Traits used by
Mendel had 2
Contrasting Forms
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Pea characteristics studied by Mendel
Monohybrid Cross
Flower color
Purple
White
Axial
Terminal
Pollen transferred
Parental
generation
Flower position
Seed color
Seed shape
Pod shape
Pod color
Yellow
Green
Round
Wrinkled
Anthers
removed
All purple flowers result
Inflated Constricted
Green
Yellow
Tall
Dwarf
Parental
generation
F1
generation
Stem length
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Results of Mendel’s Crosses
Monohybrid Cross
Parental
generation
White
Purple
F1 generation
X
F2 generation
Purple
Purple
3
Purple
White
1
2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Monohybrid Cross & Punnett Square
PP x pp
White
Flower
Parent
(pp)
Pp
Monohybrid Cross & Punnett Square
Phenotypic Ratio = 3:1
p
p
Gametes
Gametes
Genotypic Ratio = 1:2:1
P
Purple
Flower
Parent
(PP)
P
Pp
Pp
Gametes
P
Pp
Pp
F1 generation
Purple
Flower
Parent
(Pp)
Second Filial Generation (F2)
Purple
Flower
Parent
(Pp)
Gametes
P
p
PP
Pp
Pp
pp
p
F2 generation
Monohybrid Crosses
Genotype: Alleles of an individual
PP = homozygous dominant
Pp = heterozygous
pp = homozygous recessive
Phenotype: outward appearance
Purple or white pea flowers
Summary of Mendel’s Model of Inheritance
1. Parents transmit information about traits.
Each individual receives two factors
2. Mendel’s Principle of Segregation
Gametes can only receive one of two alleles.
3. Mendel’s Principle of Dominance
One factor can be preferentially expressed
4. Not all factors are identical for a given trait.
Alleles can be different
Homozygous or Heterozygous combinations
5. Alleles do not influence each other.
They remain discrete. They do not blend.
3
Examples of inherited traits in humans
Dominant Traits
Recessive Traits
Fig. 07.09
Test Cross: Confirmation of Segregation
Recessive Traits
1. Cystic fibrosis
2. Sickle cell anemia
Freckles
Dominant Traits
No freckles
1. Huntington Disease
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Dihybrid Cross
Hypothesis: Dependent
Dihybrid Cross
assortment?
ry
ry
Hypothesis:
Dependent assortment
RRYY x rryy
Parental
RRYY
RRYY
rryy
Gametes RY
X
RY X ry
ry
RrYy
F1 generation
RY
RrYy
RrYy
Sperm
Sperm
RrYy x RrYy
RY
1
1
rY
RY
4
4
1
1
ry
RY
2
2
ry
1
RY
4
RRYY
1
RY
2
Eggs
F2 generation
RrYy
rryy
F2 generation
RY
ry
rryy
P generation
RY
F1 generation
Hypothesis:
Independent assortment
1
ry
2
1
4
rY
1
4
Ry
1
4
ry
Eggs
Actual results
contradict hypothesis
1
1
ry
Ry
4
4
RyYY RRYy RrYy
RrYY
rrYY
RrYy
rrYy
RRYy
RrYy RRyy
Rryy
Rryy
rryy
RrYy
rrYy
Actual results
support hypothesis
9
16
3
16
3
16
1
16
Yellow round
Green round
Yellow wrinkled
Green wrinkled
4
Dihybrid Cross
Mendel’s Second Law of Heredity:
Principle of Independent Assortment
Hypothesis: Independent assortment
F1 cross
RRYY x rryy
RrYy X RrYy
RY
rY
Ry
Parents
1. In a dihybrid cross, alleles of each gene assort
independently.
ry
RY
RrYy
F1
rY
Yellow round
Green Round
Yellow wrinkled
Green wrinkled
Ry
ry
Incomplete Dominance – in Japanese Four O’Clock
Parental
F1
F2
2. Fate of one pair of alleles associated with one trait
does not influence the fate of another pair of
alleles associated with a different trait.
3. Genes located on different chromosomes assort
independently.
Incomplete Dominance
In Japanese Four O’Clock
heterozygote is intermediate in phenotype
between the 2 homozygotes
5
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Epistasis
Incomplete Dominance in Humans - Hypercholesterolemia
ee
No dark pigment in fur
Genotypes
HH
Hh
hh
Homozygous
for ability to make
LDL receptors
Heterozygous
Homozygous
for inability to make
LDL receptors
Yellow Lab
E_bb
eebb
Yellow fur
LDL
LDL
receptor
Cell
Normal
Mild disease
Yellow fur
E_B_
Chocolate Lab Black Lab
Brown fur
Black fur
Pigment alleles
B = Black fur
e = chocolate/brown
Severe disease
Antigens, Blood Type & Multiple Alleles
O Type Blood
eeB_
Epistasis alleles
E = express pigment in fur
e = pigment not expressed
Phenotypes
E_
Dark pigment in fur
B Type Blood
Multiple Alleles
IA = galactosamine antigen on RBC surface
IB = galactose antigen on RBC surface
i = no antigens on RBC surface
AB Type Blood
A Type Blood
Glycolipid
Sugar Exhibited
Phenotype Genotype
A
A
A
I I or I i Galactosamine
A
IB IB or IB i Galactose
B
IAIB
Galactosamine and
AB
galactose
ii
None
O
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Multiple alleles for ABO blood groups
Blood
Group
(Phenotype) Genotypes
Antibodies
Present in
Blood
ABO blood groups, Antigens and Antibodies
Reaction When Blood type Below Is
Mixed with blood type on far left column
O
A
B
AB
Galactosamine
O
A
B
Galactose
Anti-A
Anti-B
ii
IA IA
or
Anti-B
IA i
IB IB
or
Anti-A
IB i
AB
IA IB
—
= agglutination
= no agglutination
Rh factor
Polygenic Inheritance
Rh factor = protein
Genotypes
Rh+/ Rh+
Rh+/ RhRh-/ Rh-
Phenotypes
Rh positive
Rh positive
Rh negative
Rhesus monkeys
7
A model for polygenic inheritance of skin color
P generation
×
aabbcc
(very light)
AABBCC
(very dark)
F1 generation
Continuous Variation
Skin Color &
×
Polygenic Inheritance
AaBbCc
AaBbCc
Sperm
1
8
1
8
1
8
1
8
1
8
1
8
Eggs
1
64
1
8
Fraction of population
1
8
1
8
F2 generation
Environmental Influences
1
8
1
8
6
64
15
64
20
64
15
64
6
64
1
64
20
64
15
64
1
8
1
8
1
8
1
8
1
8
6
64
1
64
Skin color
Genetic Counseling
Human
Genetics
™ Cell cultures can reveal genetic disorders based on:
¾ alterations in chromosome number
¾ proper enzyme functioning
¾ association with known genetic markers
™ When?
¾ Before birth
¾ After birth
¾ Adult
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Some Important Genetic Disorders
Fig. 07.24
Sickle Cell Anemia
1100+ Recessive disorders
1400+ Dominant disorders
Phenotypes: Carrier X Carrier
Alleles: S = normal
s = Sickle cell
Genotypes:
Ss X Ss
Sickle-cell disease – Pleiotropic (multiple) effects of a single
human gene
Ultrasound
monitor
Red blood cells to become sickle-shaped
Fetus
Placenta
Anemia
Heart
failure
Paralysis
5,555 ×
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Needle inserted
through abdomen to
extract amniotic fluid
Chorionic villus sampling
Extract tissue
from chorionic villi
Ultrasound
monitor
Fetus
Placenta
Uterus
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Impaired
mental
function
Amniocentesis
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Sickle cells
Physical
weakness
Testing a fetus for genetic disorders
Uterus
Cervix
Amniotic
fluid
Cervix
Centrifugation
Chorionic
villi
Fetal
cells
Fetal
cells
Several
weeks
Tests
Several
hours
Kidney
failure
Karyotyping
9
Fig. 13.35
Prenatal Diagnosis
Autosomal Nondisjunction or Aneuploidy
Pedigree Analysis
Autosomal recessive
aa = affected
Aa = carrier (normal)
AA = normal
Adult Screening
Hexoseaminidase and Tay-Sachs Disease
Pedigree
Analysis
Autosomal
Dominant
1.
2.
3.
4.
Affected children can have parents with a normal phenotype
Heterozygotes have a normal phenotype
Two affected parents will always have affected children
Affected individuals who have non-carrier spouses will have
normal children
5. Close relatives who have children are more likely to have
affected children.
6. Equal frequency of both males and females
10
A test for red-green color blindness
Pedigree showing inheritance of deafness in a family from Martha’s Vineyard
Female Male
Fig. 07.23
Pedigree
Analysis
Sex or
X-linked
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END Mendelian & Human
Genetics
12