Download Mendelian Genetics

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.
Characteristics used
by Mendel had 2
Contrasting Forms
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Pea S by Mendel
Monohybrid Cross
Flower color
Purple
White
Flower position
Seed color
Seed shape
Pod shape
Pod color
Axial
Terminal
Yellow
Green
Round
Wrinkled
Inflated Constricted
Green
Yellow
Tall
Dwarf
Stem length
Pollen transferred
Parental
generation
Parental
generation
Anthers
removed
All purple flowers result
F1
generation
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Results of Mendel’s Crosses
Monohybrid Cross
Parental
generation
Purple
White
F1 generation
X
F2 generation
Purple
Purple
Purple
White
1
3
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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
Monohybrid Cross
Genotype: Alleles of an individual
PP = homozygous dominant
Pp = heterozygous
pp = homozygous recessive
Phenotype: outward appearance
Purple or white pea flowers
Purple
Flower
Parent
(Pp)
Second Filial Generation (F2)
Purple
Flower
Parent
(Pp)
Gametes
P
p
PP
Pp
Pp
pp
p
F2 generation
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
Dominant allele – always expressed
Recessive allele – only expressed in pairs
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.
Examples of inherited traits in humans
Dominant Traits
Recessive Traits
Fig. 07.09
Test Cross: Confirmation of Segregation
Recessive Traits
1. Cystic fibrosis
2. Tay-Sachs disease
Freckles
No freckles
Dominant Traits
1. Huntington Disease
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Dihybrid Cross
Hypothesis: Dependent
Dihybrid Cross
assortment?
ry
Hypothesis:
Dependent assortment
rryy
RRYY
ry
RRYY x rryy
Parental
F1 generation
RY
RrYy x RrYy
RrYy x RrYy
RrYy
Sperm
Sperm
RrYy x RrYy
RY
1
1
rY
RY
4
4
1
1
ry
2 RY 2
ry
1
RY
2
Eggs
RrYy
rryy
rryy
RY X ry
RY X ry
Gametes
F2 generation
RRYY
P generation
RY
F1 generation
Hypothesis:
Independent assortment
F2 generation
RY
1 ry
2
RyYY RRYy RrYy
1
4
rY
RrYY
rrYY
RrYy
rrYy
1
4
Ry
RRYy
RrYy RRyy
Rryy
1
4
ry
RrYy
rrYy
rryy
Eggs
ry
F2 generation
1
1
ry
Ry
4
4
1
RY
4
RRYY
Rryy
9
16
3
16
3
16
1
16
Yellow round
Green round
Yellow wrinkled
Green wrinkled
Actual results
support hypothesis
Mendel’s Second Law of Heredity:
Principle of Independent Assortment
Incomplete Dominance – in Japanese Four O’Clock
Parental
1. In a dihybrid cross, alleles of each gene assort
independently.
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.
F1
F2
Incomplete Dominance
Incomplete Dominance in Humans - Hypercholesterolemia
In Japanese Four O’Clock
heterozygote is intermediate in phenotype
between the 2 homozygotes
Genotypes
HH
Hh
hh
Homozygous
for ability to make
LDL receptors
Heterozygous
Homozygous
for inability to make
LDL receptors
Phenotypes
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
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Epistasis
ee
No dark pigment in fur
E_
Dark pigment in fur
What did we learn from Mendel?
Yellow Lab
E_bb
eebb
Yellow fur
eeB_
Yellow fur
Epistasis alleles
E = express pigment in fur
e = pigment not expressed
E_B_
Chocolate Lab Black Lab
Brown fur
Black fur
Pigment alleles
B = Black fur
e = chocolate/brown
Antigens, Blood Type & Multiple Alleles
O Type Blood
B Type Blood
Multiple Alleles
IA = galactosamine antigen on RBC surface
IB = galactose antigen on RBC surface
Glycolipid
i = no antigens on RBC surface
AB Type Blood
A Type Blood
Glycolipid
Phenotype Genotype
Sugar Exhibited
A
IA IA or IA i Galactosamine
B
IB IB or IB i Galactose
AB
IAIB
Galactosamine and
galactose
O
ii
None
ABO blood groups, Antigens and Antibodies
Multiple alleles for ABO blood groups
Blood
Group
(Phenotype) Genotypes
Galactosamine
Antibodies
Present in
Blood
Reaction When Blood type Below Is
Mixed with blood type on far left column
O
A
B
AB
Galactose
O
A
B
Anti-A
Anti-B
ii
IA IA
or
Anti-B
IA i
IB IB
or
Anti-A
IB i
AB
IA IB
—
= agglutination
Global Frequency of Alleles in Human Species
Rh factor
Rh factor = protein
Genotypes
Rh+/ Rh+
Rh+/ RhRh-/ Rh-
= no agglutination
Phenotypes
Rh positive
Rh positive
Rh negative
Allele
Frequency (%)
A
B
O
21
16
63
Global distribution of the B blood allele
Rhesus monkeys
Global distribution of the A blood allele
Global distribution the O blood allele
Continuous Variation & Polygenic Inheritance
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
1
8
1
8
Eggs
1
64
1
8
Fraction of population
1
8
1
8
F2 generation
Environmental Influences
×
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
™Cell culture analysis reveals genetic disorders:
¾Karyotype Æ alterations in chromosome number
¾Biochemical Æ proper enzyme functioning
¾Molecular genetic Æ association with known genetic markers
Human
Genetics
Some Important Genetic Disorders
1100+ Recessive disorders
1400+ Dominant disorders
™When can analysis occur?
¾Before birth
¾After birth
¾Adult
Sickle Cell Anemia
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
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Ultrasound
monitor
Physical
weakness
Impaired
mental
function
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
10-12 weeks into pregnancy
Extract tissue
from chorionic villi
Fetus
Placenta
Uterus
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Chorionic villus sampling
Ultrasound
monitor
Fetus
Placenta
Red blood cells to become sickle-shaped
Sickle cells
Testing a fetus for genetic disorders
Amniocentesis
Needle inserted
15-20 weeks into pregnancy through abdomen to
extract amniotic fluid
Uterus
Cervix
Amniotic
fluid
Cervix
Centrifugation
Chorionic
villi
Fetal
cells
Fetal
cells
Several
weeks
Tests
Several
hours
Kidney
failure
Karyotyping
Fig. 13.35
Prenatal Diagnosis
Autosomal Nondisjunction or Aneuploidy
Pedigree Analysis
Autosomal recessive
aa = affected
Aa = carrier (normal)
AA = normal
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
Adult Screening
Hexoseaminidase and Tay-Sachs Disease
Pedigree
Analysis
Autosomal
Dominant
A test for red - green color blindness
Pedigree showing inheritance of deafness
1
2
3
Female Male
Fig. 07.23
Pedigree
Analysis
Sex or
X-linked
END Mendelian & Human
Genetics