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Mendel’s Laws
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1
Results of Monohybrid Crosses
1. Inheritable factors or genes are
responsible for all heritable characteristics
2. Phenotype is based on Genotype
3. Each trait is based on two genes, one from
the mother and the other from the father.
4. True-breeding individuals are homozygous
( both alleles) are the same.
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2
Law of Dominance
In a cross of parents that are
pure for contrasting traits, 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)
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3
Law of Dominance
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4
Law of Segregation
• During the formation of gametes (eggs or
sperm), the two alleles responsible for a
trait separate from each other.
• Alleles for a trait are then "recombined"
at fertilization, producing the genotype for
the traits of the offspring.
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Applying the Law of Segregation
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6
Law of Independent Assortment
• Alleles for different traits are
distributed to sex cells (& offspring)
independently of one another.
• This law can be illustrated using
dihybrid crosses.
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Dihybrid Cross
• A breeding experiment that tracks the
inheritance of two traits.
• Mendel’s “Law of Independent
Assortment”
• a. Each pair of alleles segregates independently
during gamete formation
• b. Formula: 2n (n = # of heterozygotes)
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Question:
How many gametes will be produced for the
following allele arrangements?
• Remember: 2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq
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Answer:
1. RrYy: 2n = 22 = 4 gametes
RY
Ry
rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes
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Dihybrid Cross
• Traits: Seed shape & Seed color
• Alleles: R round
r wrinkled
Y yellow
y green
•
RrYy
x
RrYy
RY Ry rY ry
RY Ry rY ry
All possible gamete combinations
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Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
RRYY
RRYy
RrYY
RrYy
Round/Yellow:
RRYy
RRyy
RrYy
Rryy
Round/green:
RrYY
RrYy
RrYy
Rryy
rY
rrYY
rrYy
ry
rrYy
rryy
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9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
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Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
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Test Cross
• A mating between an individual of unknown
genotype and a homozygous recessive individual.
• Example: bbC__ x bbcc
•
•
•
BB = brown eyes
Bb = brown eyes
bb = blue eyes
•
•
•
CC = curly hair
Cc = curly hair
cc = straight hair
bC
b___
bc
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Test Cross
• Possible results:
bc
bC
b___
C
bbCc
bbCc
or
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bc
bC
b___
c
bbCc
bbcc
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Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16
pods
3/16
pods
3/16
pods
1/16
pods
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round seeds & green
round seeds & yellow
wrinkled seeds & green
wrinkled seeds & yellow
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Incomplete Dominance
• F1 hybrids have an appearance somewhat in
between the phenotypes of the two parental
varieties.
• Example: snapdragons (flower)
• red (RR) x white (rr)
r
r
•
•
RR = red flower R
rr = white flower
R
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Incomplete Dominance
r
r
R
Rr
Rr
R
Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
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Incomplete Dominance
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Codominance
• When both alleles contribute to the
phenotype of an organism
Ex. Speckled Chickens
Codominance
• Two alleles are expressed (multiple alleles) in
heterozygous individuals.
• Example: blood type
•
•
•
•
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
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Codominance Problem
• Example:
homozygous male Type B (IBIB)
•
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
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1/2 = IAIB
1/2 = IBi
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Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
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1/2 = IAi
1/2 = IBi
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Codominance
• Question:
If a boy has a blood type O and his sister
has blood type AB, what are the genotypes
and phenotypes of their parents?
• boy - type O (ii) X girl - type AB (IAIB)
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25
Codominance
• Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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Human Blood Types
Phenotype
Genotype
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
Question
• A man is suing his wife on grounds of
infidelity. The man claims that the child is
blood type O and therefore must be
fathered by someone else. Can he use
this evidence in court if he and his wife
both have heterozygous B genotypes?
• Show the cross of the two parents
Know that :
• Human hair is inherited by
incomplete dominance. Human hair
may be curly (CC) or straight (cc). The
heterozygous genotype (Cc)
produces wavy hair. Show a cross
between two parents with wavy hair
Curly Hair
Early geneticists reported that
curly hair was dominant and
strait hair was recessive. More
recent scientists believe that
more than one gene may be
involved.
Polygenic Traits
• Traits controlled by two or more
genes
Ex.) eye color, skin color
Sex-linked Traits
• Traits (genes) located on the sex
chromosomes
• Sex chromosomes are X and Y
• XX genotype for females
• XY genotype for males
• Many sex-linked traits carried on X
chromosome
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Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
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Sex-linked Trait Problem
• Example: Eye color in fruit flies
•
(red-eyes male) x (white-eyes female)
XRY
x
XrXr
• Remember: the Y chromosome in males does not
carry traits.
• RR = red eyes
Xr
Xr
• Rr = red eyes
• rr = white eyes
• XY = male
XR
• XX = female
Y
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Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyes
female
50% white eyes
male
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36
Female Carriers
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Genetic Practice Problems
Breed the P1 generation
• tall (TT) x dwarf (tt) pea plants
t
t
T
T
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Solution:
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
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39
Breed the F1 generation
• tall (Tt) vs. tall (Tt) pea plants
T
t
T
t
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Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
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41
Genetics and the Environment
• The characteristics of any organism, is
not only determined by the genes it
inherits
• Characteristics are determined by
interactions between genes and the
environment
• Ex. genes may affect a plants height but
the same characteristic is influenced by
climate, soil conditions and availability of
water.
END