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Genetics
1
Pound Puppy Genetics
Look at the Pack… What
traits do they have in
common?
copyright cmassengale
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***Genotype vs. Phenotype***
Alleles:
L = long ears dominate
l= short ears recessive
All genes occur in pairs, 2 alleles affect a trait
Allele for Long Ears
Locus for ear length gene
Homologous
pair of
chromosomes
Allele for Short Ears
Genotypes
LL
Ll
ll
Phenotypes
LONG
LONG
short
Homozygous 2 dominant or 2 recessive genes (e.g. LL or ll)
Heterozygous one dominant & one recessive allele (e.g. Li)
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Traits
Phenotype
Genotype –
Dominate/Recessive
copyright cmassengale
4
Genetic Vocabulary
Genetics: The scientific study of heredity
Heredity - passing of traits from parent to offspring
Trait - any characteristic that can be passed from parent to offspring
Genes: Point on a chromosome that controls the trait.
Allele: Alternate forms of a gene. A or a
Genotype: combination of alleles an organism has. (genetic traits e.g. RR, Rr, rr)
Phenotype: How an organism appears. (physical traits e.g. red, white)
Dominant: An allele which is expressed (masks the other capital letter (R)
Recessive: An allele which is present but remains unexpressed (masked lowercase letter (r)
Homozygous: Both alleles for a trait are the same.
Heterozygous: The organism's alleles for a trait are different.
Probability : The mathematical chance that an event will happen.
Meiosis :The cell division that produces sex cells.
Mutation : A change in the type/order of the bases in DNA: deletion, insertion or substitution.
Natural Selection : The process by which organisms with favorable traits survive and reproduce
at a higher rate than organisms without favorable traits.
Evolution :The process by which population accumulate inherited changes over time.
Genetic Vocabulary
Generations:
P1 = parental generation
F1 = 1st filial generation, progeny of the P generation
F2 = 2nd filial generation, progeny of the F1 generation (F3
and so on)
Crosses:
Monohybrid cross = single trait
Dihybrid cross = two traits
*Gregor Mendel*
(1822-1884)
Austrian monk
considered “Father
of Genetics”
Studied traits in
pea plants
Developed the
Laws of Inheritance
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 Worked with large numbers
of Plants
 counted all offspring
 made predictions and
tested them
Excellent Experimentalist
 controlled growth conditions
 focused on traits that were
easy to score
 chose to track only those
characters that varied in an
“either-or” manner
Mendel’s Methods
Statistical Analyses:
Mendel’s Studied Traits
Pea Plant Traits
Trait
Dominate
Recessive
Seed Shape
Round (R)
wrinkled (r)
Seed Color
Yellow (Y)
green (y)
Pod Shape
Smooth (S)
wrinkled (s)
Pod Color
Green (G)
Yellow (g)
Seed Coat Color
Gray (G)
White (g)
Flower Position
Axial (A)
Terminal (a)
Plant Height
Tall (T)
Short (t)
Flower Color
Purple (P)
white (p)
copyright cmassengale
10
Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
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Mendel’s Experimental Results
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***Genotype vs. Phenotype***
Alleles:
F = purple flower dominate
f = white flower recessive
All genes occur in pairs, 2 alleles affect a trait
Allele for purple flowers
Locus for flower-color gene
Homologous
pair of
chromosomes
Allele for white flowers
Genotypes
FF
Ff
ff
Phenotypes
Purple
Purple
White
Homozygous 2 dominant or 2 recessive genes (e.g. FF or ff)
Heterozygous one dominant & one recessive allele (e.g. Ff)
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Phenotype vs Genotype
Punnett Square
Used to help
solve genetics
problems
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Mendel’s Laws
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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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Applying Law of Dominance
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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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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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Applying the Law
of Independent Assortment
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
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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 round seeds & green pods
3/16 round seeds & yellow pods
3/16 wrinkled seeds & green
pods
1/16 wrinkled seeds & yellow
pods
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END DAY 1
copyright cmassengale
24
PUNNET
SQUARES
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Monohybrid
Crosses
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P1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Wrinkled seeds
RR
x
rr
r
r
Genotype: Rr
Phenotype: Round
R
Rr
Rr
Genotypic
Ratio: All alike
R
Rr
Rr
Phenotypic
Ratio: All alike
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P1 Monohybrid Cross Review
 Homozygous dominant x
Homozygous recessive
 Offspring all Heterozygous (hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE
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F1 Monohybrid Cross
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Round seeds
Rr
x
Rr
R
R
r
R
R
Rr
r
Rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
rr
P.Ratio: 3:1
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F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
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What Do the Peas Look Like?
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…And Now the Test Cross
Mendel then crossed a pure & a
hybrid from his F2 generation
This is known as an F2 or test cross
There are two possible testcrosses:
Homozygous dominant x Hybrid
Homozygous recessive x Hybrid
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F2 Monohybrid Cross (1st)
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Round seeds x Round seeds
RR
x
Rr
R
r
Genotype: RR, Rr
Phenotype: Round
R
R
R
R
R
R
Rr
Genotypic
Ratio: 1:1
Rr
Phenotypic
Ratio: All alike
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F2 Monohybrid Cross (2nd)
Trait: Seed Shape
Alleles: R – Round r – Wrinkled
Cross: Wrinkled seeds x Round seeds
rr
x
Rr
R
r
Rr
r
rr
Genotype: Rr, rr
Phenotype: Round
& Wrinkled
G. Ratio: 1:1
r
Rr
rr
P.Ratio: 1:1
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F2 Monohybrid Cross Review
 Homozygous x heterozygous(hybrid)
 Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
 Called Test Cross because the
offspring have SAME genotype as
parents
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Monohybrid cross
(cross with only 1 trait)
Problem:
Using this is a several step process, look at the
following example
Tallness (T) is dominant over shortness (t) in
pea plants. A Homozygous tall plant (TT) is
crossed with a short plant (tt). What is the
genotypic makeup of the offspring? The
phenotypic makeup?
Punnett process
1.
Determine alleles of
each parent, these are
given as TT, and tt
respectively.
2. Take each possible
allele of each parent,
separate them, and place
each allele either along
the top, or along the side
of the punnett square.
Punnett process continued
Lastly, write the letter
for each allele across
each column or down
each row.
The resultant mix is the
genotype for the
offspring. In this case,
each offspring has a Tt
(heterozygous tall)
genotype, and simply a
"Tall" phenotype.
Punnett process continued
Here we have some more
interesting results: First
we now have 3 genotypes
(TT, Tt, & tt) in a 1:2:1
genotypic ratio. We now
have 2 different
phenotypes (Tall & short)
in a 3:1 Phenotypic ratio.
This is the common
outcome from such
crosses.
Experiment: Pea Shape
Made the following observations
(example given is pea shape)
When he crossed a round pea
and wrinkled pea, the offspring
(F1 gen.) always had round
peas.
When he crossed these F1
plants, however, he would get
offspring which produced round
and wrinkled peas in a 3:1 ratio.
Genetic Practice
Problems
41
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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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
t
TT
Tt
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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Did the observed ratio match the
theoretical ratio?
The theoretical or expected ratio of plants
producing round or wrinkled seeds is 3
round :1 wrinkled
Mendel’s observed ratio was 2.96:1
The discrepancy is due to statistical error
The larger the sample the more nearly the
results approximate to the theoretical ratio
46
Dihybrid
Crosses
47
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
49
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
50
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
51
Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
52
Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
RrYy
rrYY
rrYy
wrinkled/green: 1
ry
RrYy
Rryy
rrYy
rryy
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
54
Dihybrid crosses
Dihybrid crosses are made when
phenotypes and genotypes composed of 2
independent alleles are analyzed.
Process is very similar to monohybrid
crosses.
Example:
2 traits are being analyzed
Plant height (Tt) with tall being dominant
to short,
Flower color (Ww) with Purple flowers
being dominant to white.
Dihybrid cross example
The cross with a pure-breeding (homozygous)
Tall, Purple plant with a pure-breeding Short,
white plant should look like this.
F1 generation
Mendel’s Monohybrid Cross
White
(pp)
Purple
(Pp)
Gametes
p
Purple
(PP)
p
P
Gametes
Gametes
Purple
(Pp)
P
p
PP
Pp
P
Pp
Pp
Gametes
p
P
Pp
Pp
F1 generation
All purple
Pp
pp
F2 generation
¾ purple, ¼ white
Smooth and wrinkled
parental seed strains
crossed.
Punnett square
F1 genotypes: 4/4 Ss
F1 phenotypes: 4/4
smooth
F2 With Dependent Assortment:
R
Y
r
y
R
Y
RR
YY
Rr
Yy
r
y
Rr
Yy
rr
yy
Ratio is 3 round, yellow : 1 wrinkled, green
Independent Segregation
Alleles at the 2 gene loci segregate (separate) independently, and are NOT
transmitted as a unit. Therefore, each plant would produce gametes with
allele combinations that were not present in the gametes inherited from
its parents:
Parents
Parental Gametes
F1 Offspring
F1 Offspring’s
Gametes
R
Y
R
y
RR
YY
rr
yy
R
Y
r
y
Rr
Yy
r
Y
r
y
What is the expected phenotypic ratio for the F2?
A Dihybrid Cross
How are two characters transmitted from parents to offspring?
As a package?
Independently?
A dihybrid cross
Illustrates the inheritance of two characters
Produces four phenotypes in the F2 generation
P Generation
EXPERIMENT Two true-breeding pea plants—
one with yellow-round seeds and the other with
green-wrinkled seeds—were crossed, producing
dihybrid F1 plants. Self-pollination of the F1 dihybrids,
which are heterozygous for both characters,
produced the F2 generation. The two hypotheses
predict different phenotypic ratios. Note that yellow
color (Y) and round shape (R) are dominant.
YYRR
yyrr
Gametes
F1 Generation
YR

Hypothesis of
dependent
assortment
yr
YyRr
Hypothesis of
independent
assortment
Sperm
Sperm
1⁄ YR 1⁄ yr
2
2
Eggs
1⁄
YR
F2 Generation 2
YYRR YyRr
(predicted
offspring)
1 ⁄ yr
2
YyRr yyrr
3⁄
4
CONCLUSION The results support the hypothesis
ofindependent assortment. The alleles for seed color
and seed shape sort into gametes independently of
each other.
1⁄
4
1⁄
4
1⁄
4
Yr
yR
1⁄
4
yr
Eggs
1 ⁄ YR
4
1⁄
4
Yr
1⁄
4
yR
1⁄
4
yr
1⁄
4
Phenotypic ratio 3:1
YR
9⁄
16
YYRR YYRr YyRR YyRr
YYrr
YYrr
YyRr
Yyrr
YyRR YyRr yyRR yyRr
YyRr
3⁄
16
Yyrr
yyRr
3⁄
16
yyrr
1⁄
16
Phenotypic ratio 9:3:3:1
315
108
101
32
Phenotypic ratio approximately 9:3:3:1
Dihybrid cross
F2 generation ratio:
9:3:3:1
copyright cmassengale
63
Probability
64
Laws Of Probability
The multiplication rule
States that the probability that two or more independent
events will occur together is the product of their
individual probabilities
The rule of addition
States that the probability that any one of two or more
exclusive events will occur is calculated by adding
together their individual probabilities
Laws Of Probability - Multiplication Rule
The probability of two or more independent events occurring
together is the product of the probabilities that each event
will occur by itself
Following the self-hybridization of a heterozygous purple pea
plants (Pp), what is the probability that a given offspring will
be homozygous for the production of white flowers (pp)?
Probability that a pollen seed will carry p: ½
Probability that an egg will carry p: ½
Probability that the offspring will be pp:
1/2 X 1/2 = 1/4
Laws Of Probability - Addition Rule
The probability of either of two mutually exclusive events
occurring is the sum of their individual probabilities
Following the self-hybridization of a heterozygous purple pea
plant (Pp), what is the probability that a given offspring will
be purple?
Probability of maternal P uniting with paternal P: 1/4
Probability of maternal p uniting with paternal P: 1/4
Probability of maternal P uniting with paternal p: 1/4
Probability that the offspring will be purple:
1/4 + 1/4 + 1/4 = 3/4
Probability In A Monohybrid Cross
Can be determined using these rules

Rr
Rr
Segregation of
alleles into eggs
Segregation of
alleles into sperm
Sperm
1⁄
R
2
1⁄
Eggs
1⁄
r
2
r
r
R
R
2
r
2
R
R
1⁄
1⁄
1⁄
4
R
1⁄
4
r
4
r
1⁄
4
copyright cmassengale
69
Exceptions To Mendel’s
Original Principles
Incomplete dominance
Codominance
Multiple alleles
Polygenic traits
Epistasis
Pleiotropy
Environmental effects on gene expression
Linkage
Sex linkage
INCOMPLETE DOMINANCE
copyright cmassengale
71
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
rr = white flower
R
R
72
Incomplete Dominance
r
r
R Rr
Rr
produces the
F1 generation
R Rr
Rr
All Rr = pink
(heterozygous pink)
73
Incomplete Dominance
74
Incomplete dominance
Neither allele is dominant and heterozygous individuals have an
intermediate phenotype
For example, in Japanese “Four o’clock”, plants with one red allele and one
white allele have pink flowers:
P Generation
Red
CRCR
White
CW CW

Gametes CR
CW
Pink
CRCW
F1 Generation
Gametes
Eggs
F2 Generation
1⁄
2
CR
1⁄
2
Cw
1⁄
2
1⁄
2
CR
1⁄
2
CR
CR 1⁄2 CR
CR CR CR CW
CR CW CW CW
Sperm
Incomplete Dominance
Gametes
CR
CW
CRCR
CR
CRCR
CRCW
Gametes
CW
F1 generation
All CRCW
CWCW
CRCW
CWCW
F2 generation
1:2:1
CODOMINANCE
copyright cmassengale
77
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
78
Codominance Problem
Example:homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
i
IB
IA I B
IB i
IB
IA IB
IB i
1/2 = IAIB
1/2 = IBi
79
Another Codominance Problem
• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IA i
IB i
i
IA i
IB i
1/2 = IAi
1/2 = IBi
80
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)
81
Codominance
Answer:
IA
IB
i
i
IA IB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
82
Codominance
Neither allele is dominant and both alleles are expressed in heterozygous
individuals
Example ABO blood types
Polygenic Traits
Most traits are not controlled by a single gene locus, but by
the combined interaction of many gene loci. These are
called polygenic traits.
Polygenic traits often show continuous variation, rather then a
few discrete forms:
SEX LINKED TRAITS
copyright cmassengale
85
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
86
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
87
Sex-linked Trait Problem
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x
XrXr
Remember: the Y chromosome in males
does not carry traits.
Xr
Xr
RR = red eyed
Rr = red eyed
R
X
rr = white eyed
XY = male
Y
XX = female
88
Sex-linked Trait Solution:
Xr
Xr
XR
XR Xr
XR Xr
Y
Xr Y
Xr Y
50% red eyed
female
50% white eyed
male
89
Female Carriers
90
Epistasis
Type of polygenic inheritance where the alleles at one gene
locus can hide or prevent the expression of alleles at a
second gene locus.
Labrador retrievers one gene locus affects coat color by
controlling how densely the pigment eumelanin is deposited
in the fur.
A dominant allele (B) produces a black coat while the recessive
allele (b) produces a brown coat
However, a second gene locus controls whether any eumelanin
at all is deposited in the fur. Dogs that are homozygous
recessive at this locus (ee) will have yellow fur no matter
which alleles are at the first locus:
Epistasis
ee
No dark pigment in fur
eebb
Yellow fur
eeB_
Yellow fur
E_
Dark pigment in fur
E_bb
Brown fur
E_B_
Black fur
Pleiotropy
This is when a single gene locus affects more than one trait.
For example, in Labrador retrievers the gene locus that controls how dark the
pigment in the hair will be also affects the color of the nose, lips, and eye rims.
Exceptions
To
Mendel’s
Original
Principles
Incomplete dominance
Codominance
Multiple alleles
Polygenic traits
Epistasis
Pleiotropy
Environmental effects on
gene expression
Linkage
Sex linkage