Pisum sativum
HEREDITY is the passing
of traits from parents to
offspring
Traits are controlled by
genes, so GENETICS is
the study of how traits are
inherited through the
action of alleles
An Austrian monk born in
1822 who is responsible for the
laws governing the inheritance of
traits, today known as
CLASSICAL GENETICS.
Between 1856 and 1863, Mendel
cultivated and tested over 28,000
pea plants. He was a Biology
teacher too!
Mendel performed
by cross-pollination -transferring
pollen from one plant to selected
ova of other plants, thereby
controlling which plants mixed
Plants normally
reproduce 2 ways
SELF-pollination:
pollen from one plant
fertilizes ova of SAME
plant
CROSS Pollination:
pollen from one plant
fertilizes another, that’s
why
are so
important!!!
Mendel produced pure strains
by SELECTIVE BREEDING
Mendel cut the stamens from
one plant and cross pollinated
other plants in his experiments.
Humans have been selectively
breeding organisms for
thousands of years
A PURE or TRUE BREEDING STRAIN: Always produces
offspring of the same type when bred with a member of its “breed.”
For Example: Mendel’s Plants or a German Shepard
Give some other examples-
Gregor Mendel studied 7 TRUE BREEDING characteristics
in his pea plants; Pisum sativum
He noted the CHARACTERISTICS like flower color, pea or
pod shape and position of the flowers, were able to exhibit
two distinctive TRAITS
Y
y
Mendel noted unusual patterns when he bred his pure strains
When he bred purple and white flowers, all the offspring were
purple... where did the “white” go?
Y
y
 The traits that appeared in the offspring he called
DOMINANT
 The traits that disappeared he called RECESSIVE
A
a
T
t
P generation crossed to produce F1 generation
interrupted the self-pollination process by removing male
flower parts
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.
Among the F1 generation, all plants had purple flowers
F1 plants are all heterozygous
Among the F2 generation, some plants had purple flowers and
some had white
 Mendel also noticed unusual mathematical patterns in
the offspring
What patterns do you notice in the data?
Mendel postulated that traits were controlled by
“factors” of inheritance later to be known as GENES
He reasoned these factors were carried in pairs in
normal cells and individually in gametes
MENDELS LAWS
MENDELS 1st Law- RANDOM SEGREGATION
GENES SEGREGATE RANDOMLY- In each gamete only
one copy of each parental cells genes in a 50/50 proportion
MENDEL’S 2nd Law- INDEPENDENT ASSORTMENT
GENES ASSORT INDEPENDENTLY- He observed that
traits and the genes that gave rise to them were not linked
ie; that pod color and flower color were not connected…
Can you think of any traits in humans that go together or
are LINKED ??
CHARACTER– An identifiable feature of an organism, like
flower color or height
TRAITS– any characteristic that can be passed from parents to
offspring
HEREDITY– the passing of traits from parents to offspring
DOMINANT– is always expressed; masks a recessive trait
RECESSIVE– can only be expressed if there are no dominant
alleles present
ALLELE– one form (dominant or recessive) of a gene
Sex cells have ONE form of a gene on their chromosomes
SOMATIC or Body cells have TWO alleles for a single gene
DOMINANT alleles are represented by a capital letter
RECESSIVE alleles are represented by a lower case letter
Example: B = Brown eye color (dominant)
b = Blue eye color (recessive)
HOMOZYGOUS“Purebred” organisms have two copies of the same allele
BB (homozygous dominant)
bb (homozygous recessive)
HETEROZYGOUS,
Bb Organism with two different alleles
GENOTYPE: the alleles present in the organism, BB, Bb, or bb
PHENOTYPE: the expression of the genes; observed traits
All genes occur in pairs, so TWO alleles affect a trait.
Possible combinations if:
R = Red flower
r = Yellow flower
Genotypes RR
Rr
rr
Phenotypes Red
Red
Yellow
In this cross Rr is Red therefore R is dominant to r
Monohybrid Cross: cross involving ONE trait, e.g., eye
color
B
b
B
BB
Bb
b
Bb
bb
Offspring’s genotype and phenotype is determined using a
Punnett square
Probability determines the odds of an EVENT
occurring.
Expressed as fraction or percentage
Ex: (1/4) or 25%
The probability that two or more independent
events will occur together is the PRODUCT of
their chances occurring separately
Ex: odds of having a boy = ½
Odds of having 2 boys = (1/2) x (1/2) = (1/4)
PUNNETT SQUARES ARE MODELS OF PROBABILITY!!
The probability that two or more
independent events will occur
together is the product of their
chances occurring separately
Ex: odds of having a boy = ½
Odds of having 2 boys = (1/2) x
(1/2) = (1/4)
Odds of having 3 children with a
continuous hairline:
(1/4) x (1/4) x (1/4) = (1/64)
Parental Generation (P1)
- the parental generation in a
breeding experiment
First Filial Generation (F1)
- the first generation of offspring in
a breeding experiment
Second Filial Generation (F2)
- the second generation of offspring
in a breeding experiment
Character: Seed Color
Alleles: Y – Yellow
Cross: Yellow seeds
Traits: Green or Yellow
y – Green
X
Green seeds
YY
X
yy
Crossing two true-breeding (pure) plants
Y
Y
y
Yy
Yy
y
Yy
Genotype: Yy
Phenotype: Yellow
Yy
Genotypic Ratio: 100% Yy
Phenotypic Ratio: 100% Yellow
Character: Seed Color
Traits: Yellow or Green
Alleles: Y – Yellow y – Green
Cross: Yellow seeds X
Yellow seeds
Yy
X
Yy
Genotypes: YY, Yy, yy
Crossing of heterozygotes (hybrids)
Y
y
y
Phenotypes: Yellow and
Green
Y
YY
Yy
Genotypic Ratio: 25% YY,
50% Yy, 25% yy (1:2:1)
Yy
yy
Phenotypic Ratio: 75%
Yellow, 25% Green (3:1)
WHERE IS THE F2 GENERATION??
A breeding experiment that tracks the inheritance of two traits
Remember Mendel’s “Law of Random Segregation”
Each pair of alleles segregates independently during gamete formation
therefore
Formula: 2n (n = # of heterozygotes)
Example:
1. RrYy: 2n = 22 = 4 possible gametes
RY
Ry
rY
ry
2. AaBbCC: 2n = 23 = 8 gametes
ABC
Abc
aBC
aBc
AbC
abC
ABc
abc
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
RY
Ry
rY
ry
RY
Ry
rY
ry
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RY
Ry
RY RRYY RRYy
rY
ry
Round/Yellow:
RrYY
RrYy
Round/green:
9
3
RRYy
RRyy
RrYy
Rryy
wrinkled/Yellow: 3
rY RrYY
RrYy
rrYY
rrYy
wrinkled/green: 1
RrYy
Rryy
rrYy
rryy
9:3:3:1 phenotypic
ratio
Ry
ry
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Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
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Test crosses involve breeding the individual in question with another
individual that expresses a recessive version of the same trait.
If all offspring display the dominant phenotype, the individual in
question is homozygous dominant; if the offspring display both
dominant and recessive phenotypes, then the individual is heterozygous
Y
Yy
Y
Yy
y
y
Yy
Yy
Offspring all yellow!
Y
y
y
Yy
yy
y
Yy
yy
½ Offspring yellow; ½ Offspring green!
Is an experiment that uses mating
between an individual of unknown
genotype and a homozygous
recessive individual.
bC
b___
Example: bbC__ x bbcc
BB = brown eyes
Bb = brown eyes
bb = blue eyes
CC = curly hair
Cc = curly hair
cc = straight hair
bc
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Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
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bC
b___
c
bbCc
bbcc
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The DOMINANT RECESSIVE Pattern is not
the only way genes are expressed
CODOMINANCE- both traits are expressed at
once!!!
INCOMPLETE DOMINANCE-a blend of traits
MULTIPLE ALLELES – There may be more than
just 2 alleles, often 3 or more! Example: the ABO
blood group
POLYGENIC TRAITS – Many traits are the product
of more than 1 gene, for example your intelligence,
height, facial strucure…etc.
SEX –LINKED TRAITS – Some traits are linked to
your gender for example color blindness
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F1 Hybrids have BLENDED TRAITS
Their appearance is in between the phenotypes of the
two parental varieties.
Example: Roses (flower)
Red (RR) x white (rr)
RR = red flower
rr = white flower
r
r
R
R
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F1 Hybrids have BLENDED TRAITS
Their appearance is in between the phenotypes of the
two parental varieties.
r
r
R
Rr
Rr
Produces the
F1 generation
R
Rr
Rr
All Rr = PINK
(heterozygous pink)
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BOTH ALLELES
are expressed in
heterozygous
individuals.
x
BOTH TRAITS
are observed
Example:
BUNNYS !!
F1
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Example:
Black Haired Bunny BB
White Haired Bunny bb (though b isn’t recessive)
BB
b
b
B
Bb
Bb
B
Bb
Bb
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x bb
All black and
white bunnys
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MULTIPLE ALLELES may be inherited
BOTH ALLELES are expressed in heterozygous
individuals.
BOTH TRAITS are observed
Example: THE ABO BLOOD GROUP
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
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Example:
Homozygous male Type B (IB IB)
Heterozygous female Type A (IA i)
IB IB
x IA i
IA
i
IB
IA IB
IBi
IB
IA IB
IBi
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1/2 = IAIB
1/2 = IBi
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• Example: male Type O (ii)
x
female type AB (IAIB)
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
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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Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
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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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Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
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Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XRY
x
XrX r
Remember: the Y chromosome in males does
Xr
Xr
not carry traits.
RR = red eyed
Rr = red eyed
XR
rr = white eyed
XY = male
Y
XX = female
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Xr
Xr
XR
XR Xr
XR Xr
Y
Xr Y
Xr Y
50% red eyed
female
50% white eyed
male
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XB
Sex Linked Traits - Color Blindness
An X-linked disorder
Example: Color Blindness
Xr
XB
(normal male) x (carrier-female)
XBY
x XBXb
Remember: the Y chromosome in males does
not carry traits.
XBY = Normal Male
XbY = Color-Blind Male
XBXB = Normal Female
XBXb = Carrier Female
XbXb = Color Blind Female
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Y
49
XB
Sex Linked Traits - Color Blindness
An X-linked disorder
Example: Color Blindness
Xr
XB X B XB XB Xb
(normal male) x (carrier-female)
XBY
x XBXb
Remember: the Y chromosome in males does
not carry traits.
XBY = Normal Male
XbY = Color-Blind Male
XBXB = Normal Female
XBXb = Carrier Female
XbXb = Color Blind Female
Sex-linked Trait Solution:
Female Carriers
copyright cmassengale
Y
XB Y
Xb Y
25% normal female
25% carrier female
25% normal male
25% color blind male
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Constructed to show the pattern of
inheritance of a characteristic within a family.
The particular pattern indicates the manner
in which a characteristic is inherited (suggests
X-linked, dominant, etc.)
Symbols used:
Normal Female
Carrier Female
Affected Female
Normal Male
Affected Male
INCOMPLETE DOMINANCE: Mode of gene expression in which
both traits are seen as a blend in the heterozygote
CODOMINANCE: Mode of gene expression in which both traits
are seen together in the heterozygote
MULTIPLE ALLELES: Mode of gene expression in which more
than just two alleles determine phenotype
POLYGENIC TRAITS: Mode of gene expression in which more
than one gene determines phenotype
SEX LINKED TRAITS: Mode of gene expression in which gene is
carried on sex chromosome and patterns of expression are linked
to gender
PEDIGREE CHART: A table used to illustrate the inheritance of a
particular phenotype in families