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1
Gregor Mendel
(1822-1884)
Responsible
for
discovering
the laws
governing
inheritance of
traits
Austrian monk
Studied the
inheritance of traits
in pea plants
Developed the laws
of inheritance
Mendel's work was
not recognized until
the turn of the 20th
century (1900’s)
Between 1856 and
1863, Mendel
cultivated and tested
some 28,000 pea
plants
He found that the
plants' offspring
retained traits of the
parents
He is called the
“Father of Genetics"
copyright cmassengale
Mendel stated that
physical traits are
inherited as “particles”
Mendel did not know
that the “particles”
were actually
chromosomes & DNA
Chromosome Terminology
Homologous chromosomes: a matching pair of
chromosomes that are inherited one from each parent
Sister chromatids are identical
Chromosome Terminology
Trait - any characteristic that can
be passed from parent to
offspring
Heredity - passing of traits from
parent to offspring
Genetics - study of heredity
Monohybrid cross - cross
involving a single trait
e.g. flower color
Dihybrid cross - cross involving
two traits
e.g. flower color & plant height
Used to help
solve genetics
problems
Parent
Homozygous Recessive
Homozygous Dominant
Parent
4 Heterozygous Offspring
100%
0
0%
 Alleles – possible forms of a gene
(dominant & recessive)
 Dominant - stronger of two genes
expressed in the hybrid; will mask a
recessive trait; represented by a
capital letter (R)
 Recessive - gene that shows up less
often in a cross; hidden by a
dominant trait; represented by a
lowercase letter (r)
Genotype - gene combination
for a trait (example: RR, Rr, rr)
Phenotype - the physical feature
resulting from a genotype
(example: red, white)
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2
alleles affect a characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
 Homozygous genotype - gene
combination involving 2 dominant
or 2 recessive genes (example: RR or
rr); also called pure
 Heterozygous genotype - gene
combination of one dominant &
one recessive allele (example: Rr);
also called hybrid
Can be grown in a
small area
Produce lots of
offspring
Produce pure plants
when allowed to selfpollinate for several
generations
Can be artificially
cross-pollinated
Mendel hand-pollinated
flowers using a paintbrush
He could snip the
stamens to prevent selfpollination
Covered each flower
with a cloth bag
He traced traits through
several generations
How Mendel Began
Mendel
produced
pure
strains by
allowing the
plants to
selfpollinate
for several
generations
 Flower color --- Purple (P) or white (p)
 Flower position---Axial (A) or Terminal (a)
 Seed color ---- Yellow (Y) or Green (y)
 Seed shape --- Round (R) or Wrinkled (r)
 Pod shape --- Smooth (S) or wrinkled (s)
 Pod color --- Green (G) or Yellow (g)
 Stem length --- Tall (T) or Short (t)
 Parental P1 Generation = the parental
generation in a breeding experiment
 F1 generation = the first-generation
offspring in a breeding experiment
 From breeding individuals from the P1
generation with each other
 F2 generation = the second-generation
offspring in a breeding experiment
 From breeding individuals from the F1
generation with each other
How Mendel Proceeded
Mendel crossed his parental generation (P) of a homozygous
dominant trait (XX) with his parental generation (P) of a
homozygous recessive trait (xx). (He did not know about
dominant and recessive traits, so he was lucky that the traits
he chose were ones that were controlled by one gene!) The
first generation (F1) offspring all appeared with the dominant
trait showing as a phenotype. All their genotypes were
heterozygous (Xx).
0
How Mendel Continued
Mendel then crossed his first generation
heterozygous offspring (F1) with more of his first
generation heterozygous offspring (F1). The second
generation (F2) had recessive traits show up again at
a genotype ratio of 1:2:1, meaning 1 AA, 2 Aa and
1 aa.
Ratios of 1:2:1 meaning 1AA:2Aa:1aa
Heterozygous Offspring
Heterozygous Offspring
P = Cross 2 Pure Plants
AA x aa
F1 = Results in
4 Yellow Hybrids Aa
F2 = Cross 2 Hybrids for
3 Yellow and 1 Green
1 AA, 2 Aa, 1 aa
Mendel’s Experimental Results
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
 Homozygous dominant x
Homozygous recessive
 Offspring all Heterozygous
(hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE
 Trait: Seed Shape
 Alleles: R – Round
r – Wrinkled
 Cross: Round seeds x Wrinkled seeds
RR
x
rr
R
R
r
r
Rr
Rr
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic Ratio: All alike
Phenotypic Ratio: All alike
 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
 Trait: Seed Shape
 Alleles: R – Round
r – Wrinkled
 Cross: Round seeds x Round seeds
Rr
x
Rr
R
R
r
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
Genotype Ratio: 1:2:1
Phenotype Ratio: 3:1
 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
 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
 Trait: Seed Shape
 Alleles: R – Round
r – Wrinkled
 Cross: Round seeds x Round seeds
RR
x
Rr
R
R
R
r
RR
Rr
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic Ratio: 1:1
Phenotypic Ratio: All alike
 Trait: Seed Shape
 Alleles: R – Round
r – Wrinkled
 Cross: Wrinkled seeds x Round seeds
rr
x
Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
Genotype Ratio: 1:1
Phenotype Ratio: 1:1
In a cross of parents that are
pure for contrasting traits (AA and
aa), 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)
 During the formation of gametes
(eggs or sperm), the pair of alleles
responsible for a trait separate from
each other, so each gamete only
receives one copy of a gene.
 These alleles are then randomly
united at fertilization, producing the
genotype for the traits of the
offspring.
 Alleles for different traits are
distributed to sex cells (&
offspring) independently of one
another.
 This law can be illustrated using
dihybrid crosses.
 A breeding experiment that tracks
the inheritance of two traits.
 Mendel’s Law of Independent
Assortment
 Each pair of alleles segregates
independently during gamete formation
 Formula: 2n (n = # of heterozygotes) for
all gamete possibilities
 Remember:
2n (n = # of heterozygotes)

1.
RrYy

2.
AaBbCCDd

3.
MmNnOoPPQQRrssTtQq
1. Rr Yy: 2n = 22 = 4 gametes
RY
Ry
rY ry
2. Aa Bb CC Dd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. Mm Nn Oo PP QQ Rr ss Tt Qq:
2n = 26 = 64 gamete possibilities
 Traits: Seed shape & Seed color
 Alleles: R round
Y yellow
r wrinkled
RrYy
RY Ry rY ry
x
y green
RrYy
RY Ry rY ry
All possible gamete combinations
RY
RY
Ry
rY
ry
Ry
rY
ry
RY
RY RRYY
Ry RRYy
Ry
RRYy
RRyy
rY
RrYY
RrYy
ry
RrYy
Rryy
Round/Yellow:
9
Round/green:
3
wrinkled/Yellow: 3
rY RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
ry
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
 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
bc
b___
 Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
bC
b___
c
bbCc
bbcc
LAW
DOMINANCE
(dominant trait masks a
recessive trait)
SEGREGATION
(traits separate from
each other when sorted)
INDEPENDENT
ASSORTMENT
(different traits
sort separately from
each other)
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
 F1 hybrids have an appearance somewhat in
between the phenotypes of the two
parental varieties.
 Example: snapdragons (flower)
 red (RR) x white (rr)
RR = red flower
rr = white flower
R
R
produces the
F1 generation
R
Rr
Rr
R
Rr
Rr
R
r
R R
Rr
produces the
F2 generation
Rr
rr
1:2:1
R
r
All Rr = pink
(heterozygous pink)
 Both 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
 Example: homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
1/2 = IAIB
1/2 = IBi
• 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)
 Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
 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
 Males will show recessive traits on
their X chromosome without a
matching gene on the Y chromosome
to mask it (ex: color blindness)
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
 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.
RR = red eyed
Rr = red eyed
rr = white eyed
XY = male
XX = female
Xr
XR
Y
Xr
 RR = red eyed
 Rr = red eyed
 rr = white eyed
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
 XY = male
 XX = female
50% red eyed
female
50% white eyed
male
 tall (TT) x dwarf (tt) pea plants
t
T
T
t
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
 tall (Tt) vs. tall (Tt) pea plants
T
T
t
t
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