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Mendelelian
Genetics
6.3
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Gregor Mendel
(1822-1884)
•Austrian monk
•Studied the
inheritance of
traits in pea
plants
•Developed the
laws of
inheritance
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3
4
Before Mendel,
people believed traits blended.
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Mendel Performed “crosses” on
different pea plants:
Parents: Yellow Seeds X Green Seeds
F1: All offspring had yellow seeds
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Parents:
F1:
X
X
F2: 3: 1
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Generation
Parental P1 Generation = the parental
generation
F1 generation = the first-generation
F2 generation = the second-generation
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Inheritance
Mendel found plants
were able to pass on
traits
He said these physical
traits are inherited as
“factors”
Mendel’s “factors” are
actually genes
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What we know now:
Genes code for traits
We have two copies of each gene – because
we have two copies of each chromosome
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Genetics Terms
 Gene – codes for a trait (tallness)
 Alleles - forms of a gene:
 Dominant – always expressed;(T)
 Recessive - allele that shows only if
dominant is NOT there; (t)
 Dominant MASKS the recessive
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 Homozygous – SAME alleles (TT
or tt); also called pure
 Heterozygous DIFFERENT alleles
(Tt); also called hybrid
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More Terminology
 Genotype – what GENE is
there (e.g. TT, Tt, tt)
 Phenotype - the PHysical
feature (e.g. tall, short)
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Genotype & Phenotype in Flowers
Genotype of alleles:
T = tall plant
t = short plant
Genotypes
TT
Tt
Phenotypes
tall
tall
tt
short
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Punnett Square
male
predict
outcomes of
genetic crosses
female
Used to
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Genetic Practice
Problems
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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
TT
Tt
t
Tt
tt
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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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
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
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
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
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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Mendel’s Laws
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Law of Dominance
Some alleles are dominant and other alleles are
recessive. (T or t)
An organism with at least one dominant allele for
a trait will always have that trait. (TT, Tt)
Recessive traits are only seen in homozygous
recessive organisms. (tt)
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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:
Tt can make two gametes:
T gamete
t gamete
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Applying the Law of Segregation
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Law of Independent
Assortment
Alleles for different traits are
distributed to gametes
independently of one another.
This law can be illustrated using
dihybrid crosses.
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Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
r wrinkled
Y yellow
y green
What gametes are possible in an
individual that is heterozygous for
both traits?
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Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round
Y yellow
r wrinkled
y green
FOIL
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
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Dihybrid Cross
RY
RY
Ry
rY
RRYY RRYy RrYY
ry
RrYy
Round/Yellow:
9
Ry RRYy
RRyy
RrYy
Rryy
Round/green:
rY RrYY
RrYy
rrYY
rrYy
wrinkled/Yellow: 3
RrYy
Rryy
rrYy
rryy
ry
wrinkled/green:
3
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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