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Mendelian
Genetics
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Genetic Terminology
 Heredity – passing of traits from
parent to offspring
 Trait – any characteristic that can
be passed from parent to offspring
 Genetics– study of heredity
 Gene – a unit of heredity located
on a chromosome (one protein)
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copyright cmassengale
Gregor Johann Mendel
Austrian monk
Taught high school and
at the same time kept a
garden
He found that the
plants’ offspring
retained traits of the
parents
Worked mostly with
pea plants
Called the “Father of
Genetics"
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(1822-1884)
•He worked with
pea plants because
they had many
different traits and
could cross
pollinate
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Designer “Genes”
 Gene – inheritable unit of information in DNA
Alleles – alternate expression of the same
gene; a different protein for the same trait
Ex. eye color –Brown eye allele (B),
Blue eye allele (b)
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Dominant
vs. Recessive
Dominant
vs. Recessive
Dominant - stronger of two genes
expressed, will be visible if present;
represented by a capital letter (R)
Recessive – describing a trait that is
hidden by a dominant trait; can only
be expressed when the dominant
gene (allele) is absent; (r)
Mendel found seven contrasting traits in the
peas/pea plants
T
t
7
R
Y
C
I
G
A
r
y
c
i
g
a
Each trait was always carried from parent to
the offspring. Tall parents produced tall
offspring and yellow parents produced
yellow offspring.
That
makes
Sense!
Then, Mendel
crossed plants with contrasting traits.
Tall X short (parents)
“X” means cross with, mating.
Tall
This mating produced all ______offspring.
Why aren’t
they
medium?
The trait for short plants seemed to be lost.
Mendel called the parent plants in this first
cross the P1 generation (parent)
The generation produced by this cross is the
F1 generation or first filial (family) generation.
Pure
Tall
P1
Pure
Short
X
t
t
T
Tt
Tt
T
Tt
Tt
TT x tt
F1 generation
A cross between two members of the F1
generation produced the F2 generation.
F1
Tt x Tt
X
T
t
T TT
Tt
t
tt
Tt
F2 generation
¾ tall plants and ¼ short plants in the F2
Types of Genetic Crosses
• Monohybrid cross - cross involving a
single trait
mono - one
e.g. flower color
• Dihybrid cross - cross involving two
traits
di - two
e.g. flower color & plant height
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Punnett Square
Grid system created
by R.C. Punnett in
the early 1900 to
help solve genetics
problems
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More Terminology
• Genotype – the genetic makeup of an
organism; the actual genes (alleles)
composing the organism (cannot be
determined visually)
• Represented by: Rr, RR, rr
• Phenotype – The visible characteristics
of an organism; traits which can be
observed, or the traits expressed
• such as color of hair or eyes
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More Terminology
 Genotypes can be homozygous or heterozygous for a
specific trait.
 Homozygous (pure) – the gene pair or alleles are
identical (AA, aa, BB) homo-Latin for same
 Heterozygous (hybrid) – the gene pair or alleles are
different (Aa, Bb) hetero-Latin for different
 The capital letter in a heterozygous pair is written
first.
 Is it hetero or homo? Look at the case of the letter!
Let’s Review
 Parental P1 Generation = the parental generation
in a breeding experiment.
ex. TT and tt
 F1 generation = the first-generation offspring in a
breeding experiment. (1st filial generation)
From breeding individuals from the P1 generation
ex. Tt and Tt
 F2 generation = the second-generation offspring
in a breeding experiment. (2nd filial generation)
From breeding individuals from the F1 generation
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ex. TT, Tt, tt
Following the Generations
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P1
F1
F2
Cross 2
Pure
Plants
TT x tt
Results in
all Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
Solving
Monohybrid
Crosses
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P1 Monohybrid Cross
 Step 1: Trait(s) --- Seed Shape
 Step 2: Alleles --- R – Round
r – Wrinkled
 Step 3: Cross -- Homo Round seeds x Homo Wrinkled seeds
RR
Step 4: Punnett Square
r
r
x
rr
Step 5: Genotype
Genotype: Rr-hetero round-4 (100%)
Genotypic Ratio: 4:0
R
Rr
Rr
Step 6: Phenotype
R
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Rr
Rr
Phenotype: round – 4 (100%)
wrinkled – 0 (0%)
Phenotypic ratio: 4:0
P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous
recessive
 Offspring all Heterozygous (hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL
ALIKE 4:0 or 100% of the dominant trait is
visible
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F1 Monohybrid Cross
 Step 1: Trait - Seed Shape
 Step 2: Alleles- R – Round
r –Wrinkled
 Step 3: Cross- Hetero Round seeds x Hetero Round seeds
Rr
Step 4: Punnett Square
R
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r
R
RR
Rr
r
Rr
rr
x
Rr
Step 5: Genotype
Genotype:
homo round RR-1 25%
hetero round Rr-2 50%
homo wrinkled rr-1 25%
Genotypic Ratio: 1:2:1
Step 6: Phenotype
Phenotype: Round-3 (75%) & wrinkled -1 (25%)
Phenotypic Ratio: 3:1
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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F2 Monohybrid Cross (1st)
 Step 1: Trait - Seed Shape
 Step 2: Alleles- R – Round
r –Wrinkled
 Step 3: Cross- Round seeds x Round seeds

RR
x
Rr
Step 4: Punnett Square
R
R
RR
r
Rr
Step 5 Genotype
Genotype: homo round RR-2
(50%), hetero round Rr-2 (50%)
Genotypic Ratio – 2:2
Step 6 Phenotype
R
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RR
Rr
Phenotype: Round – 4 (100%)
Phenotypic Ratio: 4:0 Round
F2 Monohybrid Cross (2nd)
 Step 1: Trait - Seed Shape
 Step 2: Alleles- R – Round
r –Wrinkled
 Step 3: Cross- homo wrinkled seeds x hetero round seeds
Step 4: Punnett Square
r
R
r
Rr
rr
rr
x
Rr
Step 5: Genotype
Genotype: Hetero round Rr (2) 50%,
Homo wrinkled- rr (2) 50%
Genotypic Ratio: 2:2 or 1:1
Step 6: Phenotype
r
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Rr
rr
Phenotype: Round (2) 50%
Wrinkled (2) 50%
Phenotypic Ratio: 2:2 or 1:1
F2 Monohybrid Cross Review
 Homozygous x heterozygous (hybrid)
 Offspring:
50% Homozygous RR or rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
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Results of Monohybrid Crosses
 Inheritable factors or genes are
responsible for all heritable
characteristics
 Phenotype is based on Genotype
 Each trait is based on two genes, one from
the mother and the other from the father
 True-breeding individuals are
homozygous ( both alleles) are the same
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Mendel’s
Three Laws of
Inheritance
Mendel’s Hypothesis
For each inherited trait, an individual has two copies
of the gene – one from each parent
There are alternate versions of genes, or alleles.
(Pea flower color can be purple or white.)
When two different alleles occur together, one of
them may be completely expressed, while the other
may have no observable effect on the organism’s
appearance. (Dominant vs Recessive)
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Mendel’s Hypothesis continued
 When gametes are formed, the alleles for each
gene in an individual separate independently of
one another. Thus, gametes only carry one allele
for each inherited trait. When gametes unite
during fertilization, each gamete contributes one
allele.
Laws of Inheritance
Mendel’s hypothesis led to laws and principles.
 Principle of Dominance
 Law of Segregation
 Law of Independent Assortment (to be
discussed later)
Principle 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. Offspring that are hybrid for a trait will have
only the dominant trait in the phenotype. (Yy – yellow)
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Law of Segregation
 During the formation of gametes (eggs or
sperm - Meiosis), 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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Mendel’s Law of Segregation
 Alleles segregate (separate) during meiosis so each
gamete gets only one allele
a
a
a
aA
Two choices for gametes:
A
A or a
A
A
Mendel’s
Law of Independent
Assortment
To be discussed later…