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Mendelian Genetics
Commitment
What is something you are really committed
to (such as a goal, a hobby, a sport or maybe
some other cause)?
Inheritance
 What was the first explanation for
resemblances between parent and
offspring?
 Theory
of blended inheritance-people
reasoned that each parent contributed
factors that were blended in their offspring.
Mendel’s Experiment
Purpose: Believed that there was a mathematical
explanation for inheritance (passing characteristics
from parents to offspring).
• He studied peas because…
• He had access to varieties with many different traits.
• Peas also reproduce by self-pollination (simple and quick)
 Small
 Easy
to grow
 Easy to reproduce in large amounts
 Mature quickly
Step 1: Purebreds
 Grew purebred pea
plants.


Purebred- produces the same
trait in its offspring for
generations.
Accomplished by selfpollination
 Allowed plants with each
trait to self pollinate for
several generations
He snipped the stamen to prevent self-pollination
and covered each flower with a bag cloth.
Materials
 Mendel grew and selected seven true breeding
varieties of peas- each had a peculiar trait
 Seed
shape: round vs. wrinkled
 Seed color: yellow vs. green
 Flower color: purple vs. white
 Pod shape: smooth vs. constricted
 Pod color: green vs. yellow
 Flower position: axial vs. terminal
 Stem length: tall vs. short
Terminal
Round
Wrinkled
Smooth
Axial
Green
Constricted
Tall
Yellow
Green
Gray
Yellow
Short
White
Step 2: Cross Fertilization
 After many
generations,
purebreds of the
same trait were
cross- pollinated
and the first
generation of
offspring resulted


Transferred pollen
using a paintbrush
Snipped the stamens to
prevent self-pollination
P= purebred/ parent generation
F1= First Generation Offspring
 Offspring only showed one of the parent’s traits
 Mendel thought that traits disappeared in the first
generation of offspring.
 He needed to know if the other trait had
disappeared completely … led to step 3
Step 3
 Mendel let the offspring (F1 generation) self-
pollinate and produce a second generation of
plants (F2 Offspring).
 Mendel discovered that
when the offspring selfpollinated, the lost trait
reappeared.
 Mendel concluded that
there are 2 building blocks
necessary to create a traithe called these genes.
Mendel’s Results
Parent Cross
Round x wrinkled
Yellow x green
Purple x white
Inflated x
constricted pod
Green x yellow pod
F1 Phenotype
Round
Yellow
Purple
Inflated
F2 data
5474 : 1850
6022 : 2001
705 : 224
882 : 299
Green
428 : 152
Axial x terminal
flower
Long x short stem
Axial
651 : 207
Long
787 : 277
3:1 Ratio
 Rewind to Experiment 1:

When Mendel crossed the purple and white purebreds, the purple
plant could only give purple genes & the white plant could only give
white genes

Mendel called these variations of genes alleles
 Rewind to Experiment 2:

When the purebreds were crossed, each offspring (F1 plant) had one
purple and one white allele

All the F1 plants were purple b/c the purple allele hid the white allele from
view
 Dominant allele- name given to the allele that masks other alleles
 Recessive allele - name given to the allele that was hidden
 Closure to Experiment 3:

When the F1 plant was self-fertilized, the dominant and recessive traits
were both observed
Conclusion
 Mendel discovered the Principle of dominance.
 The Principle of Dominance states that the
dominant allele (variation of a gene) will prevent
the recessive allele from showing up in the
offspring.

Discovered in the first cross of purebreds.
Conclusion Cont…
 Law of Segregation: states that alleles separate
when passed on from parents to offspring.

Only one allele, from a gene (for a specific trait) is passed
from the parent to the offspring.

Alleles for a trait are “recombined” at fertilization,
producing the genetic composition (genotype) for the
observable traits (phenotype) of the offspring.
Principle of Segregation
 Segregation of alleles
Conclusion Cont…
 Principle of Independent Assortment-
Mendel discovered that plants could have many
combinations of characteristics

Ie. If a pea plant is tall, it can have green seeds or yellow
seeds that are either smooth or wrinkled.
 Each trait is independent of each other
Important Vocabulary:
 Trait- A genetically determined characteristic
 Chromosome: A linear strand of DNA harboring
many genes.
 Gene: A DNA segment that gives instructions for
making protein
 Allele: An alternate form of a gene
 There are only two alleles for each gene: one allele comes
from the mother and one from the father
 Locus (Loci)- specific place where a gene occurs on
a chromosome
 Homozygous- organisms w/ two identical alleles for
a particular gene (aka purebred)

i.e. Tall & short pea plants…TT and tt
 Heterozygous- organisms w/ two different alleles
for the same gene (aka hybrid)

i.e.Tall and short pea plants… Tt
Loci with
identical alleles
Loci with
Different alleles
Important Vocab.
 Phenotype- observable physical characteristics
(Tall or short)
 Genotype- genetic composition of alleles (TT, Tt,
tt)
Punnett Square
Punnett Square- graphic organizer that shows the
gene combinations that are possible for an offspring
to inherit when two individuals mate.
P
p
P
PP
Pp
p
Pp
pp
P
P
P
PP
Pp
p
Pp
pp
Depending on the egg
that is fertilized, the
mother can give a
dominant allele found
on one chromosome
or a recessive allele
found on the other.
Depending on the sperm, the father can give a
dominant allele found on one chromosome or a
recessive allele found on the other.
Punnett Squares… show the Law of
Segregation
Monohybrid Cross
Genotypic Ratio:
1 PP:2 Pp:1 pp
Genotypic
Ratio:
25% PP
50% Pp
25% pp
P
p
P
PP
Pp
p
Pp
pp
Phenotypic
Ratio:
75% Purple
25% White
Phenotypic Ratio:
3 Purple:1 White
Monohybrid Cross
 Step One: Figure out the genotypes of the parents.
What alleles (different versions of a gene) could they pass
to their offspring
 Step Two: Set up a Punnett Square for your
mating.
 Step Three: Figure out what kinds of gametes the
parents can produce.
 Determine all the possible ways that the male sperm can
combine with the female eggs
 Step Four: Fill in the possible genotypes of the
offspring inside the table

Pea Plant Traits
 Seed Shape… Round (R) or Wrinkled (r)
 Seed color…. Yellow (Y) or Green (y)
 Pod shape… Smooth (S) or Wrinkled (s)
 Pod color…. Green (G) or Yellow (g)
 Seed Coat Color… Gray (G) or White (g)
 Flower Position… Axial (A) or Terminal (a)
 Plant Height… Tall (T) or Short (t)
 Flower color… Purple (P) or white (p)
Cross a heterozygous
plant with yellow peas
with a plant with
homozygous green
peas.
Bell Work
Self-fertilize a
heterozygous tall
flower.
Determine the genotype and phenotype ratios
of each cross!
Dihybrid Cross
Genotypes=
TtGg and
TtGg
• In this example we will
cross 2 pea plants that are
heterozygous for size &
pod color.
• FOIL it out!
Dihybrid Cross
Genotypes=
TtGg and TtGg
TG
Tg
TtGg
tG
tg
• In this example we will cross 2
pea plants that are
heterozygous for size & pod
color.
TG
• Determine what alleles would
Tg TtGg be found in all of the possible
gametes that each parent
tG
could produce
tg
Punnett
Square
In this case, each parent can make 4
different types of gametes, so the
table needs to be 4 rows by 4
columns, or 16 squares.
TG
TG
Tg
tG
tg
Tg
tG
tg
TG
Tg
tG
tg
TG
Tg
tG
tg
TTGG
TTGg
TtGG
TtGg
TTGg
TTgg
TtGg
Ttgg
TtGG
TtGg
ttGG
ttGg
TtGg
Ttgg
ttGg
ttgg
Genotypes:
TTGG:
TtGG:
TTGg:
TtGg:
TTgg:
Ttgg:
ttGG
ttGg:
ttgg:
Phenotypes:
Tall and GreenTall and YellowSmall and GreenSmall and Yellow-
Punnett
Square
Show a cross between a plant that is
homozygous dominant for round
peas, while heterozygous for yellow
peas with a plant that is
homozygous recessive for wrinkled
peas and homozygous recessive for
green peas.