Mendelian Genetics
What is Heredity?
Female
normal wing
P- generation
The passing on of traits
from one generation
to the next
X
Male double
wing
F1 – normal wing
Chapter 9, Section 1: Pre-Mendel
Early Ideas about Heredity
• Blending Theory (19th century):
– Each parent contributed “factors” that were blended in
the offspring
• What was the problem with this theory?
– All individuals of a population would eventually look
the same.
– Once blended, traits would never separate and show
up in later generations.
Chapter 9, Section 1: Mendel
Gregor Mendel (1822 – 1884)
• Augustinian monk who
studied pea plants
• Established the
particulate theory of
heredity
• Significance:
– Developed pure lines
– Counted results and kept
statistical notes (data)
• His work remained
undiscovered until 1903.
Mendel’s Research
Character
Dominant Trait
Why did he research
pea plants?
Flower Color
1. They are normally
self-pollinated,
but can be
cross-pollinated.
Flower Position
2. They have several
qualitative traits
that are easy to
distinguish
Seed Shape
i.e.,Tall vs. Short
Purple
Recessive Trait
White
Self Pollination
Side
Top
Yellow
Green
Round
Wrinkled
Inflated
Constricted
Seed Color
Pod Shape
Pod Color
Green
Cross Pollination
Yellow
Stem Length
Tall
Dwarf
Mendel’s
Research
1.
2.
3.
4.
5.
Removed stamens from
purple flower.
Transferred pollen from
stamens of white flower
to pistil of purple flower.
Pollinated flower matured
into a pod.
Planted seeds from pod.
Examined offspring:
All purple flowers…
Watch this video clip, and see
if you can explain why…
Mendel’s Conclusions
1. Law of Segregation
a. Factors (genes) for a particular trait occur in
pairs
b. For each trait, an organism inherits two
genes, one from each parent.
c. Dominant alleles mask recessive ones
i. Exception 1: Incomplete Dominance
ii. Exception 2: Co-dominance
d. Two alleles for each trait segregate
(separate) during gamete production
Law of Segregation:
Factors for a particular
trait occur in pairs
Homologous pair of
Chromosomes
Genes:
The “factors” that
control traits.
Alleles:
Different forms of a
gene.
Allele for White
Flowers (p)
Locus
for
Flower
Color
Gene
Allele for Purple
Flowers (P)
Back to Mendel’s
Conclusions
W
or
w
w
or
w
W = widow’s peak
Ww
w = no widow’s peak
ww
• Each individual is diploid
– Diploid: Containing a double-set of chromosomes (2n)
• Each gamete is haploid
– Gamete: Reproductive cell (egg or sperm)
– Haploid: Containing a single-set of chromosomes (n)
Law of Segregation:
One version of each
gene is inherited from
each parent
Back to Mendel’s
Conclusions
Dominant
alleles
mask
recessive
ones
P (Parent) Generation
True-breeding parents
(PP x pp)
705 plants had
purple flowers
224 plants had
white flowers
F1 (1st Filial) Generation
Hybrid Offspring (Pp)
What happened to the
recessive traits?
F2 (2nd Filial) Generation
What is the F2 ratio?
705:224  3:1
Dominant
alleles
mask
recessive
ones
P Generation
Phenotype (Appearance)
Genotype (Genetic Makeup)
Gametes
Purple Flowers
PP
P
White Flowers
pp
p
F1 Generation
Phenotype (Appearance)
Genotype (Genetic Makeup)
Gametes
Purple Flowers
Pp
p
P
Punnett Square
F2 Generation
What is the Genotypic Ratio
of the F2 Generation?
PP:Pp:pp  1:2:1
P
P
PP
p
Pp
p
Pp
pp
What is the Phenotypic Ratio
of the F2 Generation?
3:
1
Dominant
alleles
mask
Homozygous:
recessive
same alleles 1
ones
Heterozygous:
different alleles
Genotype
Phenotype
PP
(homozygous)
Purple
Pp
(heterozygous)
Purple
Pp
(heterozygous)
Purple
pp
(homozygous)
White
Ratio = 1:2:1
Ratio = 3:1
3
2
1
1
Vocabulary Practice
We will now play “Got Gametes?” in order
to practice understanding the following
new terms: alleles, genotype, phenotype,
homozygous, heterozygous
Vocabulary Practice
Each of you are haploid gametes carrying
single alleles – for a trait.
Your single letter can be combined with another
single letter (i.e., Hh) to form a genotype in a
diploid organism.
You will observe a series of faces. Come to the
front of the classroom if you think you have the
right genotype to match the phenotype
shown. Make sure to find the corresponding
allele for the trait you represent!
How do you set up a Punnett
Now you try!
square?
Example 1:
Hh X
Heterozygous short hair (____)
heterozygous short hair (____)
Hh
Hh
Hh
H
HH
h
Genotypic Ratio HH:Hh:hh
H
Hh
Hh
hh
1:2:1
Phenotypic Ratio Short hair:long hair
3:1
h
Dominate
alleles
mask
recessive
ones
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
What would your
What
would your
If you
have
a
dominant
phenotype
hypothesis be if the
hypothesis be if the
genotype was PP?
genotype was Pp?
(like purple flowers) how would you
determine if it was homozygous
(PP) or heterozygous
(Pp)?
What
P
p
P
p
Pp
Pp
P
p
p
p
experiment would you design?
If PP,
then all offspring purple:
Pp
Pp
If Pp,
then ½ offspring purple
and ½ offspring white:
pp
Pp
pp
Pp
This is called
a test cross
Back to Mendel’s
Conclusions
P Generation
Phenotype(s): Red and White
Genotype(s): CRCR and CWCW
Gamete of Red flower: CR
Gamete of White flower: CW
CR
CW
CR
CW
F1 Generation
Phenotype: Pink
Genotype: CRCW
Gametes: CR and CW
F2 Generation
Exception to Dominant Alleles
Masking Recessive Alleles
Incomplete Dominance:
Pink Snapdragons
Use Root Letter “C” to designate
incomplete dominance interaction
CR
CR
CRCR
CW
CRCW
CW
CRCW
CWCW
Back to Mendel’s
Conclusions
Exception to Dominant Alleles
Masking Recessive Alleles:
Co-dominance: Blood Types (video)
Back to Mendel’s
Conclusions
Use root letter “I” for dominant alleles of equal strength and “i” for recessive
Law of Segregation:
Two alleles for each trait
segregate (separate)
during gamete production
Back to Mendel’s
Conclusions
This occurs in a process called meiosis:
Specifically it is “crossing over,” which occurs very
early during Prophase I of Meiosis and separation
in Anaphase I and II
Mendel’s Conclusions
The law of segregation followed one single trait at
a time, such as seed color.
What if two traits were followed, such as seed
color and seed shape?
Are these genes somehow connected (linked) and inherited
together?
P
Following two traits:
Seed shape and seed color
Gametes
YR
F1
Ova
yr
F2
yr
YyRr
YR
YR
Sperm
yr
Hypothesis:
If the genes for seed
shape and color are
connected in some way,
then the dominant R and
Y alleles and the recessive
r and y alleles will be
matched sets in the
gametes.
For the traits of seed shape and color, this
hypothesis is NOT supported by experimental
evidence
What did the data support
instead?
What are the genotypes of
the P Generation?
YYRR
What does the P
Generation pass on to F1?
yr
YR
F1 Phenotype?
F1 Genotype?
What does the F1
Generation pass on to F2?
Combine the sperm and
ova to produce the
offspring in F2
yr
What do you expect the
phenotype will be when YR
and YR are combined?
What will the Genotype be
when YR and YR are
combined?
Law of Independent
Assortment
yyrr
Gametes
YyRr
Ova
YR
Yr
yR
YR
Yr
YYRR
YYRr
YyRR
Sperm
yR
YYRr
yr
YyRR
YYrr
Phenotypic Ratios
YyRr
YyRr
Yyrr
yyRr
9/16
Yellow & Round
3/16
Green & Round
3/16
Yellow & Wrinkled
1/16
Green & Wrinkled
Yyrr
yyRR
Now try to do it on your own!
What are the phenotypic ratios?
YyRr
YyRr
yyRr
yyrr
Mendel’s Conclusion!
Law of Independent Assortment:
If the genes are not connected, then they
should segregate independently. The
alleles are randomly packaged into
different gametes during meiosis
(For example, genes for seed shape and
color were not inherited together.)
FOIL
YyRr 
YR
Yr
yR
yr
Significance of Mendel’s Research
Punnett Square was introduced as a tool to
predict or determine the probability of an
event
Now, let’s practice dihybrid crosses!
White board exercises
One-trait cross
The allele for the hairy
trait is “H,” while the
allele for the hairless
trait is “h.”
Make a cross between
two homozygotes for
each of these traits.
Punnett Square Exercise
One-trait cross
In a population of Wisconsin fast plants, purple
color is created by the pigment anthocyan. The
gene that codes for this pigment is dominant
(A), and without this pigment, the plant is green
recessive (a).
Purple
plant:
AA
Purple
plant:
Aaoffspring from
Predict
the
Green
plant:
the
of a purple
Greencross
plant: aa
aa
homozygous plant and a
greenA plant.
a
A
aa
A
Aa
a
WhataAaif the
plant
Aa purple
Aa
50%
Purple
100%
Purple
aa
Aa
was heterozygous?
How
Aa
50%be
Green
aa offspring
would the
different?
Green
Purple
Punnett Square Exercise
One-trait cross
In populations of hamsters, brown fur is
dominant (B), and white fur is recessive (b).
Predict the offspring from
Brown
hampster:
the cross
of aBbwhite
White
hampster:
hamster
andbba brown
hamster
ifb the brown
B
Offspring
b
hamster's
mother
was
b
Bb
50% brown mice
bb
Bb
white.
bb
50% white mice
Punnett Square Exercise
One-trait cross
Henry VIII divorced six of his
wives (two of whom were
executed) for not bearing him
any sons.
Use Mendelian Genetics to prove to King
Henry that it wasn’t his wives’ fault.
Remember:
Females are XX and Males are XY.
Go to Test
Crosses
White-board exercises
Two-trait cross
Following two traits:
Kernel shape: Plump vs. withered (P and p)
and Kernel color Red vs. yellow (R and r)
Predict the cross between a homozygous
recessive corn plant and a homozygous
dominant corn plant.
Punnett Square Exercise
Two-trait cross
Now take the offspring from that cross,
and self-pollinate that plant.
What is the phenotypic
ratio of this cross?
Punnett Square Exercise
Two-trait cross
My pet guinea pigs (Joni
and Chachi) are going to
have little guinea pigs.
What is the possibility that
their offspring will have long hair, if I know that
Joni and Chachi are heterozygous for the
following traits?
Black fur (B)
White fur (b)
Long hair (L)
Short hair (l)