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Mendelian Genetics
Mendel, Pea Plants, and Inheritance
Patterns
 Mendel spent most of his adult
life in Brno, a city near Vienna
that is now part of the Czech
Republic.
 His monastery was close to
European capitals that were
centers of scientific inquiry.
 Having been raised on a farm, he
was keenly aware of agricultural
principles and their applications,
and kept abreast of literature on
breeding experiments.
 He belonged to an agricultural
society and won awards for
developing improved varieties of
vegetables and fruit.
Mendel, Pea Plants, and Inheritance
Patterns
 After entering the
monastery Mendel took
courses in mathematics,
physics, and botany at the
University of Vienna.
 Few scholars of his time
showed interest in both
plant breeding and
mathematics.
 Shortly after his university
training, Mendel started to
study Pisum stivum, the garden
pea plant.
Mendel, Pea Plants, and Inheritance
Patterns
 Why the pea plant?
 The plant is self-fertilizing.
 It’s flowers produce both male and female
gametes.
 One lineage of plants can “breed true”
 This means that successive generations will be
like parents in one or more traits, as when all
offspring grown from seeds of self-fertilized,
white flowered parent plants also have white
flowers.
 Pea plants can also cross-fertilize when
plant breeders transfer pollen from one
plant to the flower of another plant.
 Breeders open a floral bud of a plant that bred
true for white flowers or some other trait and
snip out its stamens. Now it cannot produce its
own pollen. Now, the buds can be brushed with
pollen from a plant that bred true for a different
version of the trait—say, purple flowers.
Mendel, Pea Plants, and Inheritance
Patterns
 As Mendel hypothesized, such clearly observable differences
might help him track a given trait through many generations.
 If there were patterns to the trait’s inheritance, then those
patterns might tell him something about heredity itself.
Terms Used In Modern Genetics
 In Mendel’s time, no one knew
about genes, meiosis, or
chromosomes. However, in
order for you to understand
genetics, you must know the
following terms.
 Genes
 Units of information on
heritable traits, which parents
transmit to offspring.
 For example: eye color
 Each gene has a specific
location (locus) in
chromosomal DNA.
Terms Used In Modern Genetics
 Diploid
 Pairs of genes (2n) on homologous
(coding for the same
characteristics—like eye color)
chromosomes.
 Mutation
 Alters a gene’s molecular structure
and its message about a trait.
 May cause a trait to change, as when a
gene for flower color specifies yellow
and a mutant form of the gene specifies
white.
 Alleles
 All molecular forms of a the same
gene.
 For example, eye color can range from
blue to green to brown to any number
of combinations thereof.
Terms Used in Modern Genetics
 Pure bred or true-breeding
 When offspring inherit a pair of
identical alleles for a trait
generation after generation.
 For example: Pugs are true-breeding.
Each generation has the same coat
color, body type, personality, etc.
 Hybrid
 Offspring of a cross between two
individuals that breed true for
different forms of a trait.
 Each one has inherited nonidentical alleles for the trait.
 For example: Puggles are a
combination of pure-bred beagles and
pure-bred pugs. They will display a
combination of the traits from each
breed.
Terms Used in Modern Genetics
 Homozygous
 A pair of identical alleles on a
pair of homologous
chromosomes.
 For example: A child inherits the
alleles for brown eyes from both
parents (BB)
 Heterozyous
 A pair of non-identical alleles
on a pair of homologous
chromosomes.
 For example: A child inherits the
alleles for brown eyes from his
mother and blue eyes from his
father (Bb).
Terms Used In Modern Genetics
 Dominant
 An allele that masks the effect of
any recessive allele paired with it.
 Capital letters signify dominant
alleles.
 For example: A child inherits the
alleles for brown eyes from his
mother and blue eyes from his father
(Bb), but will display brown eyes.
 Recessive
 An allele that requires homozygous
in heritance to show up physically.
 For example: A child inherits the
alleles for blue eyes from both
parents (bb) and displays blue eyes.
Terms Used in Modern Genetics
 Homozygous dominant
BB
BB
 An individual has a pair of
dominant alleles (BB) for the
trait under study.
 Homozygous recessive
bb
BB
bb
BB
 An individual has a pair of
recessive alleles (bb).
 Heterozygous
 An individual has a pair of
bb
BB
bb
bb
non-identical alleles (Bb).
Bb
Bb
Terms Used In Modern Genetics
 Genotype
bb
 Refers to the particular
alleles that an individual
carries.
 For example: BB, bb, Bb
 Phenotype
 Refers to an individual’s
observable traits.
 For example: Brown eyes or
blue eyes.
Blue eyes
Mendel’s Experimental Approach
 For Mendel’s experiments, he
PP
coded each generation of
plants with a different letter.
 P = true-breeding parents.
 Example: Mom (PP) and Dad
(pp)
 F1 = first generation offspring
 For example: Child (Pp)
 F2 = second generation
offspring of self-fertilized or
cross-fertilized F1 individuals.
 For example: Grandchild can
either be (PP), (Pp), or (pp).
Pp
P
F1
pp
Pp
F2
PP
Pp
pp
Mendel’s Theory of Segregation
 Mendel used monohybrid
PP X pp (Parental)
experiments to test a
hypothesis: Pea plants inherit
two “units” of information
(genes) for a trait, one from
each parent.
P
P
p
Pp
Pp
p
Pp
 In monohybrid experiments,
two homozygous parents differ
in a trait that is governed by
alleles of one gene (like flower
color).
 They are crossed to produce F1
offspring that are all
heterozygous.
Pp
Monohybrid Experiment Predictions
 Mendel tracked seven traits for two
generations.
Pp X Pp (F1 generation)
 In one set of experiments, he crossed
plants that bred true for purple or white
flowers. (P)
 All F1 offspring had purple flowers.
 In the next generation (F2), some
offspring had white flowers.
P
p
 Mendel was confused, what was going on?
Mendel crossed seventy plants, and
recorded the number of dominant and
recessive forms of traits in thousands of
offspring.
 On average, 3 out 4 F2 plants were
dominant, and one was recessive.
o The ratio hinted that fertilization is a
chance event having a number of
possible outcomes.

P
PP
Pp
p
Pp
pp
Monohybrid Experiment Predictions
 Mendel knew that the principles
of probability, which applied to
chance events, could help him
predict possible outcomes of
genetic crosses.
 Probability
 The chance that each outcome of
an event will occur is proportional
to the number of ways in which
the outcome can be reached.
 Mendel had to do all his probability
problems by hand, but thanks to
Reginald C. Punnett, we have the
Punnett square to help us figure out
genetic probabilities.Yay Punnett!
Probability is written as 3:4:5:6:7…
Monohybrid Experiment Predictions
 Mendel’s experiments crossing two true breeding parents
would look like this: (yes, write this down)
 Mom plant has (PP) for purple flowers.
 Dad plant has (pp) for white flowers.
p
p
P
P
Pp
Pp
Pp
Pp
All the
offspring (F1)
are
heterozygous.
The
probability for
purple flowers
in this
generation is
4:0.
Monohybrid Experiment Predictions
 Mendel’s next experiments (F1) would look like this:
 Mom plant is Pp.
 Dad plant is Pp.
P
p
P
PP
Pp
p
Pp
pp
The resulting offspring
have a 3:1 chance
(probability) of
inheriting a dominant
allele (purple).
A hint on
probability: it will
always add up to
the number of
squares in the
Punnett square,
so if you come up
with 3:2 (which
equals 5), your
probability is off!
Testcrosses
 Testcrosses help
determine whether
organisms of an
unknown genotype, but
who show dominant
characteristics, will
have Pp or PP.
 By crossing an
unknown dominant(P?)
with a known recessive
individual (pp),
genotypes can be
figured out!
•Unknown genotype (P?)--will show purple flowers
•Recessive genotype (pp)–will show white flowers
p
p
P
?
Pp
?p
Pp
?p
If 50% of
the
offspring
are white,
then the
unknown
is Pp, if
none of
the
offspring
are white,
the
unknown
is PP
Theory of Segregation
 In modern terms, the theory of
segregation is:
 Diploid cells have pairs of genes (one
from each parent) on homologous
chromosomes. The two genes of
each pair are separated from each
other during meiosis, so they end up
in different gametes.
Pp
 In other words, if you have inherited
purple flowers (P) from one parent and
white flowers (p) from another parent,
your eggs/sperm will contain the allele
for EITHER purple (P) or white (p). Not
both.
The pollen
(plant
sperm) will
each receive
either P or p
The
ovum(plant
eggs) will
each receive
either P or p
Theory of Independent Assortment
 Mendel was wondering how
two genes (like purple
flowers and round seeds) are
sorted into gametes. Do
they sort together, i.e. every
time a plant gets purple
flowers, it gets round seeds,
or separately?
 In order to figure this out,
he did dihybrid
experiments.
Dihybrid Experiments
 Dihybrid experiments
follow two different traits
from one generation to the
next.
 They start with a cross
between true-breeding
homozygous parents that
differ in two traits (i.e.
flower color and seed
shape) governed by alleles
of two genes.
 Parent 1: Purple flowers
and Round seeds (PPRR)
 Both are dominant traits.
 Parent 2: white flowers and
wrinkled seeds (pprr)
 Both are recessive traits.
 NOW, you must find all the
different combinations that
the two parents can make.
 Parent 1: PR
 Parent 2: pr
Dihybrid Experiment
 Parent 1: PPRR
 Parent 2: pprr
Each offspring
turns out to be
a hybrid for
both flower
color and pea
shape.
Notice that
each square
has 4 letters
and two alleles
for each trait!
PR
PR
PR
PR
pr
P pR r
P pR r
P p Rr
P p Rr
pr
P p Rr
P p Rr
P p Rr
Pp Rr
pr
P p Rr
P p Rr
P p Rr
Pp Rr
pr
P p Rr
Pp Rr
P p Rr
Pp Rr
Dihybrid Experiment
 Now, Mendel crossed two
 P
p
R
r
 P
p
R
r
of the F1 generation.
 Plant 1: PpRr
 Plant 2: PpRr
 Before you can start the
dihybrid cross, you must
figure out all the possible
combinations.
 PR
 Pr
 pR
 pr
Dihybrid Experiment
 Parent 1: PpRr
 Parent 1: PpRr
The four resulting
phenotypes are purple
and round, purple and
wrinkled, white and
round, white and
wrinkled.
PR
Pr
pR
pr
PR
PP RR
PP Rr
Pp RR
Pp Rr
Pr
PP Rr
PP rr
Pp Rr
Pp r r
Purple and round = 9
Purple and wrinkled = 3
White and round = 3
White and wrinkled = 1
pR
Pp RR Pp Rr
p p RR
p p Rr
The ratio will be 9:3:3:1
pr
Pp Rr
pp
pp r r
Pp
rr
Rr
Theory of Independent Assortment
 Mendel’s dihybrid
experiments pointed to the
fact that traits are not
(usually) linked. Meaning
that a plant can have purple
flowers with wrinkled seeds
or purple flowers and round
seeds.
 OR, you can have brown
hair and brown eyes, or
brown hair and blue eyes, as
the two traits are NOT
linked together.
Flower color and seed shape are
NOT linked.
Theory of Independent Assortment
 In modern terms, the theory of independent
assortment is:
Brown hair
Blue eyes
 As meiosis ends, genes on pairs of homologous
chromosomes have been sorted out for distribution
into one gamete or another, independently of gene
pairs on other chromosomes.
 In other words, when you make eggs/sperm,
your offspring can have many different
combinations of traits. They do not HAVE to
have brown eyes, simply because they have
brown hair, as the two traits are independent of
one another.
o This theory is MOSTLY true, but now we
know that some traits ARE linked, like red
hair and freckles.
Blond hair
Brown eyes
Unexpected Patterns
 Mendel just happened to focus on traits that have clearly
dominant or recessive forms.
 However, expression of genes for some traits is not as
straightforward.
Codominance
 In codominance, a pair of non-
MOM
DAD
identical alleles affecting two
phenotypes are both expressed at
the same time in heterozygotes.
 These are for traits where there is
more than one dominant option
(like eye color!)
Yy
BB
 For example, if one of your parents has
yellow hair, and the other has blue, and
you get (YB) for hair color, then you
will have both blue and yellow hair!
 Both traits show up.
YB
YB
Codominance in real life!
 Red blood cells have a type of glycolipid on
their plasma membrane that give them their
unique identity.
 The glycolipid comes in slightly different forms:
A, B, or O.
 An enzyme dictates the glycolipid’s final structure.
 Three alleles for this enzyme are present in all
populations!
 Both A and B blood types are dominant. O is recessive.


They are written as IA, IB, and i.
The occurrence of three or more alleles for a single
gene among individuals of a population is called a
multiple allele system!
Codominance in real life!
 If you inherit IAIA or IAi, you will have A blood type.
 40 % of the US population has Type A blood.
 They can receive both Type A and Type O blood in an
emergency.
 If you inherit IBIB or IBi, you will have B blood type.
 11% of the US population has Type B blood.
 They can receive both Type B and Type O blood in an
emergency.
 If you inherit ii, you will have O blood type.
 45% of the US population has Type O blood.
 They can only receive type O blood, but they are universal
donors!
 Because both A and B are dominant alleles, if you inherit IAIB, you
will have AB blood type.
 4% of the US population has Type AB blood.
 They can receive both Type AB, Type A, Type B blood and O
blood in an emergency—They are universal recipients!
Incomplete Dominance
 In incomplete dominance,
one allele of a pair is not
fully dominant over its
partner, so the
heterozygote’s phenotype is
somewhere between the two
homozygotes.
MOM
bb
DAD
Bb
 For example, if one of your
parents has yellow hair, and
the other has blue, and you
get (Bb) for hair color, then
you will have green hair!
 A combination of the two!
Bb
Bb
Incomplete Dominance in real life!
 When true-breeding (pure bred) red
and white snapdragons are bred
together, their offspring end up being
pink.
 Red snapdragons have two alleles that
left them make a lot of molecules of
red pigment.
 White snapdragons have two mutant
alleles and are pigment-free.
 Pink snapdragons have a “red” allele and a
“white” allele; and make just enough
pigment to color the flowers pink.
Genes and the Environment
 The environment can affect the
expression of genes in surprising
ways.
 For example, the himalayan rabbit
is homozygous for an enzyme that
is heat sensitive. In the winter, this
enzyme causes the rabbit to grow
white fur, in the summer, the
enzyme causes the rabbit to grow
black fur.
 In humans, you may have genes
that dictate tallness, but if you
don’t get enough food to eat
during development, you may
end up short.
All three of these
children display
the symptoms of
Rickets disease.
Rickets is the
softening of the
long bones due to
deficiency in the
diet or impaired
metabolism.