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Genetics
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What accounts for the passing of genetic traits
from parents to offspring?
Are traits blended in the offspring?
Or: are traits inherited as single, discrete and
separate units (genes)?
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Today, much of what we know about genetics
and heredity started with the work of an
Austrian monk in the 1800s—Gregor Mendel
Mendel discovered the basic principles of
heredity with his experiments with garden
peas.
Fig. 14-1
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Varieties include those with distinct heritable
characteristics (traits) such as flower color or
plant height.
Pea plants are normally self-pollinated, but can
be easily cross-pollinated by the plant breeder.
Therefore, the breeder can control which traits
are crossed.
Pea Flowers have Petals that are
closed over the Stamens and
Carpels. They are not open to
wind and other pollinators such as
insects. Therefore, they are selfpollinating. What does this mean
regarding their genetic diversity?
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Mendel chose traits that were either..or
He also chose plants that were true-breeding
for a particular trait. (Purebred).
And, since peas plants are normally selfpollinated, he knew what genes they carried.
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P
In his experiments, Mendel crossed two plants
with contrasting traits, producing offspring
called hybrids.
The true-breeding parents are the P (parental)
generation
The offspring are called the F1 generation.
When the F1 generation plants self-pollinate,
their offspring are called the F2 generation.
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When Mendel crossed contrasting, truebreeding white and purple flowered pea
plants, all of the F1 hybrids were purple
When Mendel crossed the F1 hybrids, many of
the F2 plants had purple flowers, but some
had white
Mendel discovered a ratio of about three to
one, purple to white flowers,
in the F2 generation
Fig. 14-3-3
EXPERIMENT
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)

Purple
flowers
White
flowers
All plants had
purple flowers
F2 Generation
705 purple-flowered
plants
224 white-flowered
plants
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In the first experiment, when only purple
flowers were produced, Mendel thought that
the white trait had possibly been absorbed and
had disappeared.
However, when it reappeared in the F2, he
knew it had just been hidden.
So, he called the purple color
the dominant trait and the
white color the recessive trait.
Table 14-1
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Mendel developed a hypothesis to explain the
3:1 inheritance pattern he observed in F2
offspring
Four related concepts that
make up this model can be
related to what we now
know about genes and
chromosomes
Mendel’s garden in the abbey in
Austria where he conducted his
experiments.
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Alternative versions of
genes account for
variations in traits.
For example: there are
two versions of the gene
for flower color in peas:
purple and white.
These alternative
versions are called
alleles—each of which
resides in a particular
place on a chromosome
(the locus).
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For each
characteristic, an
organism inherits two
alleles: one from the
male parent and one
from the female
parent.
The alleles may be the
same (like the truebreeding plants), or
they may be different
(like the F1 hybrids).
Mendel figured all of this out
without ever knowing
anything about chromosomes!
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If the two alleles at a
locus are different,
then one of them (the
dominant allele)
determines what the
organism will look
like.
The recessive allele
has no noticeable
affect on the
appearance.
Why are all of the F1 offspring purple?
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The law of segregation states that the two
alleles for a heritable character separate
(segregate) during gamete formation and end
up in different gametes
An egg or sperm gets only one of the two
alleles that are present in the somatic cell.
Mendel figured this out
without ever knowing
anything about
meiosis!
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How do we explain
the 3:1 results that
Mendel got in the F2
generation?
If we know the
genetics of the
parents, a Punnett
Square can show the
possible combinations
of genes the offspring
can inherit.
Use CAPITAL letters for dominant
genes, lower case letter for recessive
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Homozygous—an
organism with two
alleles for a given trait
that are identical (also
called purebred)
Heterozygous—an
organism with two
alleles for a given trait
that are different.
(these are also known
as hybrids)
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An organisms’ traits
may not always reveal
its true genetics. (think
about those hybrid
purple plants)
Phenotype: what the
organisms’ physical
traits are—what it looks
like
Genotype: what genes
the organism carries—
what are its actual
genes?
Notice that the Genotype Ratio is
1:2:1 and the Phenotype Ratio is 3:1.
Why are they different?
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How can we tell the genotype of an individual
with the dominant phenotype?
Such an individual must have one dominant
allele, but the individual could be either
homozygous dominant or heterozygous
The answer is to carry out a testcross: breeding
the mystery individual with a homozygous
recessive individual
If any offspring display the recessive
phenotype, the mystery parent must be
heterozygous
Test Cross
Technique
TECHNIQUE
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Dominant phenotype, Recessive phenotype,
unknown genotype:
known genotype:
PP or Pp?
pp
Predictions
If PP
Sperm
p
p
P
Pp
Eggs
If Pp
Sperm
p
p
or
P
Pp
Eggs
P
Pp
Pp
pp
pp
p
Pp
Pp
RESULTS
or
All offspring purple
1 /2
offspring purple and
offspring white
1 /2
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Mendel figured out the Law of Segregation by
studying the results of crosses involving only
one trait (for example, flower color).
These are called monohybrid crosses.
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Mendel derived his second law by studying
crosses involving pea plants who differed in
two traits. These are called dihybrid crosses.
An organism who is hybrid for two traits is
called a dihybrid.
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A dihybrid cross, a cross between F1 dihybrids, can
determine whether two characters are transmitted to
offspring as a package or independently
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1. Figure out the genotypes of the parents.
2. Figure out the possible combinations of
genes that could be in the gametes of these
parents. Example: TtYy could produce 4 kinds
of gametes: TY, Ty, tY, ty
3. Put the gametes into a Punnett Square &
Solve
4. Figure out genotypic
and phenotypic ratios.
Law of Independent Assortment
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Using a dihybrid cross, Mendel developed the
law of independent assortment
The Law of Independent Assortment states
that each pair of alleles segregates
independently of each other pair of alleles
during gamete formation
Remember that in Meiosis, it is
random which direction the
chromosomes go during
Anaphase I and II. There are all
kinds of possibilities!
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The Law of Independent Assortment in general
applies only to genes on different, nonhomologous chromosomes
Genes located near each other on the same
chromosome tend to be inherited together
Remember that in Prophase I of Meiosis,
synapsis and crossing over occurs. When
crossing over occurs, it could happen
anywhere on the chromosome, but it is more
likely to happen on the ends.
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Inheritance of characters by a single gene may
deviate from simple Mendelian patterns in
the following situations:
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When alleles are not completely dominant or recessive
When a gene has more than two alleles
When a gene produces multiple phenotypes
Why does everything have to be so
complicated????
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Complete dominance—when the dominant
gene totally dominates over the recessive.
Most of the traits Mendel studied showed
complete dominance.
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In incomplete dominance, the phenotype of F1
hybrids is somewhere between the phenotypes
of the two parental varieties
Incomplete
Dominance in
Snapdragons
P Generation
Red
CRCR
White
CWCW
CR
Gametes
CW
Pink
CRCW
F1 Generation
1
Gametes /2 CR
1/
CW
2
Sperm
1/
2
CR
1/
2
CW
F2 Generation
1/
2
CR
Eggs
1/
2
CRCR
CRCW
CRCW
CWCW
CW
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In codominance, two dominant alleles affect
the phenotype in separate, distinguishable
ways
There isn't a blending of the traits, but rather
both alleles are present in the phenotype.
In this flower, both the dark pink allele
and the white allele are co-dominant.
Neither one dominates over the over,
so the phenotype shows both alleles.
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Polygenic Traits are characteristics that are
affected by more than one gene.
Examples: Eye Color, Skin Color
Eye color comes from different genes which affect
tone, amount and position of the pigments.
Skin color is determined by at least 3 different genes
working together to produce a wide variety of tones.
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Most genes exist in more that two allelic forms.
A classic example of this is human blood types.
Four major blood types exist: O, A, B, and AB
They are named for the presence or absence of
certain carbohydrates on the surface of the red
blood cells.
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The gene for blood
type has 3 possible
alleles: IA, IB, and i
Both IA and IB are
dominant over i
IA and IB are codominant to each
other
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Type A Blood
Genotypes: IAIA
or IAi
Type B Blood
Genotypes: IBIB
or IBi
Type AB Blood
Genotype: IAIB
Type O Blood
Genotype: ii
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In epistasis the
expression of one
gene affects the
expression of another
gene.
Example: Coat color
in mice.
Black (B) is dominant
over brown (bb).
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However, there is also
another gene locus
independent of the first.
This gene (Cc or CC)
controls whether there
will be ANY pigment
deposited on the fur (cc
will always have white
fur)
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In pleiotropy
(“pleion”=more), a
single gene has
multiple effects on an
organism.
A good example is the
gene that causes
Sickle Cell Anemia.
This single gene
causes blood cells to
be sickle-shaped.
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This leads to many
severe problems such
as heart, lung, and
kidney damage;
muscle pain;
weakness and fatigue
It can eventually lead
to kidney failure and
death—all because of
one gene.
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Probability is a concept
important to a full
understanding of
heredity.
What is the probability
that a flipped coin will
be heads? ½
What is the probability
that 2 coins flipped will
BOTH be heads?
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A di-hybrid or other
multi-character cross
is equivalent to two or
more independent
monohybrid crosses
occurring
simultaneously.
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The multiplication rule
states that to
determine the
probability of two
events occurring
together, we multiply
the probability of one
event X the
probability of the
second event.
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What is the
probability of flipping
a coin twice and
getting “heads” both
times?
½ X ½= ¼
What is the
probability of getting
heads 3 times in a
row?
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What is the chance of
getting a recessive
trait if two
heterozygous
individuals mate?
½ X ½ = ¼ (25%)
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Each egg has a ½
chance of carrying the
“G” gene and ½
chance of carrying the
“g” gene.
The same odds apply
to the sperm cell.
The probability that
two “g” genes will
appear is 25%.
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Sometimes, the
phenotype that is
observed depends on
environmental factors
as well as the genotype.
For example: in
humans, nutrition
affects height
Even identical twins
accumulate phenotypic
differences as they
experience different
environments.
Twins holding twins
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Pedigrees are family
trees used to describe
the genetic
relationships within a
family.
Pedigrees can be used
to determine the risk
of parents passing
certain conditions to
their offspring