Essential Vocabulary
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Trait: a specific physical characteristic. ex: seed color
Gene: sections of DNA that code for the production of proteins
that will make a physical trait.
Allele: options of what a gene can say. Most genes have at least
two options for alleles. (Reminder: children inherited two of every
section of DNA, they could be the same allele or different)
Heterozygous: an individual that has inherited two different alleles
for a gene.
Homozygous: an individual that has inherited two of the same
alleles for a gene.
True-breeding: individuals who have always produced offspring
with the same version of traits as the parent.
Hybrid: individuals who are capable of producing offspring with
different versions of a trait from the parent.
Phenotype: physical expression of the genetic information.
Genotype: the two forms of genetic information an individual
contains.
History of Genetics
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Modern genetics was
founded by an Austrian
monk named Gregor
Mendel.
Mendel was in charge of
the garden and decided
to use peas to
experiment with heredity
because peas are small,
easy to grow, and
produce hundreds of
offspring.
Mendel controlled
fertilization by picking up
pollen with a paint brush
and placing it onto the
plant to be fertilized.
Mendel’s Experiments
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In Mendel’s first
experiment he tested the
inheritance of one trait
(plant height).
Mendel crossed a true
breeding tall plant with a
true breeding short
plant. All of the offspring
(F1 generation) were
tall.
Mendel allowed the F1
generation to self
pollinate and the
offspring were ¾ tall and
¼ short (even though
the parent was tall).
Mendel’s Experiments Explained
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Punnett Squares: mathematical diagram that
helps predict the probable outcomes of a cross.
Each parent has two sections of DNA for the
trait. Each section is represented by a letter so
each parent has two letters.
Dominant versions of a trait are represented by
writing the first letter of the version as a capital.
Recessive versions of a trait are represented by
writing the same letter except as a lower case.
The two letters are separated and placed on one
side of the square as the letters would be
separated during meiosis.
The letters from the two parents are combined
to make all possible combinations of children.
Remember children should have two versions of
a trait so two letters.
Simple Inheritance Examples in Humans
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Cystic fibrosis- Recessive Trait
(CF) is common among white
Americans. 1 in 28 individuals
carry the allele and 1 in 2500
babies are born with the
disease. A defect in a protein on
the plasma membrane causes
formation and accumulation of
thick mucus in the lungs and
digestive track. Physical
therapy, diet, and new drugs
have extended the life
expectancy of these individuals.
 Cross a carrier of CF with an
individual that has CF. What are
the chances the children will
have the disease?
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Simple Inheritance Examples in Humans
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Huntington's- Dominant Trait
A disorder that results in the
breakdown of areas of the brain.
This disorder is lethal. Most of the
time dominant alleles with such
severe effects (death) would result
in death before the individual was
old enough to have children
preventing the individual from
passing the allele on. But
Huntington’s does not develop until
between the ages of 30 and 50.
 Cross a person who is
heterozygous for Huntington's with
a person who does not have
Huntington's. What are the chances
the children will have the disease?
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Mendel’s Second Experiment
Mendel wanted to know if two traits
would influence each other while being
inherited (ex: are all dominant versions
of all traits are inherited together?)
 To test this Mendel followed two traits
through two generations. The first
cross was using two true breeding
plants to ensure that all of the offspring
where heterozygous. Then he allowed
these plants to self pollinate.
 To create the punnett square
remember that each person should
have two versions of a trait for each
trait (so each person will have 4 letters)
 To find the combinations of letters that
go on the side of the square
(representing the gametes made) FOIL
the letters of the parent.
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Mendel’s Conclusions
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Law of dominance: when
more than one allele for
a trait exists the trait that
is expressed when the
two versions are in the
same individual is
dominant.
Law of segregation: each
offspring will inherit only
one allele for each trait
from each parent.
Law of independent
assortment: different
traits do not influence the
inheritance of each other.
Other Types of Inheritance
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Mendel studied traits that had two alleles. One
allele was dominant over the other in all of his
experiments. In reality not all traits are inherited this
way. The pattern of inheritance depends on the trait
in question and the resulting phenotypes.
Incomplete Dominance
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In this inheritance pattern there
is one section of DNA coding
for the trait (gene) and there
are only two versions (alleles).
The difference is in this trait
neither of the versions are
dominant. This means when
the two versions are inherited
by one individual, that
organism will express a
mixture of the two versions and
create a third phenotype.
Example: snapdragon flowers
can have a red version of DNA
or a white version of DNA.
When the two versions of DNA
are inherited together the
resulting color is pink (a
mixture of red and white).
Co-dominance
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In this inheritance pattern there
is one section of DNA coding
for the trait (gene) and there
are only two versions (alleles).
The difference is in this trait
both versions are dominant.
This means when the two
versions are inherited by one
individual, that organism will
express both versions and
create a third phenotype.
Example: Chickens can have
DNA for black feathers or DNA
for white feathers. If a chicken
inherits one piece of each DNA
type that chicken will have both
black AND white feathers.
Co-dominance Example in Humans
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Sickle cell
 The allele for sickle cells is a mutation of
the gene that codes for hemoglobin.
This mutation can cause hemoglobin to
clump and form long rods giving red
blood cells a sickle shape. These cells
get stuck in capillaries and cause a lot of
pain and deprive tissues of oxygen and
nutrients. However, the negative effects
are only experienced by individuals who
are homozygous for the trait, otherwise
there are not enough sickle shaped cells
to cause the problems.
 There is a good side to this. Being
heterozygous or homozygous for the
trait makes an individual immune to
malaria.
Co-dominance Example in Humans
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Cross two people who are heterozygous for
sickle cell anemia. What are the chances
their child will be immune to malaria? What
are the chances the child will have the
negative side effects of sickle cell disease?
Multiple Alleles
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In this inheritance pattern there
is one section of DNA coding for
the trait (gene) and there are
more than two versions (alleles).
Remember that each individual
can only inherit two alleles. This
inheritance combines simple
inheritance with co-dominance.
Two of the alleles are codominant and if inherited
together both will be expressed.
The third version of the trait is
recessive and will only be
expressed if both versions of the
trait inherited are recessive.
Multiple Allele Example in Humans
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Blood Types:
Blood type has three options for alleles: The DNA that codes for
the production of A proteins, the DNA that codes for B proteins,
and the DNA that codes for no proteins (O).
 If the DNA that code for the production of A and B proteins are
inherited together, then both proteins are produced and the result
is AB blood.
 If the DNA that codes for either protein is inherited with the DNA
that codes for no proteins the proteins are produced.
 Individuals that inherit two sections of DNA that both code for no
proteins will have type O blood.
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Blood types
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The proteins on the surface of your blood cells identify your cells
for your body. Your body makes antibodies that attack anything it
recognizes as foreign. If you mix the wrong blood types the
blood cells will clump together. If this occurs in the body the
blood cannot circulate and the person dies.
Sex-linked Traits
Remember humans have 23 pairs
of chromosomes.
 The first 22 pair are called
autosomes and are homologous.
 The 23rd pair are the sex
chromosomes. If an individual
inherits two X chromosomes for the
23rd pair then they are female. If the
individual inherits an X and a Y
chromosome then they are male.
 The X and Y chromosomes do not
carry the same genetic information,
therefore males inherit only one
version of each trait on the X
chromosome making it easier for
them to inherit and express a
recessive trait.
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Sex-linked Examples in Humans
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Color blindness and
hemophilia are
recessive traits that
are carried on the X
chromosome. If an
individual inherits two
X chromosomes then
they can be
heterozygous or a
carrier and not
express the
recessive trait.
Polygenic Traits
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In this inheritance pattern there
are many section of DNA
coding for the trait (genes) and
there can be more than two
versions (alleles) for each
gene. The expression of the
trait depends on how many
dominant and recessive
versions are inherited creating
a range of phenotypes. This
type of inheritance cannot be
represented in a punnett
square, it is represented by a
bell curve graph.
Example: Human hair color,
eye color, height, and skin
color
Karyotypes
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A picture of the chromosomes of a person. This allows us to
identify gender and any chromosomal mutations.
A human mutation identified using a karyotype is Down
syndrome. This is a mutation where an individual inherits three
of the 21st chromosome causing mental retardation and
physical abnormalities.
Pedigree
This is a graphic representation of
a family tree identifying genders,
relationship, and which individuals
posses the trait in question.
 Using a pedigree it is sometimes
possible to determine the
inheritance pattern of the trait in
question.
 Note that polygenic traits cannot
be followed using a pedigree.
 Copy the information in the top left
picture onto the bottom of your
notes. Also include that
generations are represented by
roman numerals while individuals
within a generation are assigned
aerobic numbers.
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Environmental Influences
Genetic information is not the only
contributing factor to the phenotype or
appearance of an organism. The
environment can sometimes determine
when and how genetic information is
expressed.
 Internal conditions (such as other traits
and changes in hormone levels caused
by age, proper diet, and exercise) can
control when traits are expressed.
Examples include:
 Blond hair when you were young and
dark hair when older.
 Proper diet and exercise can
decrease the chances of developing
diabetes and heart disease even if
you are genetically predisposed.
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Environmental Influences
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The external environment can
also play a role in the
expression of certain traits.
Examples include:
Temperatures determining the
coat color of arctic foxes.
 Smoking increases the
expression of lung cancer.
 Sun exposure as well as
vitamin D and folic acid
deficiency can cause skin
cancer to be expressed.
 Proper diet limits the effects of
PKU, a genetic disorder in
which dairy can cause severe
brain damage. If the individual
is not exposed to dairy then
they do not develop brain
damage.
***PKU is a recessive trait***
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