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Genetics
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Gregor Mendel
Genotypes: Homozygous and Heterozygous,
Dominant, Recessive
Phenotypes: Traits you see
Law of Heredity
- law of segregation
- law of independent assortment
Probability: punnett squares
Pedigree charts: genes are sex-linked or
autosomal
Polygenetic traits, incomplete dominance,
codominance, multiple alleles.
Gregor Mendel: Father of Genetics
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Found the following using Pea plant
- Inheritance: passing of traits
- Heredity: transmission of traits from parents to offspring
Used pea plants for his experiments. WHY??
- Either the flower is either purple or white, no intermediate
colors such as pink
- You can control pollination. (able to control the mating)
- small, easily grown, matures quickly, produces many offspring
Mendel’s Experiments
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He did the experiment by both self-pollinating (pollen is not
transferred to another plant, the plant uses its pollen) and crosspollinating (cross-fertilization: transferring pollen from one plant to
the other)
His experiment
1. He crossed a purple flower with a purple flower producing
plants with purple flowers and a white flower with a white flower
producing plants with only white flowers. He referred to this as
“True-breeding” (display 1 particular trait). These plants
served as his parental generation or “P” generation
EX: White X White = All White
Purple X Purple = All Purple
Mendel’s Experiment Con’t
2. He then crossed a white P generation flower with a purple
P generation flower. He called the offspring of the P generation
the F1 Generation. All offspring were purple
EX: White X Purple == All Purple
3. He then enabled the F1 generation to self-pollinate. This produced
the F2 generation. 705 were purple and 224 flowers were white .
A ratio of 3:1
EX: Purple X purple == 3 Purple, 1 White
See the following website:
http://www2.edc.org/weblabs/Mendel/mendel.html
Mendel’s Conclusions
1. He concluded that purple was the dominant color
2. He concluded that purple was masking the white color.
3. He concluded that white was recessive because it
returned in the F2 generation
Mendel’s Hypotheses
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1. For each inherited trait, an individual has 2 copies of
the gene- one from the father one from the mother
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the process of meiosis, parents
can only contribute one allele for an
inherited trait.
 When two different alleles occur together,
only 1 may be completely expressed. The
other may not have an observable effect
on the organism’s appearance.
Other Important Info.
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Dominant: The gene (trait) that is expressed in the physical
appearance. Represented by a capitol letter (T, S, H)
Recessive: the gene (trait) that is not expressed, but you still have
the gene for the trait. It is masked by the dominant trait.
Represented by a lower case letter. (t, s, h)
Genotype: set of alleles or genes that an individual has
Phenotype: The physical appearance of an individual. (determined
by your alleles)
Alleles can be homozygous or heterozygous
-- homozygous: alleles are the same for a particular trait
ex: TT, SS, HH=== Homozygous dominant
tt, ss, hh=== Homozygous recessive
--- heterozygous: alleles are different for a particular trait
ex: Tt, Ss, Hh
Laws of Heredity
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Mendel’s findings led to the laws of heredity
2 laws
1. Law of segregation: two alleles for the same trait separate
when gametes are formed. (remember when chromosomes
separate during meiosis)
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2. Law of independent assortment: alleles of different genes (ex
hair and eye color), separate independently of one another during
gamete formation. One gene does not influence the inheritance
of the other.
Studying Heredity
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Punnett Square: Finding the probability that a
trait will be passed from one generation to the
next.
-- monohybrid: 1 trait
-- dihybrid: 2 traits
Pedigree charts: family history chart that shows
how a trait has been inherited over several
generations.
Punnett Squares
Monohybrid crosses
A man homozygous dominant for blonde hair marries a women
who in homozygous recessive for black hair. What is the
likelihood that their children will have black hair??
b
b
B Bb
Bb
Genotypes: 4 or 100% Heterozygous
Phenotypes: 100% Blonde
B Bb
Bb
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No chance of having a child with black hair
Punnett Squares Continued
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Dihybrid crosses
-- Look at 2 traits. Two heterozygous black haired and blue eyed
people marry. Look at probability of having a child with black
hair and blue eyes. B- black, b-blonde, E-blue, e-brown
-- Genotypes Father: Bb, Ee
Mother: Bb, Ee
1. find all possible genotypes: Father BE, Be, bE, be
Mother: BE, Be, bE,be
Dihybrid cross
BE
Be
bE
be
BE
Be
BBEE BBEe
BBEe BBee
BbEE BbEe
BbEe Bbee
bE
BbEE
BbEe
bbEE
bbEe
Phenotypes
9 or 56.25% black haired and blue eyed
3 or 18.75% blonde haired and blue eyed
1 or 6.25% blonde haired and brown eyed
3 or 18.75% black haired and brown eyed
be
BbEe
Bbee
bbEe
bbee
Genotypes
4 Homo dominant
8 Heter
4 Homo Recessive
Ratio: 1:2:1
Pedigree Charts
 A family history that shows how a trait has been inherited over many
generations.
 Scientist can determine the following:
Autosomal or sex-linked? If a trait is autosomal,
traits will appear in both sexes equally. If a trait is
sex-linked, males will primarily show the trait. A sexlinked trait is a recessive trait whose allele is located
on the X chromosome. Because males only have
one X chromosome, a male who carries this
recessive allele on the x chromosome will show the
trait. The only way a female will exhibit the trait if both
of her X chromosomes carries the recessive trait.
Dominant or Recessive? If the trait is autosomal
dominant, every offspring that has the trait will have a
parent with the trait. If it is recessive, the individual
will have may not have one or neither of their parents
show the trait.
Heterozygous or Homozygous? If the trait is
dominant, they will have a genotype of homozygous
dominant or heterozygous and their phenotype will
show the trait. Two people heterozygous for a
recessive trait will not show the trait but can pass it on
to their children.
Pedigree charts
 Horizontal lines: indicate matings
 Vertical lines: Offspring
Males
Male affected
females
female affected
Carrier
Complex Heredity
There is more to the patterns of heredity
than the simple dominant/recessive
patterns.
 Polygenic traits
 Incomplete dominance
 Co-dominance
 Multiple alleles
 Environmental influences
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Polygenic traits
Traits influenced by several genes
Hard to determine effects because many different
combinations occur due to independent assortment and
crossing-over.
Examples: eye color, height, weight, hair and skin color.
Incomplete Dominance
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An individual displays a trait that is intermediate
between the two parents.
Ex: Red flower is crossed with a White flower: Results
in pink flowers
Pink flowers because they show less pigment than red
but more than white.
Ex: One parent with curly hair and one parent has
straight hair and their child has wavy hair.
More Examples
•If a red tulip and a white tulip are cross
pollinated they result is a pink tulip.
 •A highly spotted cat and a cat without
spots has an offspring that has only some
spots.
 •A person with big hands and a person
with small hands have offspring with
hands of average size.
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Incomplete dominance Punnett
Square
Snapdragons are incomplete dominance in their
colors: Red, white, pink
 Red- RR
 White- WW
 Pink- RW (Blended Phenotype)
Pink and Pink?
 R
W
R RR
RW
W RW
WW
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Co-dominance
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Traits with two forms displayed at once.
2 dominant alleles displayed at the same time
EX: A homozygous white horse and a homozygous red
horse produces a heterozygous offspring. The offspring
will show both red and white hair and in equal numbers.
(called a Roan horse)
Co-dominance Example
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In shorthorn cattle, color shows co-dominance: A red
cow is RR, and white cow is WW. Heterozygous cattle
are called roan RW (red and white)
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Cross a roan bull with a roan cow
R
W
R
RR
RW
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W
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https://youtu.be/fQvER3MyI2c
RW
WW
Multiple Alleles
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Genes with 3 or more alleles.
ABO blood system: IA, IB, i
A and B are carbohydrates located on red blood cells
i does not have this carbohydrates
Nor A or B are dominate over each other: they are codominant
They are dominant over i
3 alleles can produce 4 genotypes: A, B, AB, O
IA
IB
i
IA
IAIA
IAIB
IAi
IB
IBIA
IBIB
IBi
i
IAi
IBi
ii
Blood types and their genotypes
A= AA, AO
 B- BB, BO
 AB= AB
 O=OO
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Blood Type
% of Population
Can Give
Blood to
Can Receive Blood from
Chance of Finding a
Compatible Donor
A+
34.3%
A+, AB+
A+, A-, O+, O-
80% (4 out of 5)
A-
5.7%
A+, A-,
AB+, AB-
A-, O-
13% (1 out of 8)
B+
8.6%
B+, AB+
60% (3 out of 5)
B-
1.7%
B+, B-,
AB+, AB-
B+, B-,
O+, OB-, O-
AB+
4.3%
AB+
Universal recipient (can
receive all blood types)
100%
AB-
0.7%
AB+, AB-
14% (1 out of 7)
O+
38.5%
O+, A+,
B+, AB+
AB-, A-,
B-, OO+, O-
O-
6.5%
Universal
donor (can
donate to all
types)
O-
7% (1 out of 1
9% (1 out of 12)
50% (1 out of 2)
The Rh System
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The Rh System is used to make the classification system
more precise. The Rh factor, or the positive/negative aspect
of blood, is inherited separately from the ABO classification
Mother’s Type
Rh +
Father’ Rh +
s Type Rh -
•
•
Rh -
Rh+, Rh- Rh+, Rh- Child’s
Type
Rh+, Rh- Rh-
The parents’ Rh factors may be incompatible. It is important for pregnant
women to have a blood group test so that any complications don’t go
untreated.
Rh- women who are of childbearing age should only receive Rhtransfusions to prevent complications with pregnancy.
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I woman who has blood type A has a child
with a man with O type blood. What are
the possible blood types their child could
have?
(choose the genotype that is going to produced the most possible genotypes for
offspring)
Mom- AA, AO
 Dad- OO
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O
A
AO
O
OO
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O
AO
OO
Environmental Influences
EX: Hydrangea plants: color of flowers depends
on the acidity of the soil
 Hair Color of some animals: Artic fox; during
summer months it turns reddish brown to blend
in with its environment. In the winter it is white
 Humans: skin color- exposure to the sun,
behavior, height and weight controlled by
nutrition. Identical twins are genetically the
same but can be very different
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