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Introduction to Genetics
Biology
Chapter 11
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11-1: The Work of Gregor
Mendel
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Into. To Genetics
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Genetics - the scientific study of
heredity
Gregor Mendel
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Austrian monk
Considered the “father of genetics”
The first person to succeed in predicting
how traits would be transferred from one
generation to another
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using the garden pea plant
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Parts of a Flower
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Gregor Mendel’s Peas
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Mendel used true-breeding pea plants
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Offspring is identical to the parents
Fertilization
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When the male gamete unites with the female
gamete
Self-pollination – produce true-breeding pea
plant
Cross-pollination – dusting pollen from one
plant onto the pistil or another plant
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Types of Pollination
Cross-pollination
self-pollination
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Genes and Dominance
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Trait – a specific characteristic, such as seed color or
plant height, that varies from one individual to
another
Hybrid - the offspring of parents that have different
forms of a trait, such as tall and short
Monohybrid cross (mono = one)
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The two parent plants differed by a single trait – height
P1 – parent generation
P = parent
F1 –first generation
F = “filial” son or
F2 – second generation
daughter
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Genes and Dominance
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Genes and Dominance
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The First Generation
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Crossed 2 true breeding
plants
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1 tall and 1 short
P1
X
All offspring of the 2
parent plants were tall
F1
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Genes and Dominance
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Mendel came to two conclusions:
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Factors that are passed from one generation to
the next determine inheritance.
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Scientist call the factors that determine these traits
genes.
The different forms of a single gene are called alleles.
(ex: tall & short)
The Principle of Dominance - states that some
alleles are dominant and some are recessive.
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An organism with a dominant trait will always be
dominant
An organism with a recessive trait will only show up
when a dominant allele is not present
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Dominate and Recessive
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Segregation
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Where were the recessive alleles?
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Mendel self-pollinated the plants from the first
generation
Segregation – The separation of alleles during
the formation of gametes (sex cells).
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¾ the offspring were tall
¼ the offspring were short
3:1 ratio tall to short
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Segregation
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Law of segregation
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During fertilization, male and female
gametes randomly pair to produce 3
combinations of alleles.
Concluded that each plant in the F1
generation carried one dominate allele and
one recessive allele and the F2 generation
either received 2 dominate; 2 recessive; or
one of each
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Segregation
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Recording the results for crosses
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Dominate allele is always written first
Uppercase letter is used for dominate
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T – tall
Lowercase letter is used for recessive
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t – short
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Segregation
X
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11-2: Probability and Punnett
Squares
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Genetics and Probability
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Probability – likelihood that a particular
event will occur
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Flipping a coin – 2 possible outcomes
(head or tails); with each flip there is a
50% chance it will be heads or tails
The principles of probability can be use
to predict the outcomes of genetic
crosses.
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Punnett Squares
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Punnett square – can be used to predict and
compare the genetic variations that will result
from a cross.
Homozygous – organisms that have two
identical alleles for a trait (TT or tt); truebreeding
Heterozygous – organisms that have two
different alleles for a trait (Tt); these are
hybrid
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Punnett Squares
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Phenotype
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The way an organism looks or behaves
What you see (i.e. tall, green, etc.)
Genotype
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The gene combination an organism
contains
The genetic makeup (i.e. TT, tt and Tt)
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Probability and Segregation
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If you self-pollinated a heterozygous
organisms it would result in:
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Genotypic ratio:
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¼ will be TT (both tall alleles); 2/4 will be Tt
(one of each allele); ¼ will be tt (both short
alleles)
Phenotypic ratio:
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3:1 (3/4 tall and ¼ short)
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Probability and Segregation
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White and purple garden pea flowering
plants
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Purple is dominate (P)
White is recessive (p)
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Homozygous Dominate Cross
Cross = Purple (PP) X Purple (PP)
P
P
P
P
P
P
P
P
P
P
P
P
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Homozygous Recessive Cross
Cross = White (pp) X White (pp)
p
p
p
p
p
p
p
p
p
p
p
p
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Heterozygous Cross
Cross = Purple (Pp) X Purple (Pp)
P
p
P
P
P
P
p
p
P
p
p
p
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Monohybrid Cross
Heterozygous tall parent = Tt
Cross = Tt X Tt
T
T
t
T
TT
Tt
t
Tt
tt
t
T

t
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Monohybrid Punnett Square
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11-3: Exploring Mendelian
Genetics
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Independent Assortment
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Independent assortment
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Genes from different traits are inherited
independently of each other; helps account
for the many genetic variations
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Ex. A pea plant that is RrYy, the alleles will
separate and the traits will separate
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Independent Assortment
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Dihybrid crosses - cross where the peas
differ in 2 traits
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A cross involving two traits
Ex. Seed color and Seed shape
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Independent Assortment
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The First Generation
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Two true breeding plants (P1)
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RRYY = round yellow seed
(homozygous dominate)
rryy = wrinkled green seed
(homozygous recessive)
When they were crossed all the plants had
round yellow seeds (F1)
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Independent Assortment
Dihybrid Cross = round yellow (RRYY) X wrinkled green (rryy)
RY
RY
RY
RY
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
ry
RrYy
RrYy
RrYy
RrYy
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Independent Assortment
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The Second Generation (F2)
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Self-pollinated plants from the first
generation
Resulted in 9 round yellow, 3 round green,
3 wrinkled yellow, 1 wrinkled green
A ratio of 9:3:3:1
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Independent Assortment
Heterozygous Cross = round yellow (RrYy) X round yellow (RrYy)
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
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Independent Assortment
Heterozygous round yellow seed parents = RrYy
Cross = RrYy X RrYy
RY
Ry
rY
ry
RY
Ry
rY
ry
RY
RY
RRYY
RRYy
RrYY
RrYy
Ry
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
rY
ry

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Independent Assortment
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Independent Assortment
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A Summary of Mendel’s
Principles
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The inheritance of biological characteristics is
determined by individual units known as genes.
Genes are passed from parents to their offspring.
In cases in which two or more forms (alleles) of the
gene for a single trait exist, some forms of the gene
may be dominant and others may be recessive.
In most sexually reproducing organisms, each adult
has two copies of each gene—one from each parent.
These genes are segregated from each other when
gametes are formed.
The alleles for different genes usually segregate
independently of one another.
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Beyond Dominant and
Recessive Alleles
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Some alleles are neither dominant nor
recessive, and many traits are
controlled by multiple alleles and
multiple genes
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Beyond Dominant and
Recessive Alleles
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Incomplete dominance
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Cases in which one allele is not
completely dominant over another
The phenotype of the
heterozygous is intermediate
between those of the two
homozygotes
Ex. Color of snapdragons
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Beyond Dominant and
Recessive Alleles
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Codominance
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both alleles contribute to the phenotype
Cause the phenotype of both alleles to be
expressed equally
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Beyond Dominant and
Recessive Alleles
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Multiple alleles
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Traits controlled by more than two alleles
In Labrador retriever, coat color is
determined by one gene with four different
alleles.
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Phenotype
Possible Genotypes
BBEE
BbEE
BBEe
BbEe
bbEE
bbEe
BBee
Bbee
bbee
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Beyond Dominant and
Recessive Alleles
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Polygenic traits
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The inheritance pattern of a trait that is
controlled by two or more genes
Genes may be on same or different
chromosomes
Each allele intensifies or diminishes the
phenotype
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Example: Eye color; skin color
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Warm up
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Who is the father of genetics
If a heterozygous red flower is crossed
with a white flower, what is the
percentage of white?
If a Roan cow (RW) is crossed with a
White Cow (WW), what is the
percentage Roan? (Incomplete
dominance)
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