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Heredity
It’s all in the genes
Why do I look like my Dad (or
Mom)?
My mom and dad
My brother and I
Why does my daughter look like
me (and my wife)?
The answer……
What is heredity?
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Heredity is the passing of traits from
parent to offspring
You inherit traits from your biological
parents
Both of your parents are responsible for
passing on your traits to you
What are traits?
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Characteristics that
are passed on from
parents to offspring
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Eye Color
Hair Color
Body Structure
Facial Features
Skin Color
So Who Figured all this out?
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Gregor Mendel
Austrian monk
Worked at the monastery in
the garden
Noticed that there were
patterns in the ways that the
peas reproduced
Noticed that some parent
plants looked like the offspring
and other did not
He wanted to know why!!!
Mendel chose pea plant for
several reasons
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They reproduce sexually
They have two distinct,
male and female, sex
cells called gametes
Their traits are easy to
isolate
Generation time was
short
Carrying out the crosses
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Designed a set of experiments to test his
ideas
Monohybrid crosses (involve a single trait)
Cut off the anthers of one flower and used
the pollen from these anthers to fertilize the
stamen on another plant.
Used the pollen from a plant with one trait
to fertilize the plant with the opposite trait
(purple flowers vs. white flowers)
The stigma of the flower actually turns into
seeds (peas). These seeds/peas are then
planted in the ground to produce more pea
plants.
Observed the first generation (F1) offspring
Mendel’s Pea Plants
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Noticed that some patterns of inheritance made sense and other did not.
Crossed purple and white- all of the offspring had purple flowers ????
Crossed purple offspring with each other -> ¼ offspring had white
flowers
Mendel wanted to know why?????
Mendel’s Pea Plants
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7 traits of pea plants.
Crossed plants with two
different forms of each trait
(for example,
smooth/wrinkled peas or
yellow/green peas) to
determine which traits would
appear and how often.
Determined which of the 7
traits were dominant and
which were recessive.
What did Mendel discover?
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The Rule of Unit Factors
 Each organism has two
alleles for each trait
 Alleles - different
forms of the same
gene
 Genes - located on
chromosomes, they
control how an
organism develops
What did Mendel Discover
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The Rule of Dominance
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The trait that is observed in
the offspring is the
dominant trait (uppercase
letters)
The trait that disappears in
the offspring is the
recessive trait (lowercase
letters)
What did Mendel discover?
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The Law of Segregation
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The two alleles for a trait
must separate when
gametes are formed
A parent randomly passes
only one allele for each
trait to each offspring
2 Flavors of genes:
Dominant and Recessive
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Each parent has two genes (or letters) for a trait. These
letters are called alleles.
Dominant alleles
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Represented in genotypes by CAPITAL LETTERS
When these alleles are present, they take over or show.
They are the “stronger” alleles
Recessive alleles
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Represented in genotypes by lower case letters
They are the “weaker” of the alleles.
What is homozygous?
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Both alleles [forms of the gene] are the same
When offspring inherit two dominant genes, (one
dominant gene from each parent) they are said
to be homozygous dominant (AA)
When offspring inherit two recessive genes, (one
recessive gene from each parent) they are said
to be homozygous recessive (aa)
What is heterozygous?
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When alleles occur in different forms
When offspring inherit one dominant gene
and one recessive gene, they are said to
be heterozygous (Aa)
Since the dominant gene will be
expressed, they are said to be
heterozygous dominant
Co-dominance/ Incomplete
Dominance
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Alleles not dominant or
recessive Blend of the traits
represented by the two
alleles
For example: One red allele
and one white allele-> pink
flower
Neither allele is dominant
Mendel’s impact
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Discoveries were ignored throughout the 1800s
Mendel died never knowing the importance of
what he had discovered
Work was reexamined in the early 1900s
Mendel is the known as the “Father of Genetics”
What is genetics?
Genetics is the study of how traits
are inherited
What is a gene?
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A gene is the basic unit of heredity
Genes hold the information to build and
maintain an organism's cells and pass
genetic traits to offspring.
Where do we find
these genes?
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􀂋 When organisms reproduce, traits are passed from parent to offspring.
􀂋 These traits are carried in DNA, the genetic material found in a cell’s
nucleus.
􀂋 DNA acts like a blueprint.
How are traits passed on?
2 Flavors of genes:
Dominant and Recessive
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Each parent has two genes (or letters) for a trait. These
letters are called alleles.
Dominant alleles
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Represented in genotypes by CAPITAL LETTERS
When these alleles are present, they take over or show.
They are the “stronger” alleles
Recessive alleles
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Represented in genotypes by lower case letters
They are the “weaker” of the alleles.
Examples of
Dominant and
recessive traits
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Tongue-rolling
(Roller is dominant,
non-roller is
recessive)
Widow’s peak
(dominant)
Ear Lobes
(Unattached is
dominant, attached
is recessive)
‘Bent’ phalanges
(dominant, normal is
recessive)
How are genes expressed?
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Genotype – the inherited alleles (letters)
Phenotype – what the alleles look like
when inherited.
Punnett Squares
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Using Punnett Squares we can get an idea
of what the offspring could look like
Allows us to calculate probability of certain
genotypes and phenotypes to occur
A monohybrid cross
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Using this is a several step process, look at the
following example
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Tallness (T) is dominant over shortness (t) in pea
plants. A Homozygous tall plant (TT) is crossed with
a short plant (tt). What is the genotypic makeup of
the offspring? The phenotypic makeup ?
The Punnett process
Alleles
for tall
plant
T
Alleles
for short
plant
t
t
T
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2.
Determine alleles of each
parent, these are given as
TT, and tt respectively.
Take each possible allele of
each parent, separate them,
and place each allele either
along the top, or along the
side of the punnett square
The Punnett process continued
Alleles
for tall
plant
Alleles
for short
plant
T
T
t
Tt
Tt
t
Tt
Tt
3. Lastly, write the letter for each
allele across each column or
down each row. The resultant
mix is the genotype for the
offspring. In this case, each
offspring has a Tt
(heterozygous tall) genotype,
and simply a "Tall" phenotype.
The Punnett process continued
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F1
parent
F1
parent
T
t
T
TT
Tt
t
Tt
tt
Here we have some more
interesting results: First we
now have 3 genotypes (TT, Tt,
& tt) in a 1:2:1 genotypic
ratio. We now have 2 different
phenotypes (Tall & short) in a
3:1 Phenotypic ratio. This is
the common outcome from
such crosses.
Determining Gender
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Female – XX
Male –XY
Male determines the
sex of the baby
Creating a pedigree activity
Quick Review
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Genotype=what genes someone has
Genes are usually represented by a letter, a capital letter
for the dominant trait, a small case for the recessive.
Ex. Tongue Rolling is dominant, so we use R to represent
the tongue rolling
Inability to roll your tongue is recessive so we use r to
represent the non-rolling gene
For every trait, you get a gene from each parent
Genotypes and Phenotypes
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Mom and Dad are Rr-that is
their genotype, they can also
be described as heterozygousthey have 1 of each gene
What is their PHENOTYPE?
The youngest son has a
genotype of rr-he is
Homozygous recessive-2
copies of the recessive gene
His phenotype?
Making a Pedigree chart
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Pedigree charts are
made to chart family
history and see how
traits are passed
Symbols on a pedigree chart
Homozygous dominant male
Homozygous dominant female
Heterozygous male
Heterozygous female
Unknown homozygous or
heterozygous male
Homozygous recessive male
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Unknown homozygous or heterozygous
female
Homozygous recessive female
Shade individuals that have the trait you’re studying
Males=squares , Females-circles
Drawing your pedigree
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Married-connected at side
Siblings-connected at
TOP not at the side
(hopefully not siblings
AND married)
Oldest child-to the left