Download Bio 111 Genetics Unit

Genetics
“The real key to explaining
Inheritance”
Objectives
To understand the experiments and
conclusions of Gregor Mendel
 To compare Monohybrid and Dihybrid
crosses
 To compare Complete, Incomplete and
Co-dominance
 To understand how Multiple Alleles work
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Gregor Mendel
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Austrian Monk in the
1800’s
Experimented with pea
plants
Kept detailed records of
several characteristics
for generations
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Height
Flower color
Pod color
Shape of peas and pods
Position of flowers on
the plant
Mendel’s Observations
If he had tall plants in one section of the
garden, they always produced tall
offspring
 Similarly if he had short plants in another
section the same would occur (short
offspring)
 He called these plants Pure Lines
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– Tall x Tall = Tall
– Short x Short = Short
– Mendel's Experiment
Mendel’s Experiment
He decided to cross (breed) these pure lines
Pure Line Tall x Pure Line Short
The result was ALL tall plants
He decided to cross these plants with each other
To his surprise, this resulted in both tall and short
plants
 The ratio was exactly 3 to 1 (tall to short)
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Parents
Tall (Pure Line) x Short (Pure Line)
First Generation
(F1)
All Tall Plants
Second
Generation (F2)
Tall & Short Plants
3 to 1 ratio (75% Tall & 25% Short)
Mendel’s Conclusions(4)
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All characteristics are controlled by factors
inherited from your parents
There are two factors for every trait
(characteristic)
– These traits can be hair color, length of nose,
skin tone, height, etc.
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One factor can dominate another
– ‘Mask’ the other
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These factors separate from each other
during sex cell formation (meiosis)
Modernizing Mendel’s
Conclusions
We now call these ‘factors’
Alleles
 Alleles can be Dominant or
Recessive
 We represent Alleles with
letters:
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– Capitals = Dominant Allele
– Lowercase = Recessive
Allele
– *Dominant Alleles always go
first
Genotype is the combination of
alleles you have
 Phenotype is the expressed trait
 Homozygous – 2 alleles that are
the same.
 Heterozygous – 2 alleles that are
different
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Genotype
Phenotype
TT
Tall Plant
Tt
Tall Plant
tt
Short Plant
BB
Brown Eyes
Bb
Brown Eyes
bb
Blue Eyes
Types of Genotypes
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Genotypes can also be expressed in words
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‘Homo’ – Latin prefix for ‘the same’
Genotype in
Letters
Genotype in Words Phenotype
TT
Homozygous
Dominant
Tall Plant
Tt
Heterozygous
Tall Plant
tt
Homozygous
Recessive
Short Plant
The Punnett Square
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Mendel started his
experiments crossing a
pure line of tall plants
(TT) with a pure line of
short plants (tt)
Parents
(TT x tt)
Cross
T
T
• He then took those
hybrids (Tt) and crossed
them with one another
Parents
(Tt x Tt)
Cross
t
T
t
t
T
t
Sample Problem # 1
Widow’s peak is
dominant to straight
hairline
 A homozygous
woman with widow’s
peak reproduces with
a man with a
homozygous man
with straight hairline
 What hairline will
their children (F1)
have?
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Hairline
Cross
W
W
w
w
• What genotype (in words)
can we expect from a
homozygous dominant,
homozygous recessive
cross?
• ALL offspring will be
heterozygous
Sample # 1 Continued
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Now if an offspring of that cross reproduces
with another of the same genotype, what
hairlines can we expect for their children?
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What genotype(s) (in words) can we expect
from a cross such as this?
What ratio of phenotype?
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Our Findings
Homozygous dominant crossed with
homozygous recessive yields heterozygous
offspring (100%)
 A heterozygous crossed with heterozygous yields
three options:
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– Homozygous Dominant
– Heterozygous
– Homozygous Recessive
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The ratio of those options are:
– A genotype Ratio of 1:2:1 (HD:H:HR)
– A phenotype Ratio of 3:1 (Dominant Trait :
Recessive)
Sample Problem # 2
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Attached earlobes are
recessive to un-attached
(hanging) lobes
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A homozygous dominant
man (for the trait) mates
with a heterozygous woman
(for same trait)
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What are the chances of
their offspring having
attached earlobes?
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The chances of their
child having a child
with attached
earlobes is 0%, 0 in
4, 0/4
Let’s follow Mendel’s Experiment
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P1 –
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F1 –
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F2 –
Let’s try some problems
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1. Genetics Practice Problems
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2. Bikini Bottom Genetics
Variations in Dominance
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All of the eight traits Mendel
looked at had two alleles
(dominant & recessive)
This ‘all-or-nothing’ expression
is known as complete
dominance
Another form is incomplete
dominance
This is where neither of the
alleles can be considered
dominant or recessive
The phenotype is a mixing of
the two different alleles
The classic case is with flowers
Red flowers crossed with white
flowers gives us pink flowers
Incomplete Dominance
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In order to represent these
alleles we need to make a
change
Since we’re talking about color
we’ll use the letter C
For each color we add the
capital superscript to the allele
Capital designation always
indicates Incomplete
Dominance
Therefore:
– CR = Red Allele
– CW = White Allele
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What the genotypes mean:
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CR CR = Red
CW CW = White
CR CW = Pink
CW CR = Pink
Codominance with Flowers
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With codominance neither allele is
dominant, they both show through
Lets look at flower color
In codominance a plant with red
flowers mating with one having
white flowers produces a plant
with what?
Both red & white flowers
So again we write the alleles
differently
Lower case designation indicates
Codominance
For flower color:
– Cr Cr Red Flowers
– Cw Cw = White Flowers
– Cr Cw = Red & White Flowers
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Lets cross a homozygous red
plant with a heterozygous one
What are the chances of our
baby plant having red & white
flowers?
Multiple Alleles
Some traits are controlled
by multiple alleles
 It still only takes two alleles
to make a gene
 However there are more
than two options for alleles
 For example, the eye color
of fruit flies have four
options for alleles
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E1
E2
E3
E4
=
=
=
=
Red
Apricot
Honey
White
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Each allele is dominant to
the remaining
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E1
E2
E3
E4
is
is
is
is
dominant to E2, E3, & E4
dominant to E3 & E4
dominant to E4
the recessive allele
Options for genes would be:
– E1 E4 = Red Eyes
– E2 E3 = Apricot
– E4 E4 = White
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Examples of Multiple Alleles:
– Blood Types
– Types of Corn
– Feather Color (in some
cases)
Let’s try a Multiple Allele Cross
Monohybrids & Dihybrids
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Everything we talked about so
far have been examples of
monhybrid crosses
Monohybrids – crosses that are
looking at one set of alleles
We can also do dihybrid
crosses
Dihybrids – crosses that are
looking at two sets of alleles
The following are two
examples
– A monohybrid cross
– A dihybrid cross
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The color is just coding for
different phenotypes
Independent Assortment
Dihybrid Example
In peas plants tall is dominant to short and purple flowers are dominant to
white
 So if a pea plant is heterozygous for height and heterozygous for flower
color what is their genotype?
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– TtPp
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During the creation of sex cells (meiosis) there are four combinations of
alleles the gamete (sex cell) could hold:
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TP (Dominant & Dominant alleles)
Tp (Dominant & Recessive alleles)
tP (Recessive & Dominant alleles)
tp (Two recessive alleles)
If a plant that is heterozygous for both traits reproduced with a plant
having the same genotype, what might the phenotype of their offspring be?
Dihybrid Cross Summary
If we look at the combinations of
phenotypes possible from a dihybrid cross
between two heterozygous organisms for
two traits we find a ratio of = 9:3:3:1
 9 = Tall plants with purple flowers
 3 = Tall Plants with white flowers
 3 = Short Plants with purple flowers
 1 = Short Plant with white flowers
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Test Cross
Dihybrid Cross Practice
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Dihybrid Cross Problems
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Guinea Pig Activity
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Mendel's Peas and the Pedigree Chart