Download Gregor Mendel

QOD

Who is considered to be the father of
genetics?
Gregor Mendel
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Genetics
The field of Biology
devoted to understanding
how characteristics are
passed from parents to
offspring
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Gregor Mendel
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Mendel studied heredity- which is the
transmission of characteristics from parent to
offspring
Mendel is most famous for studying pea plants
He studied what he called “factors” in pea
plants
–
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Factors would be height, pod color, flower color, etc
Gregor Mendel
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What were Mendel’s factors in reality?
– We call these traits today
– These traits are caused by genes on DNA
– Genes are segments of DNA that code for one
protein
– Mendel also used the word “factor” to describe
alleles, or alternate forms of genes
– Alleles for height would be short or tall
– Alleles for flower color would be white or purple
Some of
Mendel’s
Traits
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Gregor Mendel
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First, Mendel grew true-breeding plants
– According to Mendel true-breeding plants
are plants that will always produce
offspring with the same traits
– So a true-bred pea plant with purple
flowers will only produce plants with purple
flowers because it only has the genes for
purple (not white).
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
F2 Generation
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Purple White
flowers flowers
All plants had
purple flowers
Mendel then considered:

Phenotype- physical appearance
–
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Genotype- genetic makeup
–
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For example purple, wrinkled, tall, etc
This is usually abbreviated with letters like Gg,
FF, of hh
Gregor Mendel- P generation
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What he did not know:
– Purple flower color was dominant
– Dominant alleles mask recessive alleles
– White Flower Color was recessive
– We symbolize the dominant alleles with upper case
letters, like F
– We symbolize the recessive alleles with lower case
letters, like f
Gregor Mendel- P generation
–
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Mendel’s P generation had the genotypes
FF (for purple) and ff (for white)
 When the genotype has only one type of
allele it is called homozygous
 FF is homozygous dominant
 ff is homozygous recessive
Gregor MendelF1 generation
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All of the offspring of the P generation (which
he called the F1 generation) turned out
purple
Mendel called purple flower color the
dominant factor
He hypothesized that when the dominant
factor was present, the recessive factor
(white color) did not show.
Gregor Mendel- F1 generation
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What Mendel did not know:
– All of F1 pea plant flowers heterozygous
(two different alleles), or Ff
– That is why they were all purple
– Remember dominant alleles mask
recessive alleles
– Heterozygous organisms are also considered
called hybrids (two different alleles… one
dominate & one recessive)… the dominate
allele will show.
Let’s Review
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Phenotype – physical appearance (purple flower)
Genotype – genetic makeup (alleles) (Letters: Bb)
Dominate Allele – Will show over recessive allele
–
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Recessive Allele – Will not show with dominate allele
–
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(Lower Case Letter- b)
Homozygous – One type of allele (either both dominate or both recessive)
–
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(Capitol Letter- B)
(BB – homozygous dominate or bb – homozygous recessive)
Heterozygous – Two different alleles (one dominate & one recessive)
–
(Bb)
Let’s Practice
If red flowers (RR) are dominate over yellow (rr)
flowers then…
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RR – What is the phenotype? Is it homozygous
or heterozygous?
Rr - What is the phenotype? Is it homozygous
or heterozygous?
rr - What is the phenotype? Is it homozygous or
heterozygous?
Last slide
QOD
The offspring receives traits from
its parents this is called?
heredity
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Mendel’s Experiments
P
F1
F2
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Remember that Mendel
performed experiments
over three generations.
Genetics have a tool that
can predict the outcome
of genotypes and
phenotypes. It is called a
Punnett Square.
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Gregor MendelF1 generation
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Punnett Square:
– Shows possible
genetic
combinations in the
zygotes
– All offspring are Ff
and will therefore be
purple color leaves
F
F
f
Ff
Ff
f
Ff
Ff
Gregor MendelF2 generation
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Next, Mendel crossed the offspring from
the F1 generation (he called this the F2
generation)
– He observed that about 75% of the
flowers were purple and about 25%
were white
– This is equal to about a 3:1 ratio
Gregor MendelF2 generation
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Punnett Square:
– 3 of the 4 squares
(or zygotes) would
have purple flowers
– 1 of the 4 squares
(or zygotes) would
have white flowers
– Phenotypic ratio=
3 purple:1 white
F
f
F
FF
Ff
f
Ff
ff
Gregor Mendel- F2 generation
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Punnett Square:
– The genotypic
ratio:
 FF=1
F
f
F
FF
Ff
f
Ff
ff
 Ff=2
 ff=1
 So,
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1:2:1
Predicting the Results of Heredity
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Punnett Squares are used to predict the
outcome of genetic crosses
– The outcomes are often expressed as
percents and ratios
Let’s do a Punnett square for BB x Bb
B = black fur in bunnies
b = white fur in bunnies
Predicting the Results of Heredity
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Phenotypic ratio:
– 100% black bunnies
Genotypic ratio:
– 2BB:2Bb:0bb= 2:2:0
What are the chances
of a white bunny?
– 0%
B
B
B
BB BB
b
Bb Bb
Predicting the Results of Heredity
 Let’s
look at a heterozygous
cross
– Bb
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x Bb
Predicting the Results of Heredity
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Phenotypic ratio:
– 3 black :1 white = 3:1
Genotypic ratio:
– 1BB:2Bb:1bb = 1:2:1
What are the chances of
a white bunny?
– 25% (1 in 4)
What are the chances of
a black bunny?
– 75% (3 in 4)
B
b
B
BB Bb
b
Bb bb
Predicting the Results of Heredity
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In a monohybrid cross (only one trait is
considered)
When writing ratios:
– Phenotypic, the dominant # goes first (that’s
why it is 3:1 in the last example and not 1:3)
– Genotypic, it goes homozygous dominant
(BB), heterozygous (Bb), then homozygous
recessive (bb) ( in the last example 1:2:1)
– Numbers will always add up to 4 in
monohybrid cross!!
Predicting the Results of Heredity
What do these ratios and percents mean?
– If we flip a coin, there is a 50% chance that it will
land on heads. But it is still possible to get 5 tails in
a row (although it is highly UNLIKELY!)
– The more times you flip it, the more likely your
results will be 50:50
– If Bb and Bb bunnies mate, there is a 1:4 chance
the offspring will be white (this does NOT mean
that they will or will not have white bunnies)
– If they have LOTS of children, about 25% of them
will be white
Last slide
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QOD
What are the different forms of
a particular gene called?
Alleles
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Mendel’s Laws
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Keep in mind that Mendel knew nothing of
Punnett squares, genes, alleles, or even DNA!!!
All he could do was observe phenotypes and
record ratios and other statistics
He came up with 2 important laws as a result of
his observations:
– Law of Segregation
– Law of Independent Assortment
Mendel’s LawsLaw of Segregation
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Mendel concluded that each plant gets two
factors (alleles) for a characteristic and when the
plant reproduces, these two factors separate or
segregate. So…
– Each gamete (sex cell) gets one factor (allele)
AND therefore…
– Each offspring gets one factor from each
parent
Law of Segregation is shown in
Punnett Squares
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Notes on Mendel
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The reason alleles come in pairs is because
chromosomes come in pairs!!
– One allele on each chromosome!
– WHAT A COINCIDENCE!!!
Is inheritance always this simple?
– NOOOOO! There are many other types of inheritance
besides monogenic complete dominance
 That would be a trait controlled by only one gene,
where one allele is dominant over another
Mendel’s LawsLaw of Independent Assortment
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Mendel looked at 14 plant traits in total
He even did experiments using more than
one trait (like height and seed color)
He noticed that one trait did not influence the
inheritance of another trait
In other words, different factors separate
independently of each other during the
formation of gametes
Mendel’s LawsLaw of Independent Assortment
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Examples:
– Pea plants can be short or tall
– Their seeds can be green or yellow
– Short plants can have green or yellow seeds
– Tall plants can have green or yellow seeds
– So the inheritance of one does not affect the
inheritance of the other.
– Mendel noticed this with all the traits he
studied
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Notes on
Mendel
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Independent Assortment is not always true–
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If different genes are located on the same
chromosome, then they will most likely be
inherited together
These are called Linked Genes
The only way to separate linked genes is through
crossing over during meiosis.
Last slide
Predicting the Results of Heredity
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Dihybrid Cross- a cross in which two
characteristics are tracked
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Remember… Monohybrid Cross- a cross
in which only one characteristic is tracked
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For example- if we crossed two pea plants and
were concerned with the flower color and plant
height
For example- if we crossed two pea plants and
were only concerned with the flower color
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Test Cross
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When genotypes are not know, a testcross can be
performed to figure it out
– The organism with an unknown genotype is crossed
with a homozygous recessive individual.
– In the bunny example, if a Bb crossed with bb, white
bunnies are produced
– BUT if a BB crossed with bb, no white bunnies are
produced
– If a testcross produces white bunnies, we know the
unknown genotype is Bb; if not the genotype is BB
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QOD
The inheritance of 1 trait having no
effect on the inheritance of another is
known as the?
Law of independent
assortment
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Complex Inheritance
It’s not usually that
simple….
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Other Types of Inheritance
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Incomplete Dominance
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The phenotype of the heterozygote is
intermediate between phenotypes of the dominant
and recessive traits
Example: when a homozygous red carnation is
crossed with a homozygous white carnations,
then pink carnations are produced
Incomplete Dominance
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RR = Red
Rr = pink
rr = white
Other Types of Inheritance
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Codominance
– Occurs when both alleles for a trait are expressed in
heterozygous offspring
– Codominant alleles are often symbolized with
different letters
– For example, human blood types have 3 alleles A, B,
and O. O is recessive to A and B:
 Genotype AO or AA = A blood
 Genotype BO or BB = B blood
 Genotype OO = O blood
 Genotype AB = AB blood (both alleles expressed)
Codominance
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BB = Brown
BW = Roan
WW = White
Notice both brown and white are present in the heterozygote
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Other Types of Inheritance
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Multiple Alleles:
– Genes with 3 or more alleles (or
variations)
– Not only does human blood type show
codominance, it also has multiple
alleles- A, B, and O
Blood Type
Last slide
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QOD

What would be the blood type of a
person who inherited a B allele from
one parent and an O allele from the
other?
B Type Blood
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Other Types of Inheritance
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Sex-Linked Genes and Traits
– Remember sex chromosomes are the
chromosomes that determine the sex of an
organism
– So these are traits/genes carried on sex
chromosomes
– These traits are symbolized using a
superscript on the X or Y, such as Xr or XR
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Other Types of Inheritance
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Sex-Linked Genes and Traits Examples:
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The Y chromosomes on males carries a gene that
codes for a protein that causes the gonads
(reproductive organ) to develop as testes instead
of ovaries
Other Types of Inheritance
Sex-Linked Genes and Traits Examples:
– In fruit flies, the gene for eye color is on the X
chromosome. Red (XR) is dominant, white (Xr) is
recessive.
– To have white eyes, females must have the genotype
Xr Xr , or in other words TWO white alleles
– To have white eyes, males must have the genotype
Xr Y, or in other words ONE white allele
– This is why sex-linked traits are more common in
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males
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Other Types of Inheritance
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Sex-Linked Genes and Traits Examples:
–
–
–
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Other examples of sex-linked traits:
1. Color Blindness
2. Hemophilia – blood doesn’t clot
Other Types of Inheritance
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Polygenic Inheritance:
– Traits that are controlled by more than
one gene
– Most human traits are polygenic
– Examples are height, skin color, eye
color, and hair color
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Other Types of Inheritance
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Complex Characters:
– Characters that are influenced by
genetics AND the environment
– Skin color and height are examples
Other Types of Inheritance
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Sex-Influenced Traits:
– Traits in which males and females show
different phenotypes even though they
have the same genotypes
– Baldness is an example- it is dominant in
men, but recessive in women
– The differences are mainly due to males
and females producing different hormones
(chemical signals)
Other Types of Inheritance
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Single Allele Traits
– traits where there is only one allele
– If you have the allele you have the traitthere is no recessive
– Huntington’s disease is an example
Last slide
QOD
 Skin
color, eye color, and height
are examples of which type of
traits?
Polygenic Inheritance
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Pedigrees
Another way to show
heredity….
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Pedigree is a chart or “family tree” that tracks which
members of a family have a particular trait.
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Pedigrees
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In pedigrees carriers have one copy of the
recessive allele
– So they CARRY the trait, but they do not
show it
Pedigrees can be used to make predictions
about
– Future offspring
– The genotype of individuals in the
pedigree
#63
Ww
ww
ww
First generation
Ww (grandparents)
1
Ww ww ww Ww Ww ww
2
3
WW
or
Ww
Widow’s peak
Second generation
(parents plus aunts
and uncles)
Third
generation
(two sisters)
ww
No widow’s peak
Dominant trait (widow’s peak)
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ww= no widow’s peak
WW= widow’s peak
LE 14-14b
1
First generation
(grandparents)
Second generation
(parents plus aunts
and uncles)
Ff
Ff
?
3
FF or Ff ff
ff
Third
generation
(two sisters)
Attached earlobe
Recessive trait (attached earlobe)
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ff
Ff
Ff
ff
FF
or
Ff
ff
2
Free earlobe