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Gregor Mendel
“The father of Genetics”
1822- 1884
Mendel was experimenting with flowers in Czechoslovakia, in the
monastery's gardens, trying to develop new color variations. It was
these experiments that led to his experiments in hybridization.
One belief was that traits were stored as 'particles' in the parts of
each parent's body and 'blended' in the offspring.
This theory left many questions un-tackled, however.
Why pea plants?
MENDEL chose a common garden pea (Pisum) for his first
experiments in hybridization. These plants exhibited what are
now called "Mendelian Traits" - traits which occur in a very
simple form.
A simple trait in an organism is one which occurs either in one
variation or another, with no in-between.
In the process of experimenting, he ended up making 287
crosses between 70 different purebred plants. Approximately
28,000 pea plants were used! This does not take into account
the other species of plants he experimented on!
3 Steps in his Experiment
1. Self Fertilization = Pure Bred
P generation = Parental Generation
2. Cross fertilization - hybrid
F1 generation = First generation (filius- son / fila-daughter)
3. Cross F1 generation - two hybrids
Produced 929 F2 Plants
705 purple (76%) and 224 White (24%)
Concepts learned from Mendel
Alleles – Alternate forms of a gene. (P) or (p)
Dominant – The allele that is expressed. (P)
Recessive – Allele is present, but unexpressed unless both
alleles are recessive.(p)
Homozygous – Both alleles are the same (PP or pp)
Heterozygous – Alleles are different. (Pp)
law of segregation, alleles separate, when the F1 generation
produces gametes.
Punnett Squares- A punnett square indicates the
PROBABILITY of what the offspring from two individuals will look like
Steps to solve punnett squares:
1. Define the trait (P=purple p= white)
2. Identify the parents
3. Set-up the punnett square
4. Complete the cross
5. Determine ratios
Cross two hybrid purple flowers
1. Define the trait
P= Purple
p= white
2. What are the parents
genotypes?
3. Set-up the cross
4. Perform cross
5. Determine ratios
Pp x Pp
X
P
p
P
PP
Pp
p
Pp
pp
Genotypic ratio: 1PP: 2Pp:1pp
Phenotypic ratio 3 Purple 1:white
Monohybrid Cross
• A cross involving a simple Mendelian trait.
• A mother has a pure dominant genotype (RR) for
rolling her tongue. The father is hybrid (Rr) for
this trait. A punnett square will show the possible
offspring for each trait.
R
R
Genotypic ratio- 1RR:1Rr
R
RR
RR
Phenotypic ratio –
r
Rr
Rr
1Roller:0 non-roller
A man, heterozygous for tasting PTC, is crossed with a
woman who can also taste PTC. However, their 15 year
daughter can not taste PTC. How can this be possible?
• Phenylthiocarbamide,
also known as PTC
• About 70% of people
can taste PTC
• Used for paternity tests
before DNA testing.
T
t
T
TT
Tt
t
Tt
tt
Genotypic ratio: 1TT:2Tt:1tt
Phenotypic ratio: 3 Taster: 1 non-taster
Practice Problem
• Maya has a hitchhiker's thumb, a dominant
trait. As a matter of fact, she is a pure bred.
She decides she wants a child some day
with straight thumbs. Is this possible?
Next Hypothesis
Are 2 traits always passed on together, or are they
independent of one another?
Example: Flower color and seed coat
Will purple flowers always produce wrinkled seeds?
Conclusion:
Law of independent Assortment – genes for different
traits segregate independently of one another during
gamete formation.
Cross a pure breeding round yellow plant (RRYY)
with a wrinkled green plant (rryy)
1 RrYy:0
1Round
yellow: 0
RY
ry
RrYy
ry
RrYy
ry
RrYy
ry
RrYy
RY
RY
RY
Now complete the cross of two
F1 plants.
What are the parents genotypes?
Possible gametes for RrYy
RY
RY
Ry
rY
ry
Ry
rY
ry
Practice with gametes
• What is the genotype of a wrinkled yellow(YY) pea plant? rrYY
• What are the possible gametes? All are rY
• What is the genotype of a heterozygous Round yellow
(YY) pea plant? RrYY
• What are the possible gametes? RY or rY
• What are (is) the possible genotype(s) for a wrinkled green
pea plant? rryy
• What are the possible gametes? All are ry
Exceptions to Mendel’s Rules
• Even though Mendel did
a tremendous job
teaching us about
genetics there are some
exceptions to his work.
Some alleles are neither
dominant nor recessive,
and many traits are
controlled by multiple
alleles or multiple genes.
Incomplete Dominance
• One allele is not
completely dominant
over another so the
traits blend together
• Snapdragons are a
good example of
incomplete
dominance.
WW
RR
RW
Pink
Cross a red and a white
Parents RR x WW
R
R
W
RW
RW
W
RW
RW
Genotypic ratio – 1RW:0
Phenotypic ratio – 1pink:0 red: 0 white
Cross a pink and a red
• Parents genotypes
R
RW X RR
W
Genotypic ratio-
R
RR
R
RR
RW
RW
1RR : 1 RW
Phenotypic ratio1 Red : 1 pink : 0 White
Codominance
• both alleles contribute
to the phenotype – the
alleles are equally as
dominant.
• HCHC – Curly Hair
• HSHS – Straight Hair
• HCHS - Wavy Hair
Cross two wavy haired people
Parents Genotype HCHS X HCHS
HC
HC
HC HC
HS
HC HS
1 curly
2 wavy
1 straight
HS
HC HS
HS HS
Another exception –Sex-Linked
a trait carried on the sex chromosome
• A normal males genotype - XY
• A normal females genotype – XX
• Normal Cross
Y
X
X
XX
XY
X
XX
XY
Hemophilia
Hemophilia is known as bleeder’s disease and is inherited
as a recessive trait carried on the X chromosome.
Define the trait:
Normal female = _____
XHXH
Carrier female = _____
XHXh
Affected female = _____
XhXh
Normal Male = _____
XHY
Affected Male = _____
XhY
People with hemophilia
(hemophiliacs) have blood that clots
very slowly
Cross a Carrier female with a
normal male
• Parents genotypes
• Complete the cross
XH
Y
XHXh * XHY
# of males with hemophilia? 1
# of carrier females? 1
XH
XHXH
XHY
# of normal males? 1
# of females with hemophilia? 0
Xh
XHXh
XhY
Colorblindness
• Colorblindness is a sex-linked recessive trait
carried on the x chromosome.
• If a colorblind male is crossed with a homozygous, normal woman,
what are their possible offspring?
Determine the parents:
XBXB * XbY
Xb
XB
XB
Possible offspring
XBXb
XBXb
2 carrier females
2 normal males
Y
XBY
XBY
Colorblind Test
• Number one through four on the side of paper
• Write the number that appears
answer
answer
answer
answer
1.
2.
3.
4.
16
2
5
15 and a color deficient person will see a 13 or
17 or nothing.
Due to the fact there are so many different monitor screens
displaying different colors, the accuracy of this color
vision test is limited. This is for "screening" purposes
only, not a "diagnosis".
1
2
3
4
SEX INFLUENCED –
a trait influenced by a persons sex but is not
carried on a sex chromosome.
Early balding (pattern baldness) in humans.
• Heterozygous men (b+/b) lose their hair;
• heterozygous women do not have
significant hair loss.
• Homozygous men or women (b/b) become
bald.
• The trait is therefore dominant in men,
recessive in women.
Sex influenced – pattern baldness
Define the trait:
b+ b+ - not bald
b+ b (males) - bald
b+ b (females) - not bald
b b - bald (male or female)
Baldness problem
• Cross and
homozygous bald
male with a
heterozygous female
• Parents genotypes
• b b X b+ b
b+
b B+b
b
b+b
b
bb
bb
Multiple Alleles - the result of more than
2 types of alleles
• Blood type is determined by two alleles on
the homologous No. 9 chromosome. The
four phenotypes are type A, B, AB, and O.
Phenotyp
e
A Blood
B Blood
Genotype
Antigen in
Blood cells
Antiobodies
%
distribution
in US
AA or AO
A
B
42
B
A
7
BB or BO
AB Blood
AB
A and B
None
2
O Blood
OO
none
A and B
48
A person with type O blood has a child with a person with type
AB blood. What are the possible genotypes of their children?
AB
Parents ________
OO X ___________
50%
% type A - _____
% type B - 50%
_____
0%
% type AB - _____
0%
% type O- _____
O
O
A
AO
AO
B
BO
BO
From problem one, which parent would be the
ideal blood donor for the children in
problem 1?
•
•
Type O blood may be transfused into
all the other types. It is called the
universal donor.
Type AB blood can receive blood from
all the other blood types. It is called
the universal recipient.
3. Can a person with a type O blood have a
child with type AB blood? Explain
• NO – because a Type O person does not
contain either of the A or B alleles to pass
on.
4. I have type A blood and my brother has type AB blood
and my sister is type 0 blood? What are the possible
genotypes of my parents?
Test Cross
A
B
O
AB
AO
The parents must be
heterozygous for type A
blood and type B blood.
(AO and BO)
O
? BO
OO
Polygenic Inheritance - Many
genes control one trait.
• Human skin color is a good example of polygenic
(multiple gene) inheritance. Assume that three "dominant"
capital letter genes (A, B and C) control dark pigmentation
because more melanin is produced. The "recessive"alleles
of these three genes (a, b & c) control light pigmentation
because lower amounts of melanin are produced.
• Imagine the following couple:
AaBbCc (female) x
AaBbCc (male)
• Possible gametes?
• ABC ABc AbC Abc aBC aBc abC abc
The cross of two mulattos
Gametes
ABC
ABc
AbC
Abc
aBC
aBc
abC
abc
ABC
6
5
5
4
5
4
4
3
ABc
5
4
4
3
4
3
3
2
AbC
5
4
4
3
4
3
3
2
Abc
4
3
3
2
3
2
2
1
aBC
5
4
4
3
4
3
3
2
aBc
4
3
3
2
3
2
2
1
abC
4
3
3
2
3
2
2
1
abc
3
2
2
1
2
1
1
0
1. a What is the genotype of the darkest child they could
have?
AABBCC
b. What are the odds of producing such a child?
1/64
2. a. What is the genotype of the lightest child they could
have?
aabbcc
b. What are the odds of producing such a child?
1/64
3.What are the odds of having a child that is heterozygous
for all three genes? AaBbCc
20/64
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