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Genetics: The Science of Heredity
A Priest-Scientist
Gregor Mendel
Who was Gregor Mendel?
•
•
•
•
•
Austrian monk
Lived 1822 to 1884
Grew on the family farm
High school teacher
Gardener of monastery where he
lived as a monk
• Did experiments on pea plants
• Kept careful records & used
mathematics to make sense of what
he observed
• Considered to be the Father of
Genetics because of his extensive
research of Heredity
Heredity
• Heredity is the passing of traits
from parents to offspring.
• Mendel experimented with
heredity of certain traits found in
peas.
• Mendel studied each trait
separately and discovered
certain patterns in the way traits
are inherited in peas.
• Mendel’s work has become the
basis of genetics, the study of
heredity.
Mendel’s Pea Experiments
• Mendel chose pea plants
because their traits were easy
to see and distinguish.
• He crossed plants with two
different traits, for example
purple flowers with white
flowers.
• He started his experiments with
purebred plants.
• Purebred plants ALWAYS
produce offspring with the
same trait as the parents. For
example, if the parents are both
tall, all offspring will be tall. If
the parents are short, all
offspring will be short.
Some Pea Traits that Mendel Studied
F1 Generation
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•
•
•
Mendel called the parent plants the P generation.
He called the offspring from the parents the F1 generation.
F is from the Latin word, filial, which means son.
When Mendel crossed pure pea plants with purple flowers
with pure pea plants with white flowers, all the F1
generation had purple flowers.
P Generation
F1 Generation
F2 Generation
• When he crossed the F1 generation peas with one
another, only some of the offspring had purple
flowers. These formed the F2 generation.
• Mendel found that in the F2 generation, ¾ of the
plants had purple flowers and ¼ of them had white
flowers (3:1 ratio).
F1 Generation
F2 Generation
Review
1.Who was Gregory Mendel?
2.Why did he choose peas for his experiments?
3.What is heredity?
4.What is genetics?
5.What is a purebred plant?
6.What is the P generation?
7.What is the F1 generation?
8.What is the F2 generation?
Dominant and Recessive Traits
• It seemed to Mendel,
that for each
characteristic in peas,
one trait was stronger
than the other.
• He called the “stronger”
one, the dominant trait.
• He called the “hidden”
one, the recessive trait.
Genes and Alleles
• The traits of peas (and
yours) are controlled by
factors that scientists call
genes.
• You inherit your genes from
your parents.
• The different forms of a
gene are called alleles.
• You inherit a combination
of two alleles from your
parents.
Dominant and Recessive Traits in Peas
• For each of the 7 traits that
Mendel studied in peas, there
was always a dominant allele
and a recessive allele.
• If a plant inherits both a
dominant allele and a
recessive allele, the dominant
allele masks the recessive
allele.
• THINK- what would happen if
you would only have 2
dominant OR 2 recessive?
Would there be any masking?
Dominant and Recessive Traits in Peas
• When you have two
of the SAME alleles,
you are called a
Purebred.
• When you have two
different alleles, you
are said to be a
Hybrid.
Some Pea Traits that Mendel Studied
Understanding Mendel’s Experiments
Part I
2 alleles
for white
2 alleles
for
purple
Purebred
pp
PP
1 allele for purple. 1 allele for
white
Pp
Hybrid
Purebred
Understanding Mendel’s Experiment
1 allele for purple
Part II
1 allele for white
Hybrid
Pp
Pp Hybrid
2 alleles for purple
PP
Purebred
1 allele for purple
1 allele for white
Pp
Hybrid
2 alleles for white
pp
Purebred
Lesson 3
Probability and Genetics
Probability
• Probability is the likelihood
that a particular event will
occur.
• The laws of probability
determine what is likely to
occur, not what does occur.
• Mendel was the first scientist
that applied the principles of
probability to genetics.
Punnett Square
• Punnett square is a table
that shows all the possible
combinations of alleles that
can result when two
organisms cross.
• Using Punnett square,
geneticists can predict the
probability of occurrence of
a particular trait.
• The allele that each parent
will pass to its offspring is
based on chance, just like
tossing a coin.
Purebred
Purebred
Genotypes and Phenotypes
Phenotype
• Genotype: Indicates the
alleles that the
organism has inherited
regarding a particular
trait.
• Phenotype: The actual
visible trait of the
organism.
Genotype
Homozygous and Heterozygous
• Homozygous: An
organism with two
identical alleles for a
trait (a purebred
organism).
• Heterozygous: An
organism that has two
different alleles for a
trait (a hybrid
organism).
Codominance
• In codominance, the
alleles are neither
dominant, nor
recessive. Neither allele
is masked by the other.
Roan Cow
Is both white and red
Incomplete Dominance
• Occurs when one allele is not completely
dominant over another
• The heterozygous phenotype is somewhere in
between the two homozygous phenotypes.
• Ex.-Red flowering plant crossed with white
flowering plant yields pink flowering offspring
Complete Dominance
• Occurs when one allele is completely
dominant over another allele
• The offspring will look like the dominant
allele.
• Ex.-Homozygous tall pea plants mated with
short pea plants will yield a tall offspring.
Review
1.What is probability?
2.What is the Punnett Square? How is it helpful to
geneticists?
3.What is a genotype?
4.What is a phenotype?
5.What is a homozygous organism?
6.What is a heterozygous organism?
7.Explain codominance.
8. What is incomplete dominance?
9. Explain complete dominance.
GENETICS REVIEW
• THE STUDY OF HEREDITY.
• GREGOR MENDEL (1860’S) DISCOVERED THE
FUNDAMENTAL PRINCIPLES OF GENETICS BY
BREEDING GARDEN PEAS.
GENETICS
1.
ALLELES: ALTERNATIVE FORMS OF GENES.
2. HOMOZYGOUS ALLELES ARE EXACTLY THE SAME.
3.
DOMINANT ALLELES – CAPITALIZED
(TT - TALL PEA PLANTS)
A. HOMOZYGOUS DOMINANT
B. PUREBRED
4.
RECESSIVE ALLELES - LOWERCASE
(TT - DWARF PEA PLANTS)
A. HOMOZYGOUS RECESSIVE
B. PUREBRED
5.
HETEROZYGOUS (DIFFERENT)Tt=
Tall
PHENOTYPE
• OUTWARD APPEARANCE
• PHYSICAL CHARACTERISTICS
• EXAMPLES:
1.
TALL PEA PLANT
2.
DWARF PEA PLANT
GENOTYPE
• ARRANGEMENT OF GENES THAT PRODUCES THE PHENOTYPE
• EXAMPLE:
1.
TALL PEA PLANT
TT = TALL (HOMOZYGOUS DOMINANT)
2.
DWARF PEA PLANT
TT = DWARF (HOMOZYGOUS RECESSIVE)
3.
TALL PEA PLANT
Tt= Tall (Heterozygous)
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
You will need to refer to your vocabulary to do
this!!!!
THINK!!
Also look at the previous slides with the bubble
background!
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
1. What is the genotype of a
purebred tall plant?
You will need to refer to your vocabulary to do this!!!!
THINK!!
Also look at the previous slides with the bubble background!
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
2. What is the genotype of a
homozygous short plant?
You will need to refer to your vocabulary to do this!!!!
THINK!!
Also look at the previous slides with the bubble background!
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
3. What is the genotype of a
hybrid tall plant?
You will need to refer to your vocabulary to do this!!!!
THINK!!
Also look at the previous slides with the bubble background!
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
4. A plant has a genotype of Tt.
What is its phenotype?
You will need to refer to your vocabulary to do this!!!!
THINK!!
Also look at the previous slides with the bubble background!
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
5. A plant has a genotype of tt,
what is its phenotype?
You will need to refer to your vocabulary to do this!!!!
THINK!!
Also look at the previous slides with the bubble background!
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
6. What are the two alleles for the
height of a pea plant?
You will need to refer to your vocabulary to do this!!!!
THINK!!
Also look at the previous slides with the bubble background!
Answers:
1. TT
2. tt
3. Tt
4. Tall
5. short
6. T (tall) and t (short)
Punnett Squares
READ
Punnett Squares
• The Punnett square is the
standard way of working
out what the possible
offspring of two parents
will be.
– It is a helpful tool to show
allelic combinations and
predict offspring ratios.
Before we go further lets discuss how to set up a
Punnett Square…
We begin by constructing a grid of two
perpendicular lines.
READ
Next, put the genotype of one parent across
the top and the other along the left side.
READ
For this example lets consider a genotype of BB
crossed with bb.
1st
parent
gametes
B
b
2nd
parent
gametes
B
• Notice only one
letter goes above each
box
• It does not matter
which parent’s
genotype goes on
either side.
b
•If there is a Capital
letter, it ALWAYS goes
first!
Next, fill in the boxes by copying the column and
row head-letters down and across into
the empty spaces.
READ
B
B
b
Bb
Bb
b
Bb
Bb
Phenotype:
100% brown
Genotype:
100% Bb
Breed the P generation
• tall (TT) (x) dwarf (tt) pea plants
T
t
t
T
Fill in your P
generation
square with the
parents.
tall (TT) (x) dwarf (tt) pea plants
T
t
t
Tt
Tt
T
Tt
Tt
THE ORDER DOES
NOT MATTER IN
THE BOXES, BUT
UPPERCASE
FIRST IS A GOOD
RULE
produces the
F1 generation
Fill in your P generation square
AND record data.
Genotype:100% Tt
Phenotype: 100% tall
(heterozygous tall)
YOUR ANSWER SHEET-P Generation
C
O
P
Y
C
O
P
Y
Phenotype: 100% tall
Genotype:100% Tt heterozygous
COPY
Breed the F1 generation
• tall (Tt) (x) tall (Tt) pea plants
T
T
t
t
Fill in your F1
generation
square with
the parents
tall (Tt) (x) tall (Tt) pea plants
T
T
t
TT
Tt
t
Tt
tt
Fill in your F1
generation
square AND
record data
produces the
F2 generation
1/4 (25%) = TT=tall
1/2 (50%) = Tt=tall
1/4 (25%) = tt=dwarf
Phenotypic &
Genotypic
Percentages
THE ORDER DOES NOT MATTER IN
THE BOXES, BUT UPPERCASE
FIRST IS A GOOD RULE
YOUR ANSWER SHEET-F1 Generation
C
O
P
Y
C
O
P
Y
Phenotype: 75% tall 25% dwarf
Genotype:50% Tt, 25% TT, 25%tt
COPY
Monohybrid Cross
• A breeding experiment that tracks the inheritance of
a single trait.
• Mendel’s “principle of segregation”
a. pairs of genes separate during gamete
formation (meiosis).
b. the fusion of gametes at fertilization pairs
genes once again.
Monohybrid Cross
• Example: Cross between two heterozygotes
for brown eyes (Bb)
BB = brown eyes
Bb = brown eyes
bb = blue eyes
B
b
B
Bb x Bb
b
female gametes
male
gametes
Monohybrid Cross
Bb x Bb
B
female
gametes
B
b
BB
Bb
Fill in your
Monohybrid
cross square
AND record
data
b
Bb
bb
THE ORDER DOES NOT MATTER IN THE
BOXES, BUT UPPERCASE FIRST IS A
GOOD RULE
¼ (25%) = BB - brown eyed
½ (50%) = Bb - brown eyed
¼ (25%) = bb - blue eyed
Genotype
Phenotype
YOUR ANSWER SHEET-Monohybrid Cross
C
O
P
Y
C
O
P
Y
Phenotype: 75% brown 25% blue
Genotype:50% Bb, 25% BB, 25%bb
COPY
Practice! Practice! Practice!!!!
In cocker spaniels black (B) is
dominant to red (rust) (b).
1. What would be the phenotypic
ratio of a cross between a true
breeding black crossed with a Try first on
your own
true-breeding rust?
THEN check
answer and
steps!!
Step 1. Write the genotypes of the parents
True is another way of saying homozygous.
True breeding Black
Step 1. Write the genotypes of the parents
True is another way of saying homozygous.
True breeding Black BB
Step 1. Write the genotypes of the parents
True is another way of saying homozygous.
True breeding Black BB
True breeding rust
Step 1. Write the genotypes of the parents
True is another way of saying homozygous.
True breeding Black - BB
True breeding rust – bb
Step 2 – List the possible
gametes from each parent
BB
B
bb
B
b
b
Step 3 Draw punnett square and
place the gametes on the sides.
BB
B
bb
B
b
B
B
b
b
b
Step 4 Fill in the punnett
square to find the possible
zygotes
BB
bb
B
B
b
b
b
Bb
Bb
B Bb
Bb
B
b
THE ORDER DOES NOT
MATTER IN THE BOXES,
BUT UPPERCASE FIRST IS A
GOOD RULE
Step 5 Determine the genotypic
and Phenotypic percentages
BB
B
bb
B
b
B
Bb
B Bb
b
b
b
Bb
Phenotype
100% Black
Bb
Genotype
100% Bb
Review! Review!! Review!!!!!
Step 1. Identify the genotypes of the
parents
Step 2 Draw Punnett square and place
the gametes on the sides.
Step 3 Fill in the Punnett square to find
the possible zygotes
Step 4 Determine the genotypic and
phenotypic percentages
Practice! Practice! Practice!!!!
In cocker spaniels black (B) is
dominant to red (rust) (b).
2. What would be the phenotypic ratio of
a hybrid black and a true-breeding rust?
3. Two black cocker spaniels have eight
puppies: 5 black and 3 red. What are the
genotypes of the two parents? Hint: what
combination of parents gives both colors???
Practice! Practice! Practice!!!!
In dogs wire hair (W) is dominant
to smooth (w) hair.
4. A true-breeding wire hair is
crossed with a heterozygous wire
hair.
What will be the phenotypic
percentages of the offspring?
What percentage of puppies will
be purebred for wire hair?
Practice! Practice! Practice!!!!
In dogs wire hair (W) is dominant
to smooth (w) hair.
5. In a cross between a wire hair and a
smooth hair 6 puppies were produced.
4 puppies had wire hair and two
puppies had smooth hair.
What were the genotypes of the
parents? Hint: what combination of
parents gives both colors???
Practice! Practice! Practice!!!!
In dogs wire hair (W) is dominant
to smooth (w) hair.
In cocker spaniels black (B) is
dominant to red (rust) (b).
6.A heterozygous black dog is crossed
with a hybrid black dog. What are the
genotypic and phenotypic percentages
that would result?
MORE Practice Problems
Complete a Punnett Square for each of the following:
T = tall plant
P = purple flowers
1. PP x pp
2. Tt x TT
3. Pp x PP
t = short plant
p = white flowers
4. Pp x pp
5. tt x TT
6. Tt x Tt
Give the phenotypes and the genotypes for each!!!!!!!