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Genetics: The Science of
Heredity
A Priest-Scientist
Gregor Mendel
Who was Gregor Mendel?
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•
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
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 parent. For
example, if the parent is tall,
all offspring will be tall. If the
parent is short, all offspring
will be short.
Some Pea Traits that Mendel
Studied
F1 Generation
• 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
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
is 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.
Some Pea Traits that Mendel
Studied
Understanding Mendel’s
Experiments
Part I
2 alleles
for white
2 alleles
for
purple
pp
PP
1 allele for purple. 1 allele for
white
Pp
Understanding Mendel’s
Experiment
1 allele for purple
1 allele for white
Part II
Pp
Pp
2 alleles for purple
PP
1 allele for purple
1 allele for white
Pp
2 alleles for white
pp
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.
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.
Phenotype
• Outward appearance
• Physical characteristics
• Examples:
1.
2.
tall pea plant
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.
1. What is the genotype of a
homozygous tall plant?
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?
Practice! Practice! Practice!!!
In pea plants the Tall (T) allele
is dominant over the dwarf (t)
allele.
3. What is the genotype of a
heterozygous tall plant?
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?
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?
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?
Answers:
1. TT
2. tt
3. Tt
4. Tall
5. short
6. T (tall) and t (short)
Punnett square
• A Punnett square is used to show the possible
combinations of gametes.
Breed the P generation
• tall (TT) (x) dwarf (tt) pea plants
T
t
t
T
tall (TT) (x) dwarf (tt) pea plants
T
T
t
Tt
Tt
produces the
F1 generation
t
Tt
Tt
All Tt = tall
(heterozygous tall)
Breed the F1 generation
• tall (Tt) (x) tall (Tt) pea plants
T
T
t
t
tall (Tt) (x) tall (Tt) pea plants
T
T
t
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
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
B
b
B
BB
Bb
b
Bb
bb
Bb x Bb
1/4 = BB - brown eyed
1/2 = Bb - brown eyed
1/4 = bb - blue eyed
1:2:1 genotype
3:1 phenotype
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
true-breeding rust?
Step 1. Write the genotypes of the
parents
True breeding Black
Step 1. Write the genotypes of the
parents
True breeding Black BB
Step 1. Write the genotypes of the
parents
True breeding Black BB
True breeding rust
Step 1. Write the genotypes of the
parents
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
Step 5 Determine the genotypic
and Phenotypic ratios
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. Write the genotypes of the
parents
Step 2 – List the possible gametes from
each parent
Step 3 Draw Punnett square and place
the gametes on the sides.
Step 4 Fill in the Punnett square to
find the possible zygotes
Step 5 Determine the genotypic and
phenotypic ratios
Practice! Practice! Practice!!!!
In cocker spaniels black (B) is
dominant to red (rust) (b).
2. What would be the phenotypic
ratio of a heterozygous 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?
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 ratio
of the offspring?
What percentage of puppies will
be homozygous 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?
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
Dihybrid Cross
• A breeding experiment that tracks the inheritance
of two traits.
• Mendel’s “principle of independent assortment”
-each pair of alleles segregates independently
during gamete formation (metaphase I)
Dihybrid Cross
• Example:
R
r
Y
y
= round
= wrinkled
= yellow
= green
cross between round and yellow
heterozygous pea seeds.
RrYy x RrYy
RY Ry rY ry x RY Ry rY ry
possible gametes produced
Dihybrid Cross
RY
RY
Ry
rY
ry
Ry
rY
ry
Dihybrid Cross
RY
RY RRYY
Ry RRYy
Ry
RRYy
RRyy
rY
RrYY
RrYy
ry
RrYy
Round/Yellow:
9
Round/green:
3
Rryy
wrinkled/Yellow: 3
rY RrYY
RrYy
rrYY
rrYy
wrinkled/green:
ry
Rryy
rrYy
rryy
9:3:3:1 phenotypic ratio
RrYy
1
Incomplete Dominance
• F1 hybrids have an appearance somewhat in
between the phenotypes of the two parental
varieties.
• Example: snapdragons (flower)
• red (RR) x white (rr)
R
RR = red flower
rr = white flower
r
r
R
Incomplete Dominance
R
R
r
Rr
Rr
produces the
F1 generation
r
Rr
Rr
All Rr = pink
(heterozygous pink)
Codominance
• Two alleles are expressed (multiple alleles) in
heterozygous individuals.
• Example: blood
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
I A IB
ii
Codominance
• Example:
homozygous male B (IBIB)
x
heterozygous female A (IAi)
IB
IB
IA
IA I B
IA IB
i
IB i
IB i
1/2 = IAIB
1/2 = IBi
Codominance
• Example: male O (ii) x female AB (IAIB)
IA
IB
i
IA i
IB i
i
IA i
IB i
1/2 = IAi
1/2 = IBi
Codominance
• Question:
If a boy has a blood type O and
his sister has blood type AB,
what are the genotypes and
phenotypes of their parents.
• boy - type O (ii) X girl - type AB (IAIB)
Codominance
• Answer:
IA
IB
i
i
IA IB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
T. H. Morgan determined that…
• Males are XY and females are XX
• Genes may be found on sex
chromosomes…SEX LINKED
• Genes on an X chromosome are X-LINKED.
Genes on a Y chromosome are Y-LINKED.
• Eye color in fruit flies (Drosophila
melanogaster) is X-Linked.
Only male fruit flies have white
eyes.
Sex-linked Traits
• Traits (genes) located on the sex
chromosomes
• Example:
fruit flies
(red-eyed male) X (white-eyed female)
Sex-linked Traits
• Example:
fruit flies
(red-eyed male) X (white-eyed female)
• Remember: the Y chromosome in males
does not carry traits.
RR = red eyed
Rr = red eyed
rr = white eyed
Xy = male
XX = female
XR
Xr
Xr
y
Sex-linked Traits
XR
Xr
XR Xr
y
Xr y
1/2 red eyed and female
1/2 white eyed and male
Xr
XR Xr
Xr y
Sex linked traits
Examples of sex linked traits are
1. Blood clotting factor – this factor is located
on the X chromosome and the dominant
allele allows your blood to clot normally.
The recessive form does not allow your
blood to clot.
Two recessive alleles causes the disease
hemophilia.
Boys are more likely to get the disease
because they only have one X.
• PEDIGREES are used to map out the
inheritance patterns of a trait over several
generations.
European Royal Families and Hemophilia
Queen Victoria
Hemophilia: A sex-linked disorder
2. Red-green color vision is another sexlinked trait. The dominant allele allows
you to see reds and greens. The
recessive allele prevents seeing red or
green.
Boys are most often affected because of
having only 1 X chromosome.
Pedigree for Colorblindness,
an X-linked Recessive Trait