Background Vocabulary
True-breeds: pure gene lines – offspring
match parent
Self-pollination: pollen from flower
fertilizes the same plant
Cross-pollination: pollen will fertilize a
different plant
Hybrid: Cross between organisms with
different traits (blonde hair & brown
hair)
Trait: Physical characteristics
Gregor Mendel
• Austrian monk
• “Father of Modern
Genetics”
• Famous for his
work with peas
Mendel’s Peas
Cross-Pollinating
Mendel’s Crosses
TRAIT 1: Seed Shape
P Cross: Round v. Wrinkled
F1 Phenotype: Round
Mendel’s Crosses
TRAIT 2: Seed Color
P Cross: Green v. Yellow
F1 Phenotype: Yellow
Mendel’s Crosses
TRAIT 3: Flower Color
P Cross: Purple v. White
F1 Phenotype: Purple
Mendel’s Crosses
TRAIT 4: Pod Shape
P Cross: Inflated v. Pinched
F1 Phenotype: Inflated
Mendel’s Crosses
TRAIT 5: Pod Color
P Cross: Green v. Yellow
F1 Phenotype: Green
Mendel’s Crosses
TRAIT 6: Flower Position
P Cross: Axial v. Terminal
F1 Phenotype: Axial
Mendel’s Crosses
TRAIT 7: Plant Height
P Cross: Tall v. Short
F1 Phenotype: Tall
Mendel’s Conclusions
• Biological inheritance is determined by
chemical factors passed from one generation
to the next (Particulate hypothesis)
– Geneticists now refer to these factors as genes
– Genes can come in more than one form, each form
is an allele
ex. B or b (The “B” gene w/ 2 alleles)
The Principle of Dominance
1. Certain alleles will be expressed over
others
2. The expressed alleles are dominant to the
unexpressed recessive alleles
Self-Assessment:
• How does Mendel’s particulate hypothesis
differ from the blending hypothesis of
inheritance?
• What is the difference between self-pollination
and cross-pollination?
• Describe a pattern of inheritance shown in
Mendel’s crosses that contradicts the blending
hypothesis.
The F1 Cross
• Mendel’s experiment:
– Allow the F1 plants to self pollinate
• The results:
– The dominant trait was expressed 75% of the
time
– The recessive trait was expressed 25% of the
time
The Explanations:
1. The recessive allele was still present in the
F1 plants
2. Principle of Segregation: Each individual
inherits two alleles for each gene.
Probability
• The likelihood of an event happening
• How is it determined:
– Likelihood of rolling heads = 50%
(1 of 2 possibilities)
– Rolling heads twice?
• 50% x 50% = 25%
• To predict outcomes of genetic crosses we
use punnett squares
More genetics vocabulary…
• Homozygous: two identical alleles (AA or
aa)
• Heterozygous: two different alleles (Aa)
• Phenotype: Physical appearance
• Genotype: Genetic make-up
– Homozygous dominant (AA)
– Homozygous recessive (aa)
– Heterozygous (Aa)
The Test Cross
• In order to determine whether an individual
expressing a dominant trait is homozygous
or heterozygous, it can be crossed with an
organism expressing the recessive trait.
• If R = round seeds and r = wrinkled seeds,
show how the results of a test cross for seed
shape will differ for homozgygous v.
heterozgous genotypes for round seeds.
Self-Assessment
• What are the two possible gametes
produced by a plant that has the genotype
Aa? What is the probability of each type of
gamete?
• Use a Punnett square to predict the
genetypes produced if the plant above is
self-fertilized.
Monohybrid (1-factor) Cross Practice
KEY: G = green pods, g = yellow pods
P = purple flowers, p = white flowers
T = tall plants, t = short plants
Show a cross a plant heterozygous for green pods with a
plants with yellow pods. What are the expected genotype and
phenotype ratios?
Show a cross of a homozygous tall plant with a heterozygous
tall plant. What are the expected genotype and phenotype
ratios?
Show a cross of two heterozygous purple flowered plants.
What are the expected genotype and phenotype ratios?
Exploring Mendelian Genetics
• Does segregation of one set of alleles
influence the segregation of another pair
of alleles?
• Mendel’s Two Factor Crosses
– Followed two traits at a time.
– Same method as his original single-factor
crosses
– Cross-pollinated to produce the F1 and
allowed them to self-pollinate
The Parental Cross
_________________________________
The F1 Cross
_________________________________
A Summary of Mendel’s Principles
• Dominance: a recessive allele will be
masked by a dominant allele
• Segregation: alleles for each trait segregate
(separate) during gamete formation
• Independent Assortment: Alleles for
different traits do not influence each other’s
segregation
Mendel’s Peas were ideal for
learning about inheritance, but
they do not represent the norm…
• Traits in pea plants are determined by just
two alleles
• In peas, one allele is clearly dominant &
the other is clearly recessive
• However, things aren’t always this clearcut and simple in the world of genetics.
What if Mendel looked at mice?
 If a female black mouse and a male
white mouse were crossed, what will
the offspring look like?
- 100% are GREY
 If the F1 offspring were crossed, what
will there offspring look like?
- 25% black
- 50% grey
- 25% white
Incomplete Dominance
• A cross between two organisms with
different traits results in an offspring with
a third phenotype that is a blending of the
parental traits.
• It’s like mixing paints:
– Red + White = Pink
– Red does not totally block (dominate) white,
we end up with something in-between.
Inheritance in Snapdragons
Let’s try crossing snapdragons…
What will the genotype
and phenotype ratios be
if a red plant is crossed
with a pink plant?
(Use capital letters for alleles symbols; R & W)
G:
P:
One more….
What will the
genotype and
phenotype ratios be if
a red plant is crossed
with a white plant?
G:
P:
What does the prefix “Co-” mean?
• Consider the meaning of the
following words:
- Cooperate
- Coexist
- Cohabitat
• What about “Codominance”?
Let’s look at cattle….
This cow resulted from a cross between a cow
with red fur and a cow with white fur.
This is called ‘roan’ fur; red & white fur together.
Codominance
• Similar to incomplete dominance in that
there is a 3rd phenotype
• In COdominance, the “recessive” and
“dominant” alleles appear together in the
phenotype of hybrid organisms.
• Red x White = red & white
Let’s try crossing cattle….
What will the gentype and
phenotype ratios be if a
red cow is crossed with a
white cow?
G:
P:
One more….
What will the ratios
be if a red cow is
crossed with a roan
cow?
G:
P:
Polygenic (Multifactorial) Traits
• Phenotype is determine by more than one
gene
• Often results in gradations, where each
gene has an additive effect
Ex) If 10 gene loci are turned on plant will be
20cm tall, if only 5 loci are turned on plant
will be 10cm tall
• Results in a bell-shaped curve
– Skin color & Height are examples in humans
Phenotype Distribution:Polygenic Traits
Multiple Alleles
• More than 2 alleles for a particular trait
KEY
C = full color; dominant
to all other alleles
cch = chinchilla; partial
defect in pigmentation;
dominant to
ch and c alleles
ch = Himalayan; color in
certain parts of the
body; dominant to
c allele
chhc
ch,cCc
h
ch
AIbino:
Chinchilla:
Himalayan:
cc CC,
cc
c,hCc
, or
cch
c,hhor
cch
c
Full color:
, or
Cc
c = albino; no color;
recessive to all other
alleles
ABO Blood Types
• In addition to having multiple alleles, ABO
blood type also exhibits codominance
• ‘IA’ & ‘IB’ are codominant
• ‘i’ is recessive
ABO Blood Typing
Genotype
IA IA or IA i
IB IB or IB i
IAIB
ii
Blood Type
ABO Blood Transfusions
Blood
Type
A
B
AB*
O*
Can receive:
What does your ABO blood type mean?
 Remember the ‘flags’
on our cell membranes?
They help cells to
recognize each other.
 Some of those flags
‘announce’ your blood
type. We call these
flags antigens
Blood
Type
A
B
AB
O
Antigen
Summary of ABO Blood Types
Rh Factor
• Blood can also be categorized as + or –
• This refers to the presence (dominant) or
absence (recessive) of the Rh antigen
Phenotype Genotype(s)
Rh+
Rh-
Antigen
Rh Factor & Pregnancy
Rh Disease
• Mother's antibodies cross the placenta to fight the
Rh positive cells in the baby's body.
•As the antibodies destroy the red blood cells, the baby can
become anemic.
•The anemia can lead to other complications including
jaundice and organ enlargement
•With amniocentesis, the amniotic fluid may have a yellow
coloring and contain bilirubin.
•Ultrasound of the fetus shows enlarged liver, spleen, or heart
and fluid build up in the fetus' abdomen.
Gene Linkage & Mapping Chromosomes
• Genes on the same chromosome are more likely to be
inherited together
• Crossing over helps to increased variation, but the
closer two genes are on a chromosome the more likely
they are to be “linked”
Sex Chromosomes & Autosomes
• Two of the 46 human chromosomes are
known as sex chromosomes, because
they determine the individual’s sex.
– Females have two copies of an X
chromosome.
– Males have one X chromosome and one Y
chromosome.
• The remaining 44 chromosomes are
known as autosomal chromosomes or
autosomes.
Sex-Linked Genes
• Located on one of the
sex chromosomes
(X or Y)
• Since the X
chromosome is
longer, it has many
genes not found on
the Y chromosome.
• Most sex-linked
genes are X-linked
genes.
Sex-Linked Genes
X-Linked Inheritance
• Examples:
– Hemophilia
• The protein necessary for normal blood clotting is missing
– Colorblindness
• Defective version of one or all of the 3 genes responsible for
color vision
– Male Pattern Baldness
• Hair loss
– Duchenne Muscular Dystrophy
• Weakening and loss of skeletal muscle
• These traits are recessive & more common in males.
Why?
Possible Inheritance of Colorblindness Allele
Pedigree Analysis of Traits
Sex-Linked Pedigree Analysis
Complete the following sex-linked crosses:
Eva and Paul just had a son,
Michael. Paul is has normal
color vision, but Eva’s father
was colorblind. What is the
likelihood that Michael is
colorblind?
Laura and Steve are expecting
their first child. They are
concerned about the chances
their child might be
hemophiliac because both
Steve and Laura’s father are
hemophiliac. What is the
probability of Laura and Steve
having a hemophiliac child?
Complete the following pedigrees.
Which is for a sex-linked trait? How do you know?
Chromosomal Disorders
• The most common error in meiosis
occurs when homologous
chromosomes fail to separate.
• This is known as nondisjunction,
which means “not coming apart”
• If nondisjunction occurs, abnormal
numbers of chromosomes may
find their way into gametes and a
disorder of chromosome numbers
may occur.
• A monosomy results if an entire
chromosome is missing and a
trisomy results if there is an
additional copy of a chromosome
Other Chromosomal Mutations
Jumping Genes
• In the 1940’s, while
studying corn, Barbara
McClintock discovered that
sometimes genes could
move from one location to
another in a chromosome or
even to other chromosomes.
• The movement could result
in the genes landing in the
middle of another gene and
disrupting them.
• These “jumping genes” are
now called transposons.
Amniocentesis:
Detecting Chromosomal Disorders
Karyotypes:
Detecting Monsomies, Trisomies & Translocations
Down Syndrome
• Trisomy 21
• Produces mild to severe
mental retardation
• Characterized by:
– Increased susceptibility
to many diseases
– Higher frequency of
some birth defects.
Turner Syndrome
• Monosomy X
• Result of
nondisjunction in
females
• Characteristic physical
abnormalities, such as
short stature, broad
chest, low hairline, lowset ears, and webbed
neck
Klinefelter Syndrome
• Karyotype 47, XXY
• Extra X chromosome
interferes with meiosis
and usually prevents
these individuals from
reproducing
• Most common sex
chromosome disorder,
second most common
disorder due to the
presence of an extra
chromosome
Genes & The Environment
• Some obvious human traits are almost impossible
to associate with a single gene.
• These traits, such as the shape of your eyes or ears,
polygenic meaning they are controlled by many
genes.
• Many of your personal traits are only partially
governed by genetics.
• epigenetics is the study of heritable changes in
gene expression or cellular phenotype caused by
mechanisms other than changes in the underlying
DNA sequence
Epigenetics
NOVA Video (13min)
The Genetic Basis of Cancer
• You already know that cancer results when controls on the cell
cycle do not work properly and cells grow and divide too
quickly.
• Two classes of genes direct the production of proteins that
regulate cell growth and division:
– one produces growth factors to initiate cell division (can mutate to an
oncogene & result in too much growth factor)
– the other produces proteins to stop cell division (tumor suppressor genes)
• Cancer is always a genetic disease in that it results from
changes in DNA:
– Since the mutations do not usually arise in gametes, cancer is not
usually passed from parent to child
– However some mutations do arise in the ovaries or testes which can
give rise to gametes and result in the inheritance of cancer genes.