1. health and fitness
2. disease

Mendel and Heredity
Section 1- The Origin of Genetics
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Heredity- The passing of traits from
parents to offspring.
Example of traits: eye color, eye shape,
hair texture, height
Gregor Mendel“Father of Genetics”
Where was he from? Austria
What was his profession? Monk
What organism did he breed different varieties
of? Garden Pea
He was first to do what?
Develop rules
that accurately predict patterns of heredity
What branch of biology
focuses on heredity?
Genetics
Why were pea plants so good for
studying heredity?
1. Traits of the plant were in 2 distinct forms.
e.g. flower color was either purple or
white
2. The male and female parts are enclosed in
the same flower.
Why were pea plants so good for
studying heredity?
3. Garden peas are small and have a fast
growth rate and reproductive cycle.
What are the 7 traits that Mendel
studied in pea plants?
.
1.
2.
3.
4.
Flower color
Seed color
Seed shape
Pod color
5. Pod shape
6. Flower position
7. Plant height
Traits expressed as Simple Ratios
Monohybrid cross (mono= one)- a cross
that involves one pair of contrasting
traits
e.g. Crossing purple flower plant with a
white flower plant
3 steps of Mendel’s experiments
1. He made sure that the plants he was
going to use were true-breeding.
These original parent plants were known
as the P generation.
2. He cross-pollinated plants that had
contrasting traits. He called the
offspring of the cross the F1 generation.
He then examined each F1 plant and
recorded the number of F1 plants
expressing each trait.
3 steps of Mendel’s experiments
3.
He allowed the F1 generation to selfpollinate and called the offspring of the F1
the F2 generation. He then again counted
and characterized each plant.
3 steps of Mendel’s experiments
A ratio is a comparison of 2 numbers
and can be written as a fraction or
with a colon.
In F2 generation, 705 purple to 224
white forms a ratio of 3:1.
(3 purple: 1 white)
*****EOC Prep Question*****
When 2 true-breeding pea plants that show
contrasting forms of a trait are crossed, all of
the offspring show
A.
B.
C.
D.
Both forms of the trait
One form of the trait
One-fourth of each trait
A different trait
Section II Mendel’s Theory
Before Mendel, many people believed in
the blending hypothesis in which
offspring were a blend of their parents.
Mendel’s results did not support the
blending hypothesis.
Mendel’s Hypothesis
1. For each inherited trait, an individual has
2 copies of the gene- one from each
parent.
2. There are alternative versions of genes.
Alleles are different versions of a gene.
Mendel’s Hypothesis
3. When 2 different alleles occur
together, one of them may be
completely expressed, while the
other may have no observable
effect on the organism’s
appearance.
Mendel’s Hypothesis
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Dominant- expressed form of a trait
Dominant alleles are written as capital letters
Recessive- trait that was not expressed
Recessive alleles are written as lower-case
letters.
Mendel’s Hypothesis
4. When gametes are formed, the
alleles in each gene in an individual
separate independently of one
another. Thus, gametes only carry 1
allele for each inherited trait. When
gametes join during fertilization,
each gamete contributes 1 allele.
Mendel in Modern Times
Homozygous- 2 alleles for a
trait are the same.
Example: PP or pp; YY or yy
Heterozygous- 2 alleles for a
trait are different.
Example: Pp; Yy
Mendel in Modern Times
The allele for yellow pea seeds is dominant.
So, Y = yellow and y = green
What color seeds would YY have?
Homozygous or Heterozygous?
What color seeds would Yy have?
Homozygous or Heterozygous?
What color seeds would yy have?
Homozygous or Heterozygous?
yellow
yellow
green
Mendel in Modern Times
The allele for freckles is dominant.
So, F = freckles and f = no freckles.
Would a child with FF have freckles? yes
Homozygous or Heterozygous?
Would a child with Ff have freckles? yes
Homozygous or Heterozygous?
Would a child with ff have freckles?
no
Homozygous or Heterozygous?
*****EOC Prep Question*****
If a pea plant is heterozygous for a particular
trait, how can the alleles that control the trait
be characterized?
A. Two recessive
B. Two dominant
C. One dominant, one recessive
D. Three dominant, one recessive
Mendel in Modern Times
Genotype- the set of alleles that an individual
has
Phenotype- the physical appearance of a trait
Genotype
Phenotype
F= freckles
FF
freckles
f= no freckles
Ff
freckles
ff
no freckles
Mendel in Modern Times
Genotype Phenotype
P = purple flower
pp
white
p = white flower
Pp
purple
PP
purple
Mendel in Modern Times
Y= yellow seed
y = green seed
Genotype Phenotype
YY
yellow
Yy
yy
yellow
green
The Laws of Heredity
The Law of Segregation- 2 alleles for a
trait segregate when gametes are formed.
The Laws of Heredity
The Law of Independent
Assortment- the alleles of different
genes separate independently of one
another during gamete formation.
Example: Gene for height separate
differently from flower color or seed
shape
The Laws of Heredity

Mendel found the inheritance of one
trait did not influence the inheritance
of another trait.
For example: plant height did not
influence the inheritance of another
trait such as flower color
The Laws of Heredity

To study how different pairs of genes
are inherited Mendel conducted
dihybrid crosses. A dihybrid cross is
a cross that considers 2 pairs of
contrasting traits. For example: a
cross that considers both plant height
and flower color.
The Laws of Heredity
Example:
PPYY x ppyy
purple
white
flowers
flowers
&
&
yellow
green
seeds
seeds
Section III Studying Heredity
Punnett squares- a diagram that predicts the
outcome of a genetic cross by considering all
combinations of gametes in a cross
Punnett squares allow direct and simple
predictions. Punnett squares show probability,
NOT actual results.
Section III Studying Heredity
Probability- the likelihood that specific
events will occur.
It can be represented as a ratio or a
fraction or a percentage.
***Practice using the
examples in your notes
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Dihybrid Cross
Predictions for 2 triats:
You can predict 2 traits by making a dihybrid cross.
Use the FOIL method for setting up the dihybrid
cross.
F – First
O – outside
I – inside
L – last

Practice on the two examples.
Inheritance of Traits
Geneticists often prepare a pedigree which is a
family history that shows how a trait is
inherited. Pedigrees are particularly helpful if
the trait is a genetic disorder and the family
members want to know if they are carriers.
Carriers- individuals who are heterozygous for
an inherited disorder but do not show
symptoms
Inheritance of Traits
Albinism Pedigree
What do the squares represent? males
What do the circles represent?
females
What does it mean if the shape is shaded in?
Affected
individual
Albinism- the body
is unable to
produce an
enzyme necessary
for the production
of melanin
Albinism is a
genetic disorder
transmitted by a
recessive allele.
Examine Pedigree I, which traces the dimples trait
through three generations of a family. Blackened symbols
represent people with dimples.
Circles represent females, and squares represent males.
Although Jane and Joe Smith have dimples, their daughter, Clarissa, does not.
Joe’s father has dimples, but his mother, and his sister, grace, do not. Jane’s
father, Mr. Renaldo, her brother, Jorge, and her sister, Emily, do not have
dimples, but her mother does.
Inheritance of Traits
Sex-linked trait- a trait whose allele is
located on the X chromosome
Most sex-linked traits are recessive and
are usually only seen in males.
*****EOC Prep Question*****
A cross between two pea plants that produce
yellow seeds results in 124 offspring:
93 produce yellow seeds and
31 produce green seeds.
What are the likely genotypes of the plants
that were crossed?
A. Both Yy
C. Both yy
B. Both YY
D. One YY, one Yy
Traits Influenced by Several Genes
Polygenic trait- when several genes influence
one trait
The genes for a polygenic trait may be scattered
along the same chromosome or located on
different chromosomes. Because of
independent assortment and crossing-over,
many different combinations appear in the
offspring.
Example: eye color, skin color, height,
weight, hair color
Intermediate Traits
Incomplete dominance- when an
individual displays an intermediate trait
Example:
1. cross red snapdragons with white
snapdragons and make pink ones
2. cross straight haired parent with curly
haired parent = child with wavy hair
Sample Problem
RR = red snapdragon flower
RW = pink snapdragon flower
WW = white snapdragon flower
Parent # 1 is Pink x Parent #2 is white
RW x WW
Make a Punnet Square!!!!!!!
50 pink
0
50 white ____%
_____%
red ____%
Sample Problem
BB = black hair; BW = grey hair; WW = white hair
Parent # 1 black hair
Parent # 2 white hair
BB x WW
Offspring:
0 black 100
0 % white
___%
___ % grey ___
Make a Punnet Square!!!!
Traits Controlled by Genes with
Three or More Alleles
Multiple Alleles- genes with 3 or more alleles
Example: ABO blood types are determined by 3
alleles
4 different blood types are: A, B, AB, and O
Traits Controlled by Genes with
Three or More Alleles
IA = A; IB = B; i = O
Fill in the following blood types with the
given genotypes:
IA IA = ______
IAi = ______
A
A
B
B
IB IB = ______
IBi = ______
AB
IA IB = ______
O
ii = ______
Traits with 2 Forms Displayed at the
Same Time
Codominance- both forms of the trait are
displayed (2 different dominant alleles are
shown)
*** This is different from incomplete dominance
because both traits are displayed.
Example: ABO blood types- Type AB blood
cells carry both A and B types
Traits influenced by the
Environment
Hydrangea flowers
What color are hydrangeas
that grow in acidic soil?
___________
blue
What color are hydrangeas
that grow in neutral to
basic soils?
___________
pink
Traits influenced by
the Environment
The color of the Arctic Fox is determined by
temperature.
reddish brown
What color is the fox in summer? ____________
Enzymes that
pigments
What causes the fox’s coat to darken?make
_________
white
What color does the fox turn in the winter?____
camouflage
What advantage is this to the fox?____________
Genetic Disorders
In order for a person to develop and
function normally, the proteins encoded
by his or her genes must function
precisely. Sometimes genes get damaged
or are copied incorrectly, resulting in
faulty proteins. Changes in genetic
material are called mutations.
Genetic Disorders
Sickle-Cell
Anemiadisorder that
produces a
defective form
of the protein
hemoglobin.
Genetic Disorders
The recessive allele that
causes sickle-cell helps
protect the cells of
heterozygous
individuals from the
effects of malaria.
Malaria is a disease
caused by a parasitic
protozoan that invades
red blood cells.
Genetic Disorders
Cystic Fibrosis (CF) – a fatal recessive trait; it is
the most common fatal hereditary disorder
among Caucasians; thick accumulation of
mucus
About 1 in 2,500 Caucasian infants in the U.S. is
homozygous for the CF allele. There is no
cure.
Genetic Disorders
Hemophilia- a condition that impairs the blood’s
ability to clot.
It is a sex-linked trait. If the mutation appears on
the X chromosome, which a male receives
from his mother, he does not have a normal
gene on the Y chromosome to compensate.
Therefore, he will develop hemophilia.
Genetic Disorders
Huntington’s Disease (HD) – caused by a
dominant allele located on an autosome;
symptoms- forgetfulness, irritability, loss of
muscle control, spasms, severe mental illness
and death.
Unfortunately, most people who have the HD
allele do not know they have the disease until
after they have children. The disease is passed
unknowingly from one generation to the next.
Treating Genetic Disorders
Gene therapy- replacing defective genes with
copies of healthy ones; allows scientists to
correct certain recessive disorders.
What has been done successfully with the CF
gene?
Working cf genes were attached to a cold virus
which easily infects lung cells. Then, those
cells produce more working cells.
Genetic Disorder
Has this been successful in humans?
No
Why?
Most people have built up immunity to the
cold virus and the lungs reject the virus
and it’s cf passenger.
*****EOC Prep Question*****
The mutated allele that causes Huntington’s
disease is:
A.
B.
C.
D.
Sex-linked and recessive
Sex-linked and dominant
Autosomal and recessive
Autosomal and dominant