A. Gregor Mendel - Austrian monk who studied
how traits are inherited; known as the “Father of
Genetics”
1. Genetics - branch of biology that studies
heredity
a. Heredity - passing on of traits from parent to
offspring
B. Mendel’s Experiments
1. Studied/Researched on pea plants
a. They have many traits such as flower color
(purple or white), peas (round or wrinkled), pea
color (yellow or green) and height (tall or short).
b. Mendel bred a tall
pea plant with a short
plant (P generation).
c. The offspring in the
1st generation (F1
generation) were all
tall.
P1
Short pea plant
Tall pea plant
F1
All tall pea plants
F2
3 tall: 1 short
d. Then, he bred two of
the F1 plants.
e. The offspring in the
2nd generation (F2
generation) were 75%
tall and 25% short.
f. The short trait
disappeared in the 1st
generation and
reappeared in the 2nd
generation.
P1
Short pea plant
Tall pea plant
F1
All tall pea plants
F2
3 tall: 1 short
2. Mendel discovered that
each trait is controlled by
alleles.
a. Alleles: forms/versions
of the same gene
 Each person has TWO alleles
for each gene; 1 from
mother and 1 from father.
P1
Short pea plant
Tall pea plant
F1
All tall pea plants
 Ex: T = the allele for tall;
t = the allele for short
F2
3 tall: 1 short
◦ Some alleles are dominant and recessive
 Dominant - a trait that is always expressed (seen)
in an individual if the allele is present (capital
letter)
 Ex: T = tall allele
 Recessive - a trait that is hidden by the dominant
allele; expressed only when two copies of the
recessive allele are inherited (lowercase letter)
 Ex: t = short allele
◦ Individuals can be described by their genotype and
phenotype
 Genotype - genetic makeup (letters); alleles
present in an individual
 Ex: TT, Tt, tt
 Type of Genotypes:
 Homozygous - two of the SAME alleles for a trait
 Ex: TT or tt
 Heterozygous (hybrid) - two DIFFERENT alleles for a trait
 Ex: Tt
◦ Phenotype - physical appearance; what traits are
expressed in the individual
 Ex: tall or short plants
TT, Tt=tall tt=short
A. Punnett Square/Test Cross - a tool used to predict
the alleles/traits present in offspring
1. When parents produce gametes (sperm or
eggs), their genes separate to produce haploid
cells
a. Each gamete contains ONE allele
Homozygous Tall Dad Heterozygous Tall Dad
T
T
T
t
Homozygous Short Dad
t
t
B. How to Solve a Monohybrid Cross
1. Determine the genotypes (letters) of the
parents.
2. Set up the punnett square with one parent on
top and one parent on the side.
3. Fill out the Punnett square boxes (Look up and
over to the left to fill them in).
4. Analyze the probability (likelihood) that the
offspring would possess each specific trait.
C. Practice
Y = yellow pea color; y = green pea color
1. Cross two heterozygous pea plants.
 Parent 1 Genotype __________
Parent 2 Genotype __________
 What is the chance of the offspring having yellow pea color?
Genotypic Ratio:
YY :
Yy :
yy
Phenotypic Ratio:
yellow : green
Y = yellow pea color; y = green pea color
2. Cross a homozygous recessive pea plant with a
heterozygous pea plant.
 Parent 1 Genotype __________
Parent 2 Genotype __________
 What is the chance of the offspring having yellow pea color?
Genotypic Ratio:
YY :
Yy :
yy
Phenotypic Ratio:
yellow : green
Y = yellow pea color; y = green pea color
3. Cross a homozygous recessive pea plant with a
homozygous dominant pea plant.
 Parent 1 Genotype __________
Parent 2 Genotype __________
 What is the chance of the offspring having yellow pea color?
Genotypic Ratio:
YY :
Yy :
yy
Phenotypic Ratio:
yellow : green
D. How to Solve a Dihybrid Cross
1. Determine parent genotypes (letters).
2. Determine gamete combinations from mom and dad.
(FOIL method from math class - Use
arrows!)
3. Write the gametes from mom on 1 side of the
square and dad on the other side.
4. Fill in the boxes in the punnett square (look up and to the
left).
5. Analyze the probability (likelihood) that the offspring
would possess the specific traits.
PRACTICE:
 R = round peas, r = wrinkled peas
 Y = yellow pea color; y = green pea color
◦ Cross two heterozygous round yellow pea plants.
 Parent 1 Genotype _____
Parent 2 Genotype _____
 Parent Gamete Possibilities: FOIL
Parent #1 Gametes: RY, Ry, rY, ry
Parent #2 Gametes: RY, Ry, rY, ry
 What is the chance of the offspring having round and
yellow peas?
RY
RY
Ry
rY
ry
Ry
rY
ry
A. Autosomes - 22
pairs of body
chromosomes in a
human
22 Autosomes
Sex chromosom
B. Sex chromosomes 1 pair of chromosomes
in a human; the last
pair in a karyotype
a. Ex. XX-female XYmale
22 Autosomes
Sex chromosom

Show a punnett square crossing male and female
sex chromosomes.
d. Every time a male and
female have a baby, what is
the chance of them having a
son?
A daughter?
e. If a female has 5 sons in a
row, what is the chance of
her having another son?
A daughter?
f. Which parent determines
the sex of the child? WHY?
C. Sex-Linked Traits - traits located on the sex
chromosomes (usually the X chromosome)
a. Dad gives X chromosome to daughters and Y
chromosome to sons
b. Mom gives X chromosome to daughters and sons
 If a male inherits a sex-linked trait, which parent(s) gave him the
trait?
 MOM
 If a female inherits a sex-linked trait, which parent(s) gave her
the trait?
 MOM OR DAD
 Who (males or females) is most likely to inherit a sex-linked
trait? WHY?
 Males – they only need to inherit ONE X chromosome
c. Examples in humans
 Male pattern baldness
 Red/green colorblindness
 Hemophilia (problems with blood clotting)
 Muscular Dystrophy (muscle weakness, loss of
muscle tissue)
d. Other Examples
 Eye color in Drosophila (fruit flies)
 R = Red eye Color; r = white eye color
 Cross white eyed male (XrY) with red eyed female (XRXR)
What percentage of offspring
are likely to have red eyes?
What percentage of male
offspring are likely to have
red eyes?
What percentage of female
offspring are likely to have
red eyes?
◦ Examples
 Eye color in Drosophila (fruit flies)
 R = Red eye Color; r = white eye color
 Cross white eyed male (XrY) & heterozygous red eyed female (XRXr)
What percentage of offspring
are likely to have red eyes?
What percentage of male
offspring are likely to have
red eyes?
What percentage of female
offspring are likely to have
red eyes?
A. Incomplete Dominance - heterozygous individuals display an
intermediate (blending) phenotype of the two homozygous
individuals
1. Examples in humans
 Hair texture
 SS = straight
 CC = curly
 SC = wavy
 Tay Sachs Disease (inability to produce enzyme that breaks
down lipids)
 EE = produces enzyme
 NN = does not produce enzyme
 EN = produces half amount of enzyme
2. Other Examples
 Flower color in snapdragons
 RR = Red, WW = white, RW = pink
 Cross a red flower with a white flower.
 Parent 1 Genotype ______
Parent 2 Genotype _____
 What is the chance of producing a pink flower?
Genotypic Ratio:
RR :
RW :
WW
Phenotypic Ratio:
red:
pink: white
 Cross a pink flower with a pink flower.
 Parent 1 Genotype _____
Parent 2 Genotype _____
 What is the chance of producing a white flower?
Genotypic Ratio:
RR :
RW :
WW
Phenotypic Ratio:
red:
pink: white
B. Co-dominance - heterozygous individuals display BOTH traits of
the two homozygous individuals
1. Examples in humans
 Sickle Cell Anemia (abnormally shaped red blood cells)
 NN = normal shaped cells
 SS = sickle shaped cells
 NS = normal and sickled shaped cells
 Blood Type
 Type A
 Type B
 Type AB
◦ Hemoglobin—protein that carries oxygen in blood, makes
blood red
 In homozygous recessive individuals—hemoglobin is
defective and makes blood cells sickle (half moon) shaped
 These blood cells—cause slow blood flow, block small vessels,
tissue damage and pain
 In heterozygous individuals – both normal and sickled
hemoglobin are produced
 They produce enough normal hemoglobin that they do not have
serious health problems
2. Other Examples
 Coat color in chickens
 BB = black
 WW = white
 BW = black AND white speckled
 Cross a black rooster with a white chicken.
 Parent 1 Genotype _____
Parent 2 Genotype _____
 What is the chance of producing black and white chicks?
Genotypic Ratio:
BB:
BW :
WW
Phenotypic Ratio:
black:
black/white:
white
Complete the last cross on your own. Be ready to discuss.
A. An example of multiple alleles – more than one
allele controls the trait
B. It is determined by the presence or absence of
proteins (chains of amino acids) on the surface of red
blood cells
a. Mixing incompatible blood types can cause blood clots,
which can result in death

Human Blood Types
Phenotype
Genotype
Blood cell surface
molecules (antigens)
Type A
IAIA or IAi
A antigens
Type B
IBIB or IBi
B antigens
Type AB
IAIB
A and B antigens
Type O
ii
No antigens
C. Alleles IA and IB – are co-dominant to each
other
D. Allele i –is recessive to both IA and IB
a. Type O blood — universal donor
 Has no proteins on the blood cells so any blood type can
receive it
b. Type AB blood — universal acceptor
 Has both proteins on blood cells so this blood type can receive
any blood
E. Cross parent with A ( IAi) blood with a parent
with B blood (IBi).
IA
IB
i
i
IAIB
IBi
IAi
ii
Genotypic ratio
0 IA IA :
1 IA i :
0 IB IB :
1 IB i :
1 IA IB :
1 ii
Phenotypic ratio (blood type)—
1 type AB : 1 type A : 1 type B : 1 type O
A. Rh Positive = have proteins
a. Genotypes: Rh+/Rh+ or Rh+/RhB. Rh Negative = no proteins
b. Genotype: Rh-/Rh-
C. Cross parent heterozygous for the Rh factor with
another parent who does not have the Rh factor.
Rh+
Rh-
Rh- Rh+/Rh- Rh-/RhRh- Rh+/Rh- Rh-/RhGenotypic ratio
0 Rh+/Rh+ : 2 Rh+/Rh-: 2 Rh-/Rh-
Phenotypic ratio
2 Rh positive: 2 Rh negative