Mendelian Patterns of
Inheritance
Chapter 11
Heredity
Heredity- passing of traits from parent to
offspring
 Traits- characteristics that are inherited

– hair color, height, blood type, susceptibility to
a certain disease (diabetes, depression,
obesity, breast cancer)

Genetics- the study of heredity
Gregor Mendel
• Austrian monk who formulated
fundamental laws of heredity in
early 1860s
• Studied science and mathematics at
University of Vienna
• Conducted breeding experiments
with the garden pea (Pisum sativum)
• Gathered and documented
mathematical data from his
experiments
Mendel’s Experiments
• Garden Pea
• Easy to cultivate
• Short generation time
• Cross-pollination by hand
Mendel’s Experiments
Self-Pollination
Involves having
the pollen (male
sperm) be directly
deposited on the
female section of
the flower

Cross- Pollination
Requires the
removal of the male
stamen (makes
pollen) on 1st flower
and transferring the
pollen from a
different flower
to the first one

Blending Inheritance
• Prior to Mendel, most breeders believed
parents of contrasting appearance always
produce offspring of intermediate
appearance
• No knowledge of cells/chromosomes
• Based on the idea that offspring have traits of
both parents
• Reappearance of traits attributed to genetic
instability
Mendel’s Experiments
• Studied mathematical trends in breeding
patterns
• Looked at simple discrete traits
• Mendel’s experiments helped him
formulate the particulate theory of
inheritance
• Inheritance involves reshuffling of genes from
generation to generation
One-Trait Inheritance

Mendel performed cross-breeding
experiments between true-breeding plants
– True-breeding – parents with a certain trait
consistently pass that trait to every offspring
(homozygous)
– Chose varieties that differed in only one trait
(monohybrid cross)
Results of Mendel’s cross of true breeding short with a
true breeding tall pea plant
Mendel saw the same results in
different traits
One-Trait Inheritance
• Performed reciprocal crosses – individuals
pollinate one another
• Parental generation = P
• First generation offspring = F1
• Second generation offspring = F2
• Formulated law of segregation
Law of Segregation
•
•
•
•
Each individual has two factors for each
trait
The factors segregate during gamete
formation
Each gamete contains only one factor
from each pair of factors
Fertilization gives each new individual two
factors for each trait
Law of Segregation
Mendel’s Monohybrid Cross
Mendel’s Monohybrid Cross
Alleles on homologous chromosomes

These alternative forms of a gene that
code for a trait are called alleles. There
are 2 alleles for each trait; 1 allele for a
trait is from mom and 1 allele is from dad.
Homologous Chromosomes
Principle of Dominance
• Dominant allele (capital letter) masks the
expression of the recessive allele (lower-case)
• Alleles occur on a homologous pair of
chromosomes at a particular gene locus
(location of gene on the chromosome)
• Homozygous = identical alleles (TT, tt)
• Heterozygous = different alleles (Tt)
Genotype Vs. Phenotype
• Genotype
• Refers to the alleles an individual receives at
fertilization
• If alleles are identical, genotype is
homozygous
• If alleles are different, genotype is
heterozygous
• Phenotype
• Refers to the physical appearance of the
individual
Practice
In humans, the ability to taste the
bitterness of PTC paper is dominant (T) to
not being able to taste the bitterness of
PTC paper (t).
GENOTYPE
GENOTYPE IN
SCIENCE TERMS
PHENOTYPE
Punnett Square
• All possible genotypes of
sperm are lined up on one
axis, and all possible
genotypes of eggs are
lined up on the other axis
• Every possible combination
of alleles (zygote
genotypes) placed within
the squares
Formative Assessment

Get your clickers!
Alternate forms of a gene are
called
5%
5%
He
t
er
oz
yg
ot
e
s
es
Ga
m
et
le
le
s
0%
Al
es
D.
os
om
C.
91%
om
B.
Chromosomes
Alleles
Gametes
Heterozygotes
Ch
r
A.
Only one ________ allele is
needed in order for that trait to
be expressed in the phenotype.
5%
0%
Ho
m
oz
yg
ou
s
ou
s
er
oz
yg
He
t
in
an
t
0%
Do
m
D.
ss
ive
C.
ce
B.
Recessive
Dominant
Heterozygous
Homozygous
Re
A.
95%
1.
2.
3.
Which of the following
genotypes is homozygous
recessive?
RR
Rr
rr
96%
4%
1
0%
2
3
The genotype TT is
1.
2.
3.
Homozygous
recessive
Homozygous
dominant
Heterozygous
100%
0%
1
0%
2
3
Which of the following
genotypes is heterozygous?
1.
2.
3.
BB
Bb
bb
100%
0%
1
0%
2
3
Brown eyes (B) are dominant over blue eyes
(b). Mr. Mallin has blue eyes. What is his
genotype?
100%
1.
2.
3.
BB
Bb
bb
0%
1
0%
2
3
Brown eyes (B) are dominant over blue eyes
(b). Channing Tatum has brown eyes. What
is his genotype?
1.
2.
3.
4.
91%
BB
Bb
bb
BB or Bb
9%
0%
1
2
0%
3
4
Brown eyes (B) are dominant over blue eyes
(b). Assume Mr. Ward is homozygous
dominant for brown eyes. What is his son’s
phenotype?
1. Brown eyes
83%
2. Blue eyes
3. Bb
4. BB
17%
0%
1
2
0%
3
4
Free earlobes (E) are dominant over attached
earlobes (e). Ms. Palmeri’s phenotype is free
earlobes and her genotype is heterozygous (Ee).
Which of these statements is true?
1.
2.
3.
Both of her parents
have attached ear
lobes
At least one of her
parents has free
earlobes
Both of her parents
are homozygous
recessive
91%
9%
0%
1
2
3
If a persons genotype is EE, what
percentage of their gametes (produced by
meiosis) would contain the recessive
allele?
A. 100%
B. 50%
92%
C. 25%
D. 0%
8%
0%
0%
25
%
50
%
10
0%
0%
The previous question is a direct
application of which of Mendel’s
laws?
nc
e
of
Do
m
in
a
at
io
re
g
Se
g
le
cip
La
w
of
Pr
in
of
w
La
sD
o.
..
oz
yg
ou
Ho
m
ep
en
de
n
ta
ss
...
n
9%
in
d
D.
32%
23%
of
C.
36%
w
B.
Law of
independent
assortment
Law of
Homozygous
Dominance
Law of
Segregation
Principle of
Dominance
La
A.
Testcross
• A monohybrid testcross is used to determine if
an individual with the dominant phenotype is
homozygous dominant or heterozygous for a
particular trait
• Individuals with recessive phenotype always have the
homozygous recessive genotype
• However, Individuals with dominant phenotype have
indeterminate genotype
• May be homozygous dominant, or
• Heterozygous
Testcross
Dihybrid Cross
Dihybrid Cross
Two-Trait Testcross
•
A two-trait testcross is used to determine
if an individual is homozygous dominant or
heterozygous for either of the two traits
Two-Trait Testcross
Law of Independent Assortment
Pairs of alleles for different traits separate
independently of one another during gamete
formation (meiosis).
 In other words the inheritance of one trait has
no influence on the inheritance of another trait.
