Punnett Squares
How can I predict the appearance
of offspring based on the traits of
the parents?
Two kinds of inheritance:

Mendelian
inheritance includes
any trait which has
only a pair of
contrasting alleles
and one of the
alleles is dominant
to the other allele.
 These traits will
follow Mendel’s
principles of
heredity.
Two kinds of inheritance:

Non-Mendelian
inheritance includes
traits which may share
dominance, be linked
with a second trait
(such as sex), rely on
multiple genes within
the chromosomes, or
have multiple forms
(alleles) which may be
inherited.
 These traits do not
follow all of Mendel’s
principles of
inheritance.
Probability & inheritance

Punnett developed
a graphical method
to predict the
results of a cross
between two parent
organisms.
 These are called
Punnett squares.
Probability & inheritance

A Punnett Square
shows all of the
possible outcomes
each time gametes
from the two
parents combine.
Steps to solving a Punnett square:

Assign each allele
(form of the trait) a
letter

Problem:
tongue rolling is
dominant to nonrolling
Tongue rolling = R
Non-rolling = r
Steps to solving a Punnett square:

Determine the
genotype of each
parent based on
the information in
the problem.

Problem:
cross two
heterozygous
individuals
Parent 1 = Rr
Parent 2 = Rr
Steps to solving a Punnett square:

Set up the Punnett
square by putting one
parent’s genotype
across the top and
the other down the
side of the square.
 These represent
possible gametes.
R
R
r
r
Steps to solving a Punnett square:

Complete the
Punnett square by
recording the letter
on top of the
column and on the
side of the row.
 Always put the
capital letter first.
R
r
R
RR
Rr
r
Rr
rr
Steps to solving a Punnett square:


Use the laws of
probability and the
Punnett square to
answer any
questions posed in
the problem.
Problem 1: what is
the genotype ratio?
 Answer:
1RR : 2Rr : 1rr
1:2:1
R
r
R
R
R
Rr
r
Rr
rr
Steps to solving a Punnett square:


Problem 2: what is
the phenotype
ratio?
Answer:
3 Tongue rolling : 1
Non-rolling
3:1
R
r
R
R
R
Rr
r
Rr
rr
Steps to solving a Punnett square:


Problem 3: what is
the chance the
couple will have a
non-rolling child?
Answer:
1/4 or 25%
R
r
R
R
R
Rr
r
Rr
rr
Fill It In …
Genotype Ratio follows the format:
______ : ______ : ______
Phenotype Ratio follows the format:
______ : ______
Hybrid crosses:

Punnett squares
can be used to
solve crosses
involving only one
trait (called a
monohybrid cross)
or crosses involving
two traits (called a
dihybrid cross).
Monohybrid cross

The monohybrid
cross requires four
squares to
represent all
possible gamete
combinations.
Dihybrid cross

The dihybrid cross
requires sixteen
squares to
represent all
possible gamete
combinations.
What are the different patterns of
inheritance?




Mendelian/simple
 Solution:
dominance
tall = H short = h
Dominant masks
P1 = HH P2 = hh
ressessive alleles
H
H
Monohybrid
Problem: tall pea plant
Hh
Hh
h
height is dominant dominant
to short pea plant height.
Cross a pure breeding tall
Hh
Hh
pea plant with a pure
h
breeding short pea plant.
Give the genotypic and
G: 0 : 4 : 0 (HH : Hh : hh)
phenotypic ratio.
P: 4 : 0 (tall : short)
Non-Mendelian - 5 possible paths of
inheritance


Codominance
Both alleles are
equally dominant
and so both are
equally expressed.
 To represent the
equal dominance
each allele is
assigned a different
capital letter.
 **use as
superscripts!!!
Fill It In …
WORD HELP!
Co = ______________
Non-Mendelian - 5 possible paths of
inheritance


Codominance
Problem:
Black feathers and
white feathers are
codominant in
chickens. Cross a
chicken with black and
white feathers and a
chicken with only black
feathers. What is the
chance they will have a
chick with only white
feathers?
black feathers = B
white feathers = W
P1 = BW P2 = BB
B
W
B
BB
BW
B
BB
BW
0% chance of chick with
white feathers
Non-Mendelian - 5 possible paths of
inheritance



Incomplete dominance
Neither allele is sufficiently dominant to mask the
other allele.
When both alleles are present in an individual’s
genotype (heterozygous) an entirely different,
blended phenotype appears.
Non-Mendelian - 5 possible paths of
inheritance


Incomplete dominance
To represent the incomplete dominance both share
the same capital letter, but one is assigned a
“prime” symbol.
RR
RR’
R’R’
Non-Mendelian - 5 possible
paths of inheritance


Incomplete
dominance
Problem:
In four o’clock flowers
red petals and white
petals are incompletely
dominant. Cross a red
flower and a white
flower. What is the
resulting phenotype of
all offspring?
red = R
white = R’
P1 (red) = RR
P2 (white) = R’R’
R
R
R’
RR’
RR’
R’
RR’
RR’
All offspring are pink.
Non-Mendelian - 5 possible
paths of inheritance




Multiple alleles
More than two alleles
exist within the
population for the
given trait.
However, each
individual may only
inherit two of the
possible alleles
To represent the
multiple alleles, a base
letter is used for each
allele and the allele is
represented by a
unique superscript
letter.

Problem:
blood type in humans
is determined by
multiple alleles: IA, IB, i
In addition, IA and IB
are codominant, while
i is recessive to both.
Below is a chart
representing all the
possible genotypes
and resulting
phenotypes ….
Non-Mendelian - 5 possible
paths of inheritance

Blood Type (phenotype)
Type A
Type B
Type AB
Type O
Possible Genotypes
IAIA or IAi
IBIB or IBi
IAIB
ii
Fill It In …
Draw a blood cell for each type:
ABO**Add AB -
Codominance
Fill It In …
Alleles for Blood Type:
Allele for A =
Allele for B =
Allele for O =
Non-Mendelian - 5 possible
paths of inheritance

Multiple alleles

Problem:
Cross a heterozygous Type A female with a Type O
male. What are all the possible blood types of the
offspring?
A
I
i
A
P1 = I i
P2 = ii
i
I Ai
ii
i
I Ai
ii
Type A and Type O are the possible blood types.
Non-Mendelian - 5 possible
paths of inheritance

Sex-linkage
 One of the pairs of
chromosomes in an
organism
determines the sex.
 This pair is called
the sex
chromosomes.
Sex-Linked Disorders
Non-Mendelian - 5 possible
paths of inheritance


Sex-linkage
In humans, the two
types of sex
chromosomes are
represented by an X
and a Y
 Inheriting two X
chromosomes
makes a female;
inheriting an X and a
Y chromosomes
makes a male.
Non-Mendelian - 5 possible
paths of inheritance


Sex-linkage
The genes located
on a sex
chromosome
(almost always the
X chromosome) are
called sex-linked
genes.
 Thus, females
inherit two alleles
for the trait, while
males only inherit
one allele for the
trait.
Fill It In …
What makes sex-linked genes
DIFFERENT from other genes?
Non-Mendelian - 5 possible
paths of inheritance


Sex-linkage
Problem:
Colorblindness is a
recessive sex-linked
trait. A colorblind man
has a child with a
woman who is a carrier
(heterozygous) for
colorblindness. What
is the chance they will
have a colorblind son?
“normal” vision = XB
colorblind = Xb
P1 = XbY
P2 = XBXb
Xb
Y
XB
XBXb
XBY
Xb
XbXb
XbY
There is a 25% chance
they will have a
colorblind son
Non-Mendelian - 5 possible
paths of inheritance

Sex-linkage
Non-Mendelian - 5 possible
paths of inheritance

Polygenic
inheritance
 Many traits actually
depend on several
genes.
 The interaction of
the many genes
within one
individual creates a
range of
phenotypes
Non-Mendelian - 5 possible
paths of inheritance



Polygenic inheritance
For example, height,
skin color and the size
of your foot depend on
the total number of
dominant alleles
inherited for these
traits.
This means someone
with four dominant
alleles will have a
slightly bigger foot than
someone with three
dominant alleles.
Non-Mendelian - 5 possible
paths of inheritance
Polygenic
inheritance
 This results in a
characteristic
graphical pattern
seen below:
Number of Individuals

Skin Color
Fill It In …
The SIX paths of inheritance:
1. M_______________ (A, a)
2. C_______________ (A, B)
3. I________________
D_______________ (A, A’)
4. M_______________
A_______________ (IA, IB, i)
5. S____ - l_________ (XA, Xa)
6. P_______________ (AaBbCc)
Paths of Inheritance
Path Type
Characteristics
Mendelian
1 allele is dominant and 1 allele is
recessive
Codominant
2 alleles – both are equally
expressed
Incomplete
dominance
2 alleles – if heterozygous, alleles
blend together
Multiple alleles
More than 2 alleles possible for a
trait (but each person gets 2)
Polygenic
A traits is controlled by more than
one gene in a person
Check Yourself!
1.
Name the five paths of non-Mendelian inheritance.
2.
Which pattern of inheritance results in a third,
blended phenotype of individuals?
3.
What two patterns of inheritance does blood type
follow?
4.
Where are the sex-linked traits located?
5.
Name three traits that follow polygenic inheritance?
Check Yourself!
1.
2.
Name the five paths of non-Mendelian inheritance.
CODOMINANCE, INCOMPLETE DOMINANCE,
MULTIPLE ALLELES, SEX-LINKED, &
POLYGENIC
Which pattern of inheritance results in a third,
blended phenotype of individuals?
3.
What two patterns of inheritance does blood type
follow?
4.
Where are the sex-linked traits located?
5.
Name three traits that follow polygenic inheritance?
Check Yourself!
1.
2.
3.
Name the five paths of non-Mendelian inheritance.
CODOMINANCE, INCOMPLETE DOMINANCE,
MULTIPLE ALLELES, SEX-LINKED, &
POLYGENIC
Which pattern of inheritance results in a third,
blended phenotype of individuals?
INCOMPLETE DOMINANCE
What two patterns of inheritance does blood type
follow?
4.
Where are the sex-linked traits located?
5.
Name three traits that follow polygenic inheritance?
Check Yourself!
4.
Name the five paths of non-Mendelian inheritance.
CODOMINANCE, INCOMPLETE DOMINANCE,
MULTIPLE ALLELES, SEX-LINKED, &
POLYGENIC
Which pattern of inheritance results in a third,
blended phenotype of individuals?
INCOMPLETE DOMINANCE
What two patterns of inheritance does blood type
follow?
MULTIPLE ALLELES & CODOMINANCE
Where are the sex-linked traits located?
5.
Name three traits that follow polygenic inheritance?
1.
2.
3.
Check Yourself!
1.
2.
3.
4.
5.
Name the five paths of non-Mendelian inheritance.
CODOMINANCE, INCOMPLETE DOMINANCE,
MULTIPLE ALLELES, SEX-LINKED, &
POLYGENIC
Which pattern of inheritance results in a third,
blended phenotype of individuals?
INCOMPLETE DOMINANCE
What two patterns of inheritance does blood type
follow?
MULTIPLE ALLELES & CODOMINANCE
Where are the sex-linked traits located? ON THE
SEX CHROMOSOMES (ESP THE X)
Name three traits that follow polygenic inheritance?
Check Yourself!
1.
2.
3.
4.
5.
Name the five paths of non-Mendelian inheritance.
CODOMINANCE, INCOMPLETE DOMINANCE,
MULTIPLE ALLELES, SEX-LINKED, &
POLYGENIC
Which pattern of inheritance results in a third,
blended phenotype of individuals?
INCOMPLETE DOMINANCE
What two patterns of inheritance does blood type
follow?
MULTIPLE ALLELES & CODOMINANCE
Where are the sex-linked traits located? ON THE
SEX CHROMOSOMES (ESP THE X)
Name three traits that follow polygenic inheritance?
(ANSWERS WILL VARY)
Bell Ringer
In pea plants round seeds (R) are
dominant to wrinkled seeds (r) and
yellow seed color (Y) is dominant to
green seed color (y).
 If a plant heterozygous for both is
crossed with a plant that has wrinkled,
green seeds, what is the phenotypic
ratio of their offspring?

Bell Ringer

In goats, a recessive gene causes the
goats to “faint” when they are startled.
A farmer breeds two goats (that have
never fainted) and their first offspring
faints two days after its birth. What
must the parent’s genotypes have
been? Show the cross to defend your
answer.
Fainting goats