Warm-up (use pencil today)
1.What type of cross does the visual below represent?
(Hint: see word wall terms)
2. How many traits are being crossed?
3. What are the genotypes of the traits being crossed?
4. What words from the word wall represent the parent
genotypes?
PART 2
Mendel’s experimentation with peas continued:

Mendel performed another set of crosses in
which he used peas that differed from each other
in two traits rather than only one.

A cross involving two pairs of contrasting
traits is called a dihybrid cross (because di
means “two”). example: seed shape (smooth
vs. wrinkled) and seed color (yellow vs. green).

Mendel wanted to find out if the two traits would
stay together in the next generation (do they
influence one another).

After Mendel experimented with the dihybrid
crosses, he found that alleles sorted
independently.
“Mendel’s Law of Independent
Assortment”: two alleles for different traits
can segregate independently during the
formation of gametes.
How do we use a Punnett square
to conduct Dihybrid Crosses?
We start by determining the possible gamete
combination of each parent’s genotype.
#1. Possible Gametes:
To determine the number of different gametes a parent can have, we use the
acronym FOIL (First – Outer – Inner – Last) with the pair of traits.
rrEe
rE
re
rE
re
There are 2 possible gametes:
rE and re
#2. Possible Gametes:
To determine the number of different gametes a parent can have, we use the
acronym FOIL (First – Outer – Inner – Last) with the pair of traits.
RREe
RE
Re
RE
Re
There are 2 possible gametes:
Re and RE
#3. Possible Gametes:
To determine the number of different gametes a parent can have, we use the
acronym FOIL (First – Outer – Inner – Last) with the pair of traits.
RREE
RE
RE
RE
RE
There is 1 possible gamete:
All gametes are RE
Determine the different types of gametes
Hint: Use the FOIL method.
Parent genotype
Number of gamete variations
#4.
TTYY
=
#5.
TtYY
=
two (TY and tY)
TtYy
=
four (TY,Ty, tY, ty,)
#6.
one (TY)
Determining Gametes:
What are the different possible gametes combinations for
these parents (R=round, r=wrinkled, T=tall, t=short) ?
RRTT x RrTt
RT
RT, Rt, rT, rt
There are 4 total different gametes
#7. Determine the possible gametes and offspring by completing
the cross below (R=round, r=wrinkled, T=tall, t=short).
RRTT x RrTt
Genotypic ratio?
4 RRTT
4 RRTt
4 RrTT
4 RrTt
RT
Rt
rT
rt
RT
RRTT
RRTt
RrTT
RrTt
RT
RRTT
RRTt
RrTT
RrTt
RT
RRTT
RRTt
RrTT
RrTt
RT
RRTT
RRTt
RrTT
RrTt
Phenotypic ratio?
16 Round/Tall
#8. Determine the possible gametes and offspring by
completing the cross below (R=round, r=wrinkled, T=tall, t=short).
rrTt x RRTt
Genotypic ratio?
4 RrTT
8 RrTt
4 Rrtt
RT
Rt
RT
Rt
rT
RrTT
RrTt
RrTT
RrTt
rt
RrTt
Rrtt
RrTt
Rrtt
rT
RrTT
RrTt
RrTT
RrTt
rt
RrTt
Rrtt
RrTt
Rrtt
Phenotypic ratio?
12 Round/Tall
4 Round/short
#9. Determine the possible gametes and offspring by completing
the cross below (R=round, r=wrinkled, T=tall, t=short).
RrTt x RrTt
Genotypic ratio?
1 RRTT
2 RRTt
2 RrTT
4 RrTt
1 RRtt
2 Rrtt
1 rrTT
2 rrTt
1 rrtt
RT
Rt
rT
rt
RT
RRTT
RRTt
RrTT
RrTt
Rt
RRTt
RRtt
RrTt
Rrtt
rT
RrTT
RrTt
rrTT
rrTt
rt
RrTt
Rrtt
rrTt
rrtt
Phenotypic ratio?
9 Round/Tall
3 Round/short
3 wrinkled/Tall
1 wrinkled/short
RESPONSE BOARD
ASSESSMENT
ARE YOU READY?
Warm-up 1-10-13
According to the table above which of the following
phenotypes would probably occur in all the offspring
from the parents shown above?
a. Solid gray fur
b. Striped gray fur
c. Green eyes
d. Blue eyes
A pea plant with the genotype TtWW is
crossed with a pea plant with the
genotype ttWw. How many different
genotypes can be expressed in the
offspring?
A.
B.
C.
D.
1
2
3
4
Two cats heterozygous for short hair and
tabby striping mate. Short hair is dominant
over long hair. Tabby striping is dominant
over plain coat.
1. What is the phenotypic
ratio?
9 Short/Tabby
3 Short/Plain
3 long/Tabby
1 long/plain
2. What is the probability of
producing a long haired, tabby cat?
3 out of 16
Online Dihybrid Cross Practice
http://biology.clc.uc.edu/courses/bio105/geneprob.htm
Why Did Mendel Conclude That The
Inheritance of one Trait is Independent
of Another?
Phenotype Ratio:
9 yellow/smooth
3 yellow/wrinkled
3 green/smooth
1 green/wrinkled
Because it’s the
only way to explain
the pattern of
inheritance.
What Works
for Peas Also
Works for
Humans
Consider a cross
between parents
heterozygous for
both deafness and
albinism.
This is the same
9:3:3:1 ratio seen
for Mendel’s cross
involving pea
color and shape.
Going Beyond
Dominant and Recessive
There are always exceptions to
the rule…
Some alleles are related through
Incomplete Dominance
a cross between organisms
with two different phenotypes
produces hybrid offspring with
a third phenotype that is a
blending of the parental
traits.
Remember Incomplete Dominance
RED Flower x WHITE Flower ---> PINK Flower
WW
white
Codominance
A cross between two different
phenotypes produces a third
phenotype where both traits
appear “together” in a hybrid
offspring.
A.
B.
A. Cross between a homozygous red flower
and a homozygous white flower.
B. Cross between homozygous IA blood type
and homozygous IB blood type.
Remember codominance
red x white ---> red & white spotted
Codominance cont’d
Cross between a homozygous black horse and a
homozygous white horse.
W
B
B
W
BW
BW
BW
BW
Incomplete or Codominant?
+
Incomplete or Codominance?
X
Polygenic Inheritance –
when a single trait is influenced
by many genes
Polygenic Inheritance
Examples: Height
Hair texture
Skin color
eye color
Multiple Alleles
Many genes are present in 3 or
more alleles (versions). This is
known as multiple alleles.
The human ABO blood group
is determined by three alleles
(IA, IB, and i) of a single gene.
Blood
Type
A
B
AB
O
Genotype
I AI A
IAi
IBIB
IBi
I AI B
ii
AA
AO
BB
BO
AB
OO
Can Receive
Blood From:
A or O
B or O
A, B, AB, O
O
Human Blood Types
Sex-Linked Traits
Characteristics that are
inherited from genes
found on the sex
chromosomes.
Sex-Linked Traits
• Sex Determination
– Females – XX
– Males – XY
• Almost all sex-linked traits are found on
the X chromosome
• Y chromosome contains very few genes
and is mainly involved in sex
determination
Intro to Human a Karyotype
Normal Male and Female
Karyotypes
See page 341 in your biology textbook
In a human karyotype, the first 22 pairs of chromosomes are the
autosomes. The 23rd pair of chromosomes are the sex chromosomes
Examples of X-linked traits:
1. Color Blindness
2. Hemophilia
3. Muscular Dystrophy
Colorblindness
• A person with normal color vision sees a number seven in
the circle above.
• Those who are color blind usually do not see any number
at all.
Colorblindness
• RED-GREEN
COLORBLINDNESS:
• People with red-green
color blindness see
either a three or
nothing at all.
• Those with normal
color vision see an 8.
Carrier – person who has one recessive
allele and one dominant allele for a trait or
heterozygous for that trait.
•
Example
Hemophiliac carrier XHXh
Colorblind carrier
XCXc
http://webexhibits.org/causesofcolor/2.html
Sex-linked punnett square
Question: What is the
probability that a carrier female
and a colorblind male will have
a girl who is colorblind
(c = colorblind, C = normal)?
Xc
Y
XC
XCXc
XCY
Xc
XcXc
XcY
Human Pedigree
See page 342-343 in your biology textbook