MENDEL’S GENETICS
Punnett Squares
& Probability
Before we begin,
let’s get a few terms straight…
Phenotype: The PHysical APPEARANCE
of an inherited trait. What you SEE.
Hint: look for descriptions (adjectives) or pictures.
Examples: blue eyes, black hair, tall, short, hairy, smooth…
Genotype: Describes the 2 alleles you have
for a gene. Your genetic make-up.
Hint: look for 2 letters together (eg. AA, aa, or Aa)
or the words “homozygous” or “heterozygous”
EXAMPLE…
Gg = green pea pods
Genotype
(heterozygous)
Phenotype
PREPARE TO PRACTICE…
PLEASE FIND
A WHITEBOARD AND A MARKER
at your table!
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
RR
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
RR
Genotype!
(specifically, a homozygous
dominant genotype!)
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
Red hair
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
Red hair
Phenotype!
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
Phenotype!
(An albino lion!)
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
Heterozygous
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
Heterozygous
Genotype!
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
Practice: Genotype or Phenotype?
(write G.T. or P.T. on whiteboard)
Phenotype!
DOMINANT: A version of the gene that is
ALWAYS shown if even one allele is present.
Bossy, pushy, insists on being written 1st in a genotype…
Hint: look for a CAPITAL LETTER if given.
Remember: Dominant traits don’t surprise youone of the parents MUST have shown the
dominant phenotype if they had a
dominant allele to pass on.
recessive: A version of the gene that’s totally
HIDDEN if any dominant alleles are present.
Hints: Look for a lower case letter if given. The ONLY
way to LOOK recessive is to have NO dominant allele.
Remember: If traits that seem to “pop up” or “skip a
generation” because they weren’t seen in the parents,
but appear in offspring, the “surprise” trait is recessive
AND the parents were both heterozygous!
Practice: Dominant/Recessive
(write D or R on whiteboard)
f
Practice: Dominant/Recessive
(write D or R on whiteboard)
f
recessive
(represented by a lower case letter)
Practice: Dominant/Recessive
(write D or R on whiteboard)
Tt will look…
Practice: Dominant/Recessive
(write D or R on whiteboard)
Tt will look…
Dominant! It has a T
Practice: Dominant/Recessive
(write D or R on whiteboard)

If a dove who is heterozygous for beak
color has an orange beak, are orange
beaks DOMINANT or RECESSIVE?
Practice: Dominant/Recessive
(write D or R on whiteboard)
If a dove who is heterozygous for beak
color has an orange beak, are orange
beaks DOMINANT or RECESSIVE?
DOMINANT
(Heterozygous will always show
the dominant phenotype)
PLEASE PUT AWAY
YOUR WHITEBOARD
AND MARKER !
(done with whiteboards, that isthe rest will be done on paper).
Genotypes: 3 options…
- HOMOzygous Dominant (like AA):
“homo”= same (both alleles are dominant)
*Phenotype will show DOMINANT
- HOMOzygous Recessive (like aa):
“homo”= same (both alleles are recessive)
*only way to see a RECESSIVE phenotype.
- HETEROzygous (like Aa): “hetero”= mixed
A mixture of 1 dominant & 1 recessive.
*Phenotype will show DOMINANT
Other common terms seen in
genetics problems….
- True-breeding: Means homozygous.
(Describes a parent which has
consistently similar offspring).
- Purebred: Means homozygous.
- Carrier: Means heterozygous.
(They appear dominant, but also
“carry” that sneaky, undetectable
recessive allele which may be
passed on to the next generation).
Test Cross
If something SHOWS a dominant
phenotype, you can’t be sure if they are AA
or Aa.
 You can do a “test cross” by breeding it
with one that is homozygous recessive
(aa).
 If the dominant parent was Aa, some of the
offspring will look recessive.
 If AA, none of the offspring will be
recessive.

Test
Cross
P = purple
 p = white

Practice decoding
genetics problems…
“A red flower that is heterozygous is
crossed with a white flower plant.”
a. Pick any letter of the alphabet & tell
me the genotype (alleles) of the red
plant: ____
Practice decoding
genetics problems…
“A red flower that is heterozygous is
crossed with a white flower plant.”
a. Pick any letter of the alphabet & tell
me the genotype (alleles) of the red
plant: _Rr_
(whatever letter you chose is fine, but it is
HETEROZYGOUS, so you need a capital
& a lower case of SAME LETTER)
Practice decoding
genetics problems…
“A red flower that is heterozygous is
crossed with a white flower plant.”
a. Pick any letter of the alphabet & tell
me the genotype (alleles) of the red
plant: _Rr_ *(any letter is fine)
b. What is the genotype of the white
plant?: ______
How can you know?
Practice decoding
genetics problems…
“A red flower that is heterozygous is
crossed with a white flower plant.”
a. Pick any letter of the alphabet & tell
me the genotype (alleles) of the red
plant: _Rr_ *(any letter is fine)
b. What is the genotype of the white
plant?: __rr__ (homozygous rec.)
How can you know?
It doesn’t show the dominant trait
(red)
More practice decoding
genetics problems…
“Two hybrid aliens with purple eyes
mate. 3 of their 13 offspring have
orange eyes.”
a.) What is the genotype of the parents?
____ Keyword that helped: _______
More practice decoding
genetics problems…
“Two hybrid aliens with purple eyes
mate. 3 of their 13 offspring have
orange eyes.”
I chose P=purple, p=orange.
a.) What is the genotype of the parents?
_Pp_ Keyword that helped: _hybrid_
More practice decoding
genetics problems…
“Two hybrid aliens with purple eyes
mate. 3 of their 13 offspring have
orange eyes.”
I chose P=purple, p=orange.
a.) What is the genotype of the parents?
_Pp_ Keyword that helped: _hybrid_
b.) What is the genotype of the orangeeyed offspring? _______
How can you know?
More practice decoding
genetics problems…
“Two hybrid aliens with purple eyes
mate. 3 of their 13 offspring have
orange eyes.”
I chose P=purple, p=orange.
a.) What is the genotype of the parents?
_Pp_ Keyword that helped: _hybrid_
b.) What is the genotype of the orangeeyed offspring? __pp__
How can you know? It’s a “surprise”
phenotype, not seen in parents.
STEPS for solving genetics problems…
#1: Read the question CAREFULLY!
#2: Decide which phenotype is dominant, & which
is recessive (see hints in definitions above!).
#3: Make a key (like H= hairy, h= no hair),
be sure to use only 1 letter of the alphabet per trait.
#4: Decide & write out what the genotypes of all
the individuals involved are, if possible.
(see hints in definitions above!)
#5: Do a PUNNETT SQUARE if needed.
#6: READ question AGAIN- Does the answer make sense?
PUNNETT SQUARES: why?
- If you know the exact genotype of two
parents, you can predict the possible
genotypes and phenotypes of their
offspring!
- Punnett squares can show the
probability (likelihood) of an offspring’s
genotype.
PUNNETT SQUARES: how to
- Draw a Punnett Square frame.
- Write the 2 alleles
A
of 1 parent on
A
the side of
the square.
- Write the 2 alleles
of the other parent
on the top of the a
square.
Example: Mom= Aa & Dad= Aa
a
PUNNETT SQUARES: how to
Fill in the boxes by:
- Copying top alleles into
each box in the
column below
A
a
A
A
a
a
A
a
PUNNETT SQUARES: how to
Fill in the boxes by:
- Copying top alleles into
each box in the
column below
- Copy side alleles
A
into each box in
the row next to them
* There should be
a
2 letters in each box
when you are done!
A
a
A A
A a
A a
a a
PUNNETT SQUARES:
Are used to determine Probability
- The 2 letters in each box represent a possible
genotype of an offspring.
- To figure out the chances (probability) of a
certain genotype being produced by a cross,
1. Do the Punnett Square
2. Count how many boxes show each genotype or
phenotype
3. Divide that number by 4 (for monohybrid
crosses)
-
Probability can also be shown as a ratio
(like “3 to 1” or 3:1)
PUNNETT SQUARES:
Are used to determine Probability
which can be shown as a fraction,
%, or ratio.
There are 4 boxes in a
basic Punnett Square.
each box is ¼, or 25%
¼ =_______%
2/4 = _____%
¾ = ______ %
4/4= ______%
Fill in the Punnett Square
to
answer the questions.

If 1 parent’s genotype is Jj, and the other
parent’s genotype is jj… (“Jj x jj”)
1. What percent of offspring
is expected to be
heterozygous?
________%
2. What percent of offspring
is expected to show the
dominant phenotype?
________%
Fill in the Punnett Square
to
answer the questions.

If 1 parent’s genotype is Jj, and the other
parent’s genotype is jj… (“Jj x jj”)
J
j
j
j
1. What percent of offspring
is expected to be
heterozygous?
________%
2. What percent of offspring
is expected to show the
dominant phenotype?
________%
Fill in the Punnett Square
to
answer the questions.

If 1 parent’s genotype is Jj, and the other
parent’s genotype is jj… (“Jj x jj”)
J
j Jj
j Jj
j
jj
jj
1. What percent of offspring
is expected to be
heterozygous?
________%
2. What percent of offspring
is expected to show the
dominant phenotype?
________%
Fill in the Punnett Square
to
answer the questions.

If 1 parent’s genotype is Jj, and the other
parent’s genotype is jj… (“Jj x jj”)
J
j Jj
j Jj
j
jj
jj
1. What percent of offspring
is expected to be
heterozygous?
(Jj)
50
________%
2. What percent of offspring
is expected to show the
dominant phenotype?
________%
Fill in the Punnett Square
to
answer the questions.

If 1 parent’s genotype is Jj, and the other
parent’s genotype is jj… (“Jj x jj”)
J
j Jj
j Jj
j
jj
jj
1. What percent of offspring
is expected to be
heterozygous?
(Jj)
50
________%
2. What percent of offspring
is expected to show the
dominant phenotype?
50
________%
The secret to mastering
PUNNETT SQUARES:
PRACTICE
PRACTICE
PRACTICE!!!
Image sources & links:
Give credit where credit is due!

Albino lion:
http://4.bp.blogspot.com/_vnyw0Q0l7uU/RzBOCa5kTvI/AAAAAAAABEc/vG3L6_wfjLY/s400/Al
binoLeo.jpg

Six fingers: http://img5.travelblog.org/Photos/32088/214128/t/1647731-The-Six-fingeredman.com

Red hair: http://home.overflow.net.au/~nedwood/pauline.jpg
 Whiteboard done:
http://cache.lifehacker.com/assets/resources/2007/08/white-board.png
 Tim Russert: http://www.tv.com/news-documentary/tim-russertswhiteboard
 Punnett Square Practice:
http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/sciber/punnett.htm

Clay Genetics: http://www.youtube.com/watch?v=IPPljCw5AW8