T
t
T
Tt
t
MIDDLE SCHOOL
GENETICS
SINGLE TRAIT PUNNETT
SQUARE
¼
TOOLS TO KNOW
A PUNNET SQUARE
IS A TOOL USED TO
PREDICT THE
POSSIBLE
GENOTYPES FOR
THE OFFSPRING OF
TWO KNOWN
PARENTS.
PARENT’S GENES
PARENT’S GENES
TERMS TO KNOW
ALLELES
DIFFERENT FORMS OF A
TRAIT THAT A GENE MAY
HAVE
HOMOZYGOUS
AN ORGANISM WITH
TWO ALLELES THAT ARE
THE SAME
HETEROZYGOUS
AN ORGANISM WITH
TWO DIFFERENT
ALLELES FOR A TRAIT
T,t
TT, tt
Tt, Gg
TERMS TO KNOW
HYBRID
SAME AS
HETEROZYGOUS
DOMINANT
A TRAIT THAT
DOMINATES OR COVERS
UP THE OTHER FORM OF
THE TRAIT
Tt, Gg
REPRESENTED BY AN
UPPERCASE LETTER
T G
OR
RECESSIVE
THE TRAIT BEING
DOMINATED OR
COVERED UP BY THE
DOMINATE TRAIT
REPRESENTED BY A
LOWER CASE LETTER
t g
or
TERMS TO KNOW
PHENOTYPE
THE PHYSICAL
APPEARANCE OF AN
ORGANISM
(WHAT IT LOOKS LIKE)
TALL, SHORT,
GREEN,
WRINKLED
GENOTYPE
THE GENE ORDER OF AN
ORGANISM
(WHAT ITS GENES LOOK
LIKE)
TT, GG, Tt, gg
Gg, tt
RATIO
THE RELATIONSHIP IN
NUMBERS BETWEEN
TWO OR MORE THINGS
3:1, 2:2, 1:2:1
HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT
SQUARE
THE PARENTS’ ALLELES
GO ON THE OUTSIDE OF
THE SQUARE
BB X bb
b
b
B
B
HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT
SQUARE
THE PARENTS’ ALLELES
GO ON THE OUTSIDE OF
THE SQUARE
b
THE ORDER DOES
NOT MATTER IN
THE BOXES, BUT
UPPERCASE
FIRST IS A GOOD
RULE
b
B
B b
B b
B
B b
B b
DROP THE
LETTERS ON
THE TOP, INTO
EACH SQUARE
MOVE EACH
LETTER ON
THE SIDE,
INTO EACH
SQUARE
HOW TO USE A MONOHYBRID (ONE TRAIT) PUNNETT
SQUARE
WHAT DO THE
RESULTS SHOW?
RESULTS:
B
B
PHENOTYPIC:
IF B IS THE
DOMINANT
ALLELE FOR
BLACK
AND b IS THE
RECESSIVE
ALLELE FOR
BROWN
100% BLACK
b
Bb
GENOTYPIC:
100% Bb
b
Bb
THEN WE CAN
MAKE
PREDICTIONS
ABOUT THE
OUTCOMES
Bb
4:0 RATIO, BLACK
TO BROWN
Bb
4:0 ALL Bb
HOW TO USE A PUNNETT SQUARE
LET’S LOOK AT
ANOTHER
PUNNETT
SQUARE AND
PREDICT THE
OUTCOME
WHAT ARE THE
RESULTS?
T
t
PHENOTYPIC:
T
T T
Tt
T IS THE
DOMINANT
ALLELE FOR
TALLNESS
t IS THE
RECESSIVE
ALLELE
FOR
SHORTNESS
75% TALL
25% SHORT
3 TO 1 RATIO: TALL
TO SHORT
GENOTYPIC:
t
T
t
t t
1TT: 2Tt: 1tt
1:2:1 RATIO
25 %TT,
50% Tt, 25% tt
PRACTICAL APPLICATION OF
PUNNETT SQUARES
THE ALLELES OF A
PARTICULAR SPECIES OF DOG
CAN BE EITHER D (NORMAL
HEIGHTH) OR d (DWARF). THE
HETEROZYGOUS (Dd) AND
HOMOZYGOUS DOMINANT (DD)
FORM OF THIS DOG LOOK THE
SAME (TALL). IF YOU FOUND A
STRAY DOG OF THIS BREED,
HOW COULD YOU DETERMINE
ITS GENOTYPE?
PRACTICAL APPLICATION OF
PUNNETT SQUARES
COULD A DOG BE
CROSSED WITH ANOTHER
DOG TO DETERMINE IF HE
WAS PUREBRED FOR
TALLNESS?
WHAT GENOTYPE
SHOULD THE DOG HAVE
THAT IS BEING USED
FOR THE CROSS?
PRACTICAL APPLICATION OF
PUNNETT SQUARES
IF THE DOG IS PUREBRED
(DD), IT DOESN’T MATTER
WHAT YOU CROSS IT WITH,
THE OFFSPRING WILL
ALWAYS LOOK LIKE THE
DOMINANT.
D
D
DD
d
Dd
Dd
D
D
DD
DD
D
DD
DD
D
D
d
Dd
Dd
d
Dd
D
DD
D
Dd
PRACTICAL APPLICATION OF
PUNNETT SQUARES
WHAT WOULD BE THE MOST
EFFECTIVE CROSS FOR
DETERMINING IF THE DOG IS
HETEROZYGOUS (Dd) ?
CROSSING IT WITH A
PUREBRED (DD) WILL NOT
HELP.
D
WHAT WOULD THE RESULTS BE IF
YOU CROSSED IT WITH ANOTHER
HETEROZYGOUS?
WHAT WOULD THE RESULTS BE IF
YOU CROSSED IT WITH A
HOMOZYGOUS RECESSIVE (dd)?
d
D
DD
Dd
d
Dd
dd
d
d
D
d
Dd
dd
Dd
dd
PRACTICAL APPLICATION OF
PUNNETT SQUARES
D
d
D
DD
Dd
d
d
Dd
dd
d
THE HETEROZYGOUS
CROSS WOULD ONLY GIVE
YOU A 25% CHANCE OF THE
RECESSIVE TRAIT
APPEARING.
D
d
Dd
dd
Dd
dd
THE MOST EFFECTIVE CROSS WAS
USING THE HOMOZYGOUS
RECESSIVE. THIS WOULD GIVE A 50%
CHANCE OF THE RECESSIVE TRAIT
APPEARING.
THIS PROCESS IS CALLED A TEST CROSS. IN A LITTER OF DOGS,
IF A RECESSIVE DOG APPEARS, THEN YOU KNOW THAT THE
ORIGINAL DOG WAS NOT A PUREBRED.
TAKS FORMATTED ITEMS
IN DROSOPHILA MELANOGASTER
(FRUIT FLIES), RED EYE COLOR
(R) IS DOMINANT OVER BROWN
EYE COLOR (r). IF THE FLIES IN
THE PICTURE WERE CROSSED,
WHAT PERCENT OF THEIR
OFFSPRING WOULD BE
EXPECTED TO HAVE BROWN
EYES?
ANSWER: 50%
TAKS FORMATTED ITEMS
H
1. WHICH OF THE FOLLOWING
HAS THE hh GENOTYPE?
A.
B.
C.
D.
1&3
2
4
NONE
H
4
1
h
3
2
2. WHICH OF THE FOLLOWING IS A TRUE
STATEMENT?
A.
B.
C.
D.
h
INDIVIDUAL 4 IS RECESSIVE
INDIVIDUALS 1 & 3 ARE HETEROZYGOUS
INDIVIDUAL 2 IS DOMINANT
ALL INDIVIDUALS ARE FEMALE
TAKS FORMATTED ITEMS
3. IF B IS THE ALLELE FOR BLACK
FUR AND b IS THE ALLELE FOR
WHITE FUR, WHAT PERCENT
WOULD BE BLACK?
A.
B.
C.
D.
25%
50%
100%
75%
B
b
B
BB
Bb
b
Bb
bb
4. WHAT FRACTION IS HOMOZYGOUS DOMINANT IN THE ABOVE CROSS?
A.
B.
C.
D.
1/2
1/4
1/3
3/4
TAKS FORMATTED ITEMS
B
5. IN THIS CROSS, WHAT IS THE
RATIO OF BB TO Bb?
A.
B.
C.
D.
3:1
4:1
2:2
0:4
B
B
BB
BB
b
Bb
Bb
BIBLIOGRAPHY
• Mendel picture provided by wikipedia.com
•Dog and tree pictures provided by Microsoft ClipArt
•Graphics from TEA/ TAKS information booklet Middle School Science
Grade 8
•Dice graphic from Microsoft Clipart