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S A N C L E M E N T E H I G H S C H O O L
Punnett Squares
Punnett Squares Made Easy!Download "cheat sheet"
Below is a sampling of Punnett Square problems that you will be
expected to solve. In order to do this, you will have to understand themeaning of the terms below.
Genotype: The letters that make up the individual. E.g. TT or TtPhenotype: The physical characteristics of the particular trait. E.g. Tall orshor tDominant trait: Signified by capital letter-E.g. T. If the traits you are using aredominant or recessive, this trait will "overpower" the recessive trait and will beexpressed. E.g. TtRecessive trait: Signified by small case letter-e.g. t. An organism with arecessive allele for a particular form of a trait will have that form only when thedominant allele for the trait is not presentHomozygous: Has same letters. E.g. TTort t (same alleles for trait)Heterozygous: Has different letters. E.g. Tt (different alleles for trait)Purebred trait: Also known as true breeding. Individuals genotype is
homozygous and will only make one type of gamete. E.g TT will alwaysproduces T, and T. tt will always produce t, and t.
Gamete: sex cells. Represented by letter N (meaning they are haploid-contain half the chromosomesP generation: The parental generation (Usually the first one in a genetic cross)
http://docushare.capousd.org/docushare/dsweb/Get/Document-3933/PUNNETT+SQUARE+CHEAT+SHEET.pdfhttp://docushare.capousd.org/docushare/dsweb/Get/Document-3933/PUNNETT+SQUARE+CHEAT+SHEET.pdfhttp://sctritons.schoolloop.com/http://sctritonscience.com/Wilson/fun_stuff.htmhttp://sctritonscience.com/Wilson/summer%20school.htmhttp://sctritonscience.com/Wilson/biology.htmhttp://sctritonscience.com/Wilson/physics.htmhttp://sctritonscience.com/Wilson/indexwilson.htmhttp://sctritons.schoolloop.com/http://sctritonscience.com/Wilson/fun_stuff.htmhttp://sctritonscience.com/Wilson/summer%20school.htmhttp://sctritonscience.com/Wilson/biology.htmhttp://sctritonscience.com/Wilson/physics.htmhttp://sctritonscience.com/Wilson/indexwilson.htmhttp://sctritons.schoolloop.com/http://sctritonscience.com/Wilson/fun_stuff.htmhttp://sctritonscience.com/Wilson/summer%20school.htmhttp://sctritonscience.com/Wilson/biology.htmhttp://sctritonscience.com/Wilson/physics.htmhttp://sctritonscience.com/Wilson/indexwilson.htmhttp://sctritons.schoolloop.com/http://sctritonscience.com/Wilson/fun_stuff.htmhttp://sctritonscience.com/Wilson/summer%20school.htmhttp://sctritonscience.com/Wilson/biology.htmhttp://sctritonscience.com/Wilson/physics.htmhttp://sctritonscience.com/Wilson/indexwilson.htmhttp://sctritons.schoolloop.com/http://sctritonscience.com/Wilson/fun_stuff.htmhttp://sctritonscience.com/Wilson/summer%20school.htmhttp://sctritonscience.com/Wilson/biology.htmhttp://sctritonscience.com/Wilson/physics.htmhttp://sctritonscience.com/Wilson/indexwilson.htmhttp://sctritons.schoolloop.com/http://sctritonscience.com/Wilson/fun_stuff.htmhttp://sctritonscience.com/Wilson/summer%20school.htmhttp://sctritonscience.com/Wilson/biology.htmhttp://sctritonscience.com/Wilson/physics.htmhttp://sctritonscience.com/Wilson/indexwilson.htmhttp://docushare.capousd.org/docushare/dsweb/Get/Document-3933/PUNNETT+SQUARE+CHEAT+SHEET.pdf8/22/2019 S A N C L E M E N T E H I G H S C H O O L.docx
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F1 generation: The f i rs tgeneration of offspring from P generation (means firstfilial: Latin for "son")F2 generation: The secondgeneration of offspring from P generation (meansfirst filial: Latin for "son")Monohybrid Cross: Also known as a Single-Factor Cross. Only one trait isused in the genetic cross. E.g. T=Tall, t=short. Example: Tt x TtDihybrid Cross: Also known as a Two-factor Cross. Two trait are used in thegenetic cross. E.g. T=Tall, t=short & B=Black fur, b=white fur. Example TtBbx TTBBIncomplete Dominance: One allele is not completely dominant over theother. There is a blending with the heterozygous offspring. E.g. RR=Red,Rr=Pink, and rr=whiteCo-dominance: Both alleles contribute to the phenotype. Offspring will havecombination of two alleles. E.g. RR=Red hair, Rr=Roan (mix of red and whitehairs-almost looks pink), and rr=whiteSex-linked trait: Genes located on the sex-chromosomes called sex-l inkedgenes. Usually found on the X chromosome. X-linked alleles are alwaysexpressed in males because males have only one X chromosome.Multiple Alleles: There are more than two-choices for the allele. Example is
human blood group genes. There are three possible alleles for this gene. I
A
, I
B
,and i. IA and IBare co-dominant. There are four possible phenotypes: A, B,
AB, and O.
Genotypic ratios: The ratio of different genotype in the offspring from a geneticcross. E.g 1:2:1Phenotypic ratios: The ratio of different phenotypes in the offspring from agenetic cross. E.g 3:1
Sample Problems
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Monohybrid(Dominant andRecessive)PROBLEM: Cro
ss twoheterozygousTall peaplants. Givegenotypic andphenotypicratios.STEP1: Determinewhat kind of
problem you aretrying to solve.
Does itinvolvesimpledominantandrecessivetraits, Incomplete
dominance, or Co-dominance?
Is it amonohybrid ordihyrid?
In thiscase thereis only one
trait..........this isa monohybr id cross
involv ing
dominant
an d
recessive
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trai ts.STEP2: Determineletters you will
use to specifytraits.
In thiscase it isdominantandrecessive,so you canuse Tandt.
STEP3: Determineparentsgenotypes.
In thiscase youwere toldtheparents
wereheterozygous. Youthereforeknow thattheparentsmustbe TtandTt
The Cross
is: Tt X Tt
STEP 4: Makeyour punnetsquare and makegametes (thesego on the top andside of your
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punnett square. Ttwould
makea Tand a t
STEP5: Completecross anddeterminepossibleoffspring.STEP6: Determinegenotypic and
phenotypic ratios. Genotypic
ratio: Make a list ofall thedifferentgenotypes(the lettercombinations) and
determinehow manyof eachyou have.
o Inyourproblemthis
wouldbe:TT
=
1,
Tt =
2, a
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nd tt -
1.Thegen
otypicratiowouldthereforebe1 :
2 :1
Phenotypicratio: Make a list ofall thedifferentphenotypes (physicalcharacteris
tics.o In
youproblemthiswouldbe:Ta l
l =3,
and
sho
rt =
1.Thephe
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notypicratiowou
ldthereforebe3 :
1
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Monohybrid(IncompleteDominance)PROBLEM: Cr
oss twoheterozygousplants. Givegenotypic andphenotypicratios. The tallgene is thisplant isincompletelydominant overthe short
gene. TT=Tall,Tt=medium,and tt=short.STEP1: Determinewhat kind ofproblem youare trying tosolve.
Does itinvolvesimpledominantandrecessivetraits, Incompletedominance, or
Co-dominance?
Is it amonohybrid ordihyrid?
In this
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casethere isonly onetrait..........this is
a monohybr id
cross
involv in
g
incompl
et e
domina
nt trai ts .STEP
2: Determineletters you willuse to specifytraits.
In thiscase it isaincompletedominant
problem. TT=tall,Tt=medi
um , and
tt=shortSTEP3: Determineparentsgenotypes.
In thiscase youwere toldtheparentswereheterozygous. Y
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outhereforeknowthat theparents
mustbe Ttand Tt
TheCrossis: Tt XTt
STEP 4: Makeyour punnetsquare and
make gametes(these go onthe top and sideof your punnettsquare.
Ttwouldmakea Tanda t
STEP5: Completecross anddeterminepossibleoffspring.STEP6: Determinegenotypic andphenotypic
ratios.
Genotypicratio: Make a listof all thedifferent
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genotypes (thelettercombinations) and
determine howmany ofeach youhave.
o Inyourproble
mthiswouldbe:T
T
=
1,
Tt
=
2,
andtt
-
1.
Thegenotypicra
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tiowouldth
ereforebe1:
2
:
1
Phenoty
picratio: Make a listof all thedifferentphenotypes(physicalcharacteristics.
o In
youproblemthiswould
be:T
all
=
1,
m
e
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d i
u
m
=
2,
an
d
s
h
or
t
=
1.
T
hephenotypicratiowould
thereforebe1:
2
:
1
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Monohybrid(Co-dominant)PROBLEM: Cr
oss twoheterozygoustan cows. Givegenotypic andphenotypicratios. BB=Black, BW=tan,and WW=whiteSTEP1: Determine
what kind ofproblem you aretrying to solve.
Does itinvolvesimpledominantandrecessivetraits, Inc
ompletedominance, or Co-dominance?
Is it amonohybrid ordihyrid?
In thiscase
there isonly onetrait..........this isa monohybr id
cross
involv in
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g co-
dominan
t traits.STEP
2: Determineletters you willuse to specifytraits.
In thiscase it isaincompletedominant
problem.TT=tall,
Tt=medi
um , and
tt=shortSTEP3: Determineparentsgenotypes.
In thiscase youwere toldtheparentswereheterozygous. Youthereforeknow that
theparentsmustbe BWand BW
TheCrossis: BW x
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BWSTEP 4: Makeyour punnetsquare and
make gametes(these go on thetop and side ofyour punnettsquare.
BWwould makea Banda Wforboth
parentsSTEP5: Completecross anddeterminepossibleoffspring.STEP6: Determine
genotypic andphenotypicratios.
Genotypicratio: Make a listof all thedifferentgenotype
s (thelettercombinations) anddetermine howmany ofeach you
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have.o In
yourpr
oblemthiswouldbe:B
B
=1,
B
W
=
2,
andW
W
=1.
Thegenotypicratio
wouldthereforebe
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1: 2
:
1
Phenoty
picratio: Make a listof all thedifferentphenotypes(physicalcharacteristics.
o In
youproblemthiswouldbe
:B lac
k
=
1,
Ta
n
=
2,
an
dW
hit
e
=
1.
Th
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ephenotypic
ratiowouldthereforebe1
: 2: 1
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Monohybrid(sex-linked)PROBLEM: Crossa female carrier for
hemophilia with amale withhemophilia. H=normal, andh=hemophiliaSTEP 1: Determinewhat kind ofproblem you aretrying to solve.
Does itinvolvesimpledominant andrecessivetraits, Incompletedominance,or Co-dominance?
Is it a
monohybridor dihyrid?
In this casethere is onlyonetrait..........thisisa monohybr id cross
involv ing
sex l inked
traits.STEP 2: Determineletters you will useto specify traits.
In this case itis a sex-
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linkedproblem. Rememberthat XXisfemale,
and XYismale. H=normal andh=hemophilia. Normallyyou wouldnot write thecapital letterson thegenotypes,only the
small case(therecessivegeneresponsiblefor thedisorder)
STEP 3: Determineparents genotypes.
In this caseyou were toldthe parentswere: Female carr ier =
XhX, and
a male withhemophi l ia
= XhY.
The Crossis: XhXx Xh
Y
STEP 4: Make yourpunnet square andmake gametes(these go on the topand side of your
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punnett square. X
hXwould
makea Xhand X
Xh
Ywouldmakea Xhand Y
NOTE: Thefemalesgametesalways go ontop of thepunnettsquare andthe males on
the side.
STEP 5: Completecross and determinepossible offspring.STEP 6: Determinegenotypic andphenotypic ratios.
Genotypic
ratio: Makea list of allthe differentgenotypes(the lettercombinations) anddeterminehow many ofeach youhave.
o
In yourproblem thiswouldbe: Xh
X =
1, Xh
Xh=
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1, andXY= 1,
and XhY =
1The
genotypicratiowouldthereforebe 1 :1 : 1 :
1
Phenotypicratio: Make
a list of allthe differentphenotypes(physicalcharacteristics.
o In youproblem thiswouldbe: F
emaleCarrier= 1,Female w/hemophilia =1,Normal male= 1,
andMalew/hemophilia =1.
The
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phenotypicratiowouldbe 1 :
1 : 1 :1
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Monohybrid(Multiple Alleles)PROBLEM: Cross aperson with type AB
blood with a personwho is heterozygousfor type A blood.STEP 1: Determinewhat kind of problemyou are trying tosolve.
Does it involvesimple dominant
and recessivetraits, Incomplete dominance,or Co-dominance, ormultiple alleles?
Is it amonohybrid ordihyrid?
In this casethere is only
onetrait..........this isa monohybr idcross
involv ing mul t i
ple allelesSTEP 2: Determineletters you will use tospecify traits.
There are morethan two-choices for theallele. Exampleis human bloodgroupgenes. Thereare three
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possible allelesfor thisgene. IA , IB,an d
i. IA and IBare
co-dominant . There are fourpossiblephenotypes: A,B, AB, and O.
STEP 3: Determineparents genotypes.
In this case you
were told onehas type ABblood
= IAIB. The
otherisheterozygousfor type A = I
Ai
STEP 4: Make yourpunnet square andmake gametes (these
go on the top and sideof your punnett square. I
AIBwould make
a IA and IB I
Aiwould make
a IAand i
STEP 5: Completecross and determinepossible offspring.STEP 6: Determinegenotypic andphenotypic ratios.
Genotypicratio: Make alist of all the
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differentgenotypes (thelettercombinations)and determine
how many ofeach you have.
o In yourproblemthiswouldbe: IA IA =1, I
AIB=
1, and IA i
= 1,
andIBi
= 1 Thegenotypicratiowouldthereforebe 1 : 1 :1 : 1
Phenotypicratio: Make alist of all the
differentphenotypes(physicalcharacteristics.
o In youproblemthiswouldbe: TypeA = 2,
Type AB
= 1, andType B =
1
o Therefore thephenotypic ratiois: 2 : 1
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: 1
Dihybrid
(Dominant andRecessive)PROBLEM: Cross twoheterozygousTall Blackcows. Tall isdominant overshort, and Blackis dominant over
white. Givegenotypic andphenotypic ratiosof offspring.STEP1: Determinewhat kind ofproblem you aretrying to solve.
Does itinvolvesimpledominantandrecessivetraits, Incompletedominance,or Co-dominance
? Is it a
monohybridor dihyrid?
In this casethere aretwotraits.........t
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his isa dihybr idcross
involv ing
dominant
an drecessive
traits.STEP2: Determineletters you will useto specify traits.
In this casethere are
two traitsyou willneed lettersfor. Use T= Tall, and
t = shortforone trait,and B =Black, and
b =
whi tefor
the secondtrait. Theseare bothdominant/recessivegenes
STEP3: Determineparentsgenotypes.
In this caseyou weretold theparentswereheterozygous for both
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traits. Youthereforeknow thatthe parentsmust
be TtBband TtBb
The Crossis: TtBb XTtBb
STEP 4: Makeyour punnetsquare and makegametes (these goon the top and
side of yourpunnett square.
Makinggametes fora dihybridcrossrequiresyouuse FOIL (first-outside-
inside-last)
TtBbwouldmake fourdifferentgametes= TB, Tb,tB, tb
STEP5: Complete crossand determine
possible offspring.
STEP6: Determinegenotypic andphenotypic ratios.
Genotypic
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ratio: Make a list of allthe differentgenotypes(the letter
combinations) anddeterminehow manyof each youhave.
o Thegenotypicratiowoul
dthereforebe 1:2:2:1
:4:1:
2:3:1 Phenotypic
ratio: Make a list of allthe different
phenotypes(physicalcharacteristics.
o Inyouproblemthiswould
be:Tall/Blac
k =
9,
and
Tall/
whi t
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e =
3,
shor
t/Bla
ck =
3,and
shor
t/whi
te =
1.o The
phenotypicratio
wouldthereforebe 9:3:3:1
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Dihybrid(DominantandRecessiveand Sex-linked)PROBLEM: Crossa homozygousTall femalecarrier forhemophilia witha short normalmale. Givegenotypic andphenotypicratios ofoffspring.STEP1: Determinewhat kind ofproblem you aretrying to solve.
Does itinvolvesimpledominantandrecessivetraits, Incompletedominance, or Co-dominance, or sex-linked?
Is it amonohybrid ordihyrid?
In thiscase
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there aretwotraits.........this isa dihybr i
d crossinvolv ing
dominant
an d
recessive
traits and
sex-
l inked
trait for
hemophi l
ia. STEP2: Determineletters you willuse to specifytraits.
In thiscasethere aretwo traits
you willneedlettersfor. UseT = Tall,
and t =
shor tforone trait,and H=normal ,
and h =
hemophi liafor thesecondsex-linkedtrait. These arebothdominant/
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recessivegenes
STEP3: Determine
parentsgenotypes.
In thiscase youwere toldtheparentswereheterozygous for
bothtraits. Youthereforeknow thattheparentsmustbe XhXTTand XYtt
The Cross
is: Xh
XTTX XYtt
STEP 4: Makeyour punnetsquare andmake gametes(these go on thetop and side ofyour punnettsquare.
Makinggametesfor adihybridcrossrequiresyou
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use FOIL (first-outside-inside-last)
Xh
XTTwould makefourdifferentgametes= XhT,XhT, XT,
and XT XYttwoul
d makefour
differentgametes= Xt, Xt,Yt, and Yt
STEP5: Completecross anddeterminepossibleoffspring. When
you have a sex-linked trait, makesure you put thefemales gameteson top of thepunnett squareand the male'son the side.STEP6: Determine
genotypic andphenotypicratios.
Genotypicratio: Make a list of
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all thedifferentgenotypes(the lettercombinati
ons) anddeterminehow manyof eachyou have.
o Thegenotypic
ratiowouldthereforebe4:4
:4:
4
or1:1
:1:
1 Phenotyp
icratio: Make a list ofall thedifferentphenotyp
es(physicalcharacteristics.
o Inyouproble
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mthiswouldbe:
Tall/
fe
ma
le
car
rier
=
4,
Ta l
l/n
orma
l
fe
ma
le
=
4,
Ta l
l/m
ale
w /he
mo
ph i
l ia
=
4,
an
d
Ta l
l/n
orma
l
ma
le
=
4.o Th
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ephenotypic
ratiowouldthereforebe4:4
:4:
4
or1:1
:1:
1
H E Y ! M R . W I L S O N
Website byDuncan WilsonPage last updated January 07, 2012
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activity at all times.
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