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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 the meaning of the terms below. 

Genotype: The letters that make up the individual.  E.g. TT or Tt

Phenotype: The physical characteristics of the particular trait.  E.g. Tall or short

Dominant trait:  Signified by capital letter-E.g. T.  If the traits you are using are dominant or recessive, this trait will "overpower" the recessive trait and will be expressed.  E.g. Tt

Recessive trait:  Signified by small case letter-e.g. t.  An organism with a recessive allele for a particular form of a trait will have that form only when the dominant allele for the trait is not present

Homozygous:  Has same letters.  E.g. TT or tt  (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 always produces T, and T.  tt will always produce t, and t.  

Gamete:  sex cells.  Represented by letter N (meaning they are haploid-contain half the chromosomes

P generation:  The parental generation (Usually the first one in a genetic cross)

F1 generation:  The first generation of offspring from P generation  (means first filial: Latin for "son")

F2 generation:  The second generation of offspring from P generation  (means first filial: Latin for "son")

Monohybrid Cross:  Also known as a Single-Factor Cross.  Only one trait is used in the genetic cross.  E.g. T=Tall, t=short.  Example:  Tt x Tt

Dihybrid Cross:  Also known as a Two-factor Cross.  Two trait are used in the genetic cross.  E.g.  T=Tall, t=short  &  B=Black fur, b=white fur.  Example  TtBb x TTBB

Incomplete Dominance:  One allele is not completely dominant over the other.  There is a blending with the heterozygous offspring.  E.g. RR=Red, Rr=Pink, and rr=white

Co-dominance:  Both alleles contribute to the phenotype.  Offspring will have combination of two alleles.  E.g. RR=Red hair, Rr=Roan (mix of red and white hairs-almost looks pink), and rr=white

Sex-linked trait:  Genes located on the sex-chromosomes called sex-linked genes.  Usually found on the X chromosome.  X-linked alleles are always expressed 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.  IA, IB, and i.  IA and IB are co-dominant.  There are four possible phenotypes: A, B, AB, and O.  

Genotypic ratios: The ratio of different genotype in the offspring from a genetic cross.  E.g 1:2:1

Phenotypic ratios:  The ratio of different phenotypes in the offspring from a genetic cross.  E.g 3:1

Sample Problems

Monohybrid (Dominant and Recessive)

PROBLEM:  Cross two heterozygous Tall pea plants.  Give genotypic and phenotypic ratios.

STEP 1:  Determine what kind of problem you are trying to solve.  

  • Does it involve simple dominant and recessive traits,  Incomplete dominance, or Co-dominance?

  • Is it a monohybrid or dihyrid?

  • In this case there is only one trait..........this is a monohybrid cross involving dominant and recessive traits.

STEP 2:  Determine letters you will use to specify traits.  

  • In this case it is dominant and recessive, so you can use T and t.

STEP 3:  Determine parents genotypes.  

  • In this case you were told the parents were heterozygous.  You therefore know that the parents must be Tt and Tt

  • The Cross is:  Tt X Tt

STEP 4:  Make your punnet square and make gametes (these go on the top and side of your punnett square.

  • Tt would make a T and a t

STEP 5:  Complete cross and determine possible offspring.

STEP 6:  Determine genotypic and phenotypic ratios.

  • Genotypic ratio:  Make a list of all the different genotypes (the letter combinations) and determine how many of each you have.

    • In your problem this would be:  TT = 1,  Tt = 2, and tt - 1.  The genotypic ratio would therefore be 1 : 2 : 1 

  • Phenotypic ratio:  Make a list of all the different phenotypes (physical characteristics.

    • In you problem this would be:  Tall = 3, and short = 1.  The phenotypic ratio would therefore be 3 : 1

Monohybrid (Incomplete Dominance)

PROBLEM:  Cross two heterozygous plants.  Give genotypic and phenotypic ratios.  The tall gene is this plant is incompletely dominant over the short gene.  TT=Tall, Tt=medium, and tt=short.

STEP 1:  Determine what kind of problem you are trying to solve.  

  • Does it involve simple dominant and recessive traits,  Incomplete dominance, or Co-dominance?

  • Is it a monohybrid or dihyrid?

  • In this case there is only one trait..........this is a monohybrid cross involving incomplete dominant traits.

STEP 2:  Determine letters you will use to specify traits.  

  • In this case it is a incomplete dominant problem.  TT=tall, Tt=medium, and tt=short

STEP 3:  Determine parents genotypes.  

  • In this case you were told the parents were heterozygous.  You therefore know that the parents must be Tt and Tt

  • The Cross is:  Tt X Tt

STEP 4:  Make your punnet square and make gametes (these go on the top and side of your punnett square.

  • Tt would make a T and a t

STEP 5:  Complete cross and determine possible offspring.

STEP 6:  Determine genotypic and phenotypic ratios.

  • Genotypic ratio:  Make a list of all the different genotypes (the letter combinations) and determine how many of each you have.

    • In your problem this would be:  TT = 1,  Tt = 2, and tt - 1.  The genotypic ratio would therefore be 1 : 2 : 1 

  • Phenotypic ratio:  Make a list of all the different phenotypes (physical characteristics.

    • In you problem this would be:  Tall = 1, medium = 2, and short = 1.  The phenotypic ratio would therefore be 1 : 2 : 1

 

Monohybrid (Co-dominant)

PROBLEM:  Cross two heterozygous tan cows.  Give genotypic and phenotypic ratios.  BB=Black, BW=tan, and WW=white

STEP 1:  Determine what kind of problem you are trying to solve.  

  • Does it involve simple dominant and recessive traits,  Incomplete dominance, or Co-dominance?

  • Is it a monohybrid or dihyrid?

  • In this case there is only one trait..........this is a monohybrid cross involving co-dominant traits.

STEP 2:  Determine letters you will use to specify traits.  

  • In this case it is a incomplete dominant problem.  TT=tall, Tt=medium, and tt=short

STEP 3:  Determine parents genotypes.  

  • In this case you were told the parents were heterozygous.  You therefore know that the parents must be BW and BW

  • The Cross is:  BW x BW

STEP 4:  Make your punnet square and make gametes (these go on the top and side of your punnett square.

  • BW would make a B and a W for both parents

STEP 5:  Complete cross and determine possible offspring.

STEP 6:  Determine genotypic and phenotypic ratios.

  • Genotypic ratio:  Make a list of all the different genotypes (the letter combinations) and determine how many of each you have.

    • In your problem this would be:  BB = 1,  BW = 2, and WW = 1.  The genotypic ratio would therefore be 1 : 2 : 1 

  • Phenotypic ratio:  Make a list of all the different phenotypes (physical characteristics.

    • In you problem this would be:  Black = 1, Tan = 2, and White = 1.  The phenotypic ratio would therefore be 1 : 2 : 1

 

Monohybrid (sex-linked)

PROBLEM:  Cross a female carrier for hemophilia with a male with hemophilia.  H=normal, and h=hemophilia

STEP 1:  Determine what kind of problem you are trying to solve.  

  • Does it involve simple dominant and recessive traits,  Incomplete dominance, or Co-dominance?

  • Is it a monohybrid or dihyrid?

  • In this case there is only one trait..........this is a monohybrid cross involving sex linked traits.

STEP 2:  Determine letters you will use to specify traits.  

  • In this case it is a sex-linked problem.  Remember that XX is female, and XY is male.  H=normal and h=hemophilia.  Normally you would not write the capital letters on the genotypes, only the small case (the recessive gene responsible for the disorder)

STEP 3:  Determine parents genotypes.  

  • In this case you were told the parents were: Female carrier = XhX, and a male with hemophilia = XhY.  

  • The Cross is:  XhX  x  XhY

STEP 4:  Make your punnet square and make gametes (these go on the top and side of your punnett square.

  • XhX  would make a Xh and X    

  • XhY would make a Xh and Y   

  • NOTE:  The females gametes always go on top of the punnett square and the males on the side. 

STEP 5:  Complete cross and determine possible offspring.

STEP 6:  Determine genotypic and phenotypic ratios.

  • Genotypic ratio:  Make a list of all the different genotypes (the letter combinations) and determine how many of each you have.

    • In your problem this would be:  XhX  = 1,  XhXh  = 1, and XY= 1, and XhY = 1 The genotypic ratio would therefore be 1 : 1 : 1 : 1 

  • Phenotypic ratio:  Make a list of all the different phenotypes (physical characteristics.

    • In you problem this would be:  Female Carrier = 1, Female w/ hemophilia = 1, Normal male = 1, and Male w/ hemophilia = 1.  

      The phenotypic ratio would be 1 : 1 : 1 : 1 

Monohybrid (Multiple Alleles)

PROBLEM:  Cross a person with type AB blood with a person who is heterozygous for type A blood.

STEP 1:  Determine what kind of problem you are trying to solve.  

  • Does it involve simple dominant and recessive traits,  Incomplete dominance, or Co-dominance, or multiple alleles?

  • Is it a monohybrid or dihyrid?

  • In this case there is only one trait..........this is a monohybrid cross involving multiple alleles

STEP 2:  Determine letters you will use to specify traits.  

  • There are more than two-choices for the allele.  Example is human blood group genes.  There are three possible alleles for this gene.  IA, IB, and i.  IA and IB are co-dominant.  There are four possible phenotypes: A, B, AB, and O.  

STEP 3:  Determine parents genotypes.  

  • In this case you were told one has type AB blood = IAIB.  The other is heterozygous for type A = IAi

STEP 4:  Make your punnet square and make gametes (these go on the top and side of your punnett square.

  • IAIB would make a IA and IB

  • IAi would make a IA and i  

STEP 5:  Complete cross and determine possible offspring.

STEP 6:  Determine genotypic and phenotypic ratios.

  • Genotypic ratio:  Make a list of all the different genotypes (the letter combinations) and determine how many of each you have.

    • In your problem this would be:  IAIA = 1,  IAIB= 1, and IAi = 1, and IBi = 1     The genotypic ratio would therefore be 1 : 1 : 1 : 1 

  • Phenotypic ratio:  Make a list of all the different phenotypes (physical characteristics.

    • In you problem this would be:  Type A = 2, Type AB = 1, and Type B = 1 

    • Therefore the phenotypic ratio is:   2 : 1 : 1 

 

Dihybrid (Dominant and Recessive)

PROBLEM:  Cross two heterozygous Tall Black cows.  Tall is dominant over short, and Black is dominant over white.  Give genotypic and phenotypic ratios of offspring.

STEP 1:  Determine what kind of problem you are trying to solve.  

  • Does it involve simple dominant and recessive traits,  Incomplete dominance, or Co-dominance?

  • Is it a monohybrid or dihyrid?

  • In this case there are two traits.........this is a dihybrid cross involving dominant and recessive traits.

STEP 2:  Determine letters you will use to specify traits.  

  • In this case there are two traits you will need letters for.  Use T = Tall, and t = short for one trait, and B = Black, and b = white for the second trait.  These are both dominant/recessive genes

STEP 3:  Determine parents genotypes.  

  • In this case you were told the parents were heterozygous for both traits.  You therefore know that the parents must be TtBb and TtBb 

  • The Cross is:  TtBb  X TtBb 

STEP 4:  Make your punnet square and make gametes (these go on the top and side of your punnett square.

  • Making gametes for a dihybrid cross requires you use FOIL (first-outside-inside-last)

  • TtBb would make four different gametes =  TB, Tb, tB, tb

STEP 5:  Complete cross and determine possible offspring.

STEP 6:  Determine genotypic and phenotypic ratios.

  • Genotypic ratio:  Make a list of all the different genotypes (the letter combinations) and determine how many of each you have.

    • The genotypic ratio would therefore be 1:2:2:1:4:1:2:3:1

  • Phenotypic ratio:  Make a list of all the different phenotypes (physical characteristics.

    • In you problem this would be:  Tall/Black = 9, and Tall/white = 3, short/Black = 3, and short/white = 1.  

    • The phenotypic ratio would therefore be 9:3:3:1

 

Dihybrid (Dominant and Recessive and Sex-linked)

PROBLEM:  Cross a  homozygous Tall female carrier for hemophilia with a short normal male.  Give genotypic and phenotypic ratios of offspring.

STEP 1:  Determine what kind of problem you are trying to solve.  

  • Does it involve simple dominant and recessive traits,  Incomplete dominance, or Co-dominance, or sex-linked?

  • Is it a monohybrid or dihyrid?

  • In this case there are two traits.........this is a dihybrid cross involving dominant and recessive traits and sex-linked trait for hemophilia.

STEP 2:  Determine letters you will use to specify traits.  

  • In this case there are two traits you will need letters for.  Use T = Tall, and t = short for one trait, and H =normal, and h = hemophilia for the second sex-linked trait.  These are both dominant/recessive genes

STEP 3:  Determine parents genotypes.  

  • In this case you were told the parents were heterozygous for both traits.  You therefore know that the parents must be XhXTT and XYtt 

  • The Cross is:  XhXTT X XYtt 

STEP 4:  Make your punnet square and make gametes (these go on the top and side of your punnett square.

  • Making gametes for a dihybrid cross requires you use FOIL (first-outside-inside-last)

  • XhXTT would make four different gametes =  XhT, XhT, XT, and XT

  • XYtt would make four different gametes =  Xt, Xt, Yt, and Yt

STEP 5:  Complete cross and determine possible offspring.  When you have a sex-linked trait, make sure you put the females gametes on top of the punnett square and the male's on the side.

STEP 6:  Determine genotypic and phenotypic ratios.

  • Genotypic ratio:  Make a list of all the different genotypes (the letter combinations) and determine how many of each you have.

    • The genotypic ratio would therefore be 4:4:4:4 or 1:1:1:1

  • Phenotypic ratio:  Make a list of all the different phenotypes (physical characteristics.

    • In you problem this would be:  Tall/female carrier = 4, Tall/normal female = 4, Tall/male w/ hemophilia = 4, and Tall/normal male = 4.

    • The phenotypic ratio would therefore be 4:4:4:4 or 1:1:1:1

 

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Website by Duncan Wilson

Page last updated January 07, 2012

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