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2 Factor Cross on Punnett Squares

If you follow the necessary steps in solving genetic problems with Punnett Squares, life will be a breeze for you.  It is important that you follow the necessary steps!

First you have to establish your parental cross, or P1.

P1  ________________________X_________________________

Next you need to make a 16 square Punnett Square for your 2 traits you want to cross.

The next step is to determine the genotypes of the two parents and assign them letters to represent the alleles.  Example:  In pigs, curly tails (C) are dominant over straight tails (c).  Pink color (P) is dominate over gray (p).  You are to cross a male pig that is homozygous dominate for both traits with a female pig that is homozygous recessive for type of tail and heterozygous for body color.  Let's look at how we will assign the letters to represent the alleles and fill in the P1 cross.

P1    CCPP  X   ccPp

Since the male piggy is homozygous dominate for both traits, the letters assigned for both traits must be capital letters.  Remember, it takes 2 letters (alleles) to represent each trait.  For the female pig she is homozygous recessive for type of tail (2 lowercase letters) and heterozygous for color (One capital and one lowercase letter.  Follow the directions in the next step to prepare how you will take the possbile combinations of alleles for each parent to place them on your punnett square.

As you can see, the possible combinations of alleles for the male pig are:  CP, CP, CP, and CP.  The possible combinations for the female pig are: cP,cp,cP, and cp.  Now you are ready to move them down to start your

punnett square.  Align each combination of alleles for each parent along the side and top of the 16 squares.  Take the letters that correspond with each box and bring the letters into the boxes.  Remember, Capital letters always go in front of lowercase letters since they dominate over them.  Keep same letters together to represent the trait assigned for each of the 16 offspring.

GENOTYPES:  CcPP and CcPp               GENOTYPIC RATIO:  8:8


The results of this punnett square indicates that with the mating of two pigs with the above genotypes, the 16 possible offspring would be 1/2 CcPP and 1/2 CcPp as their genotypes, and all fo the offspring of the F1 (first family) would be 16 that are curly and pink.  We have no piggy babies that would have gray color or have straight tails.

Now do the following crosses on your own to practice 2 factor punnett squares.  Use the example above to help you solve the problems.  Do your P1 cross, punnett square, come up with your genotypes, phenotypes, and the ratios for both. Do not check out the answers until you do your problem.

Problem 1:  In humans, brown hair is dominate over blonde hair.  Unattached earlobes are dominate over attached.  Cross a male that is heterozygous for both traits with a female that is heterozygous for both traits.  (This is called a dihybrid cross).  This one will be one of the hardest crosses to accomplish correctly.

Problem 2:  Walrus have tusks and that is what most of them have.Walrus without tusks are very rare in the world.  Thick fur occurs more often than being bald on their bodies.  You are to cross a male walrus that is homozygous dominate for tusks and heterozygous for body fur with a female that is has no tusks and is bald! (A real cutie in the animal world).
Problem 3:  In pea plants, green peas are dominate over yellow peas.  Tall plants occur more often than short ones.  Cross one pea plant that is homozygous recessive or both traits with another plant that is homozygous recessive for color of pea and homozygous dominate for height.

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