Mr. Kousen is    Water Man

Baby Steps Through the
PUNNETT SQUARE


(Get it? "Square" = nerd. Ha ha ha ha ha ...)

No this page is not a place to pick on those students who you will one day call "boss".  This is a place for some serious practice with a very useful tool for completing genetics problems, the Punnett Square (P-Square for short).
The basic naked p-square looks like a window pane :
Aaaah, reminds me of my bedroom window when I was a kid. The
very window I would peer out on snowy mornings and hope to myself:
"snow day, snow day, snow day".  But I digress....
When given enough info about two parent organisms, we can use this window pane to
predict the genotypes & phenotypes of their offspring. ExcIting, ain't it?


Very quick rehash (review):


Here are the basic steps to using a Punnett Square when solving a genetics question.  After you get good at this you should never miss a genetic question involving the cross of two organisms.
 
BABY STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a p-square
4. "split" the letters of the genotype for each parent & put them "outside" the p-square
5. determine the possible genotypes of the offspring by filling in the p-square
6. summarize results (genotypes & phenotypes of offspring)
7. bask in the glow of your accomplishment !
Step #1: Determine the genotypes of the parent organisms. Step #2: Write down your "cross" (mating).  Write the genotypes of the parents in the form of letters (ex: Tt x tt).

Step #3: Draw a p-square

Step #4: "Split" the letters of the genotype for each parent & put them "outside" the p-square.


 
 

Step #5: Determine the possible genotypes of the offspring by filling in the p-square.

Filling in the top-left box:

Filling in the bottom-left box:

Filling in the top-right box:

Filling in the bottom-right box:

One from the left, one from the top... one from the left, one from the top...one from the left, one from the top...one from the left, one from the top.
 
Step #6: Summarize the results (genotypes & phenotypes of offspring).


Step #7: Bask in the glow of your accomplishment !


A little scientific side-note:

You know how, in Step #4, when we "split" the letters of the genotype & put them outside the p-square?  What that step illustrates is the process of gametogenesis (the production of sex cells, egg & sperm).  Gametogenesis is a cell division thing (also called meiosis) that divides an organism's chromosome number in half.  For example, in humans, body cells have 46 chromosomes a piece.  However, when sperm or eggs are produced (by gametogenesis/meiosis) they get only 23 chromosomes each.  This makes sense (believe it or not), because now, when the sperm & egg fuse at fertilization, the new cell formed (called a zygote) will have 23 + 23 = 46 chromosomes.  Cool, huh?

So, when the chromosome number is split in half, all of the two letter genotypes for every trait of that person (or organism) get separated.  Which is why we do what we do in Step #4.


TAKE WHAT YOU'VE LEARNED & DAZZLE SOME PEOPLE.



 
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On to a Punnett Square Practice Page>


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