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Mendelian Genetics, Punnet square.

  1. Dec 4, 2007 #1
    I started Genetics a couple of weeks ago and it is like a bajillion times harder than Honors Genetics Bio. I would appreciate if someone could clarify somethings and help give me an explanation on how the addition and multiplication rule works. Furthermore, my teacher says that during the AP test there won't be time for me to make a punnet square so I need to know how to find the amount of phenotype and/or genotype without making a punnet square. I can do the little ones like if it's Bs x Bs and B=Black hair theres going to be a 3/4 chance that the offspring receiving Black hair. I also know that codominance follow the ratio 1:2:1. But what can I do if it was a larger punnet square like AABBCCDDEE x aabbccddee? If it was a problem like BbSs x BbSS and we needed to find the number of offspring of F2 generation with a certain trait, my teacher would go something like (3/4) x (1/4) and get (3/16). How does he know that that certain trait is gonna produce 3/4 of the trait and other 1/4. Is there a pattern within the number of each punnet square or is it just something one has to remember?
  2. jcsd
  3. Dec 4, 2007 #2
    I think your teacher is applying the Binomial theorem. And the hypothetical question you posted is not going to be on a test. Anyway if you have something like Bs x Bs= B^2 2Bs s^2,which essentially is the Binomial theorem looked at closely. at least I think, it has been a while since I did genetics.
  4. Dec 5, 2007 #3
    If the traits aren't linked, then just separate them.

    That is, with BbSs x BbSS, if you want to know the offsprings' chance of being B phenotype and Ss genotype, just do:

    - B phenotype is 3/4 (you said you already know this)

    - Ss genotype is 1/2 (I assume you can do this)

    Together, we get (3/4)x(1/2).

    As for patterns, they must exist so long as you do AaBbCc x AaBbCc or AABBCC x aabbcc, etc. Do a few and it should be obvious eventually.
  5. Dec 7, 2007 #4


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    When not given time for writing out something like a Punnett square on an exam, the problems are usually kept sufficiently simple that you should have seen plenty of examples to know the answer without going through all the steps again. At most, they'll ask you to figure out a dihybrid, sex-linked cross.

    Same goes for the math you'll need to do without a calculator...they won't set you up with a problem that asks you to find the square root of 137.96, it'll be something like finding the square root of 100 or 144 (this is a good thing to keep in mind when taking a test like that...if the math gets too complex and you're not allowed a calculator, you've probably done something wrong to make it more complicated than it should be).
  6. Dec 19, 2007 #5
    Thanks everybody for responding.
    But for the multiplication rule and addition rule, is the only time one would use the multiplication rule is when you are trying to find the phenotype of a dihybrid and addition for the Phenotype of a monohybrid?

    In addition, for X inactivation, when does that apply? Is it only used for specific species like female calico cats? In what situations would one of the X chromosome shut down? Is this a result of epistasis?
  7. Dec 20, 2007 #6
    You use multiplication whenever you want to find the overall probability for event A AND event B. So if you broke down a dihybrid into 2 monohybrid crosses, then you could multiply results there. This is just the basics of probability, unrelated to biology.

    X-inactivation doesn't seem related, IMO. It happens in each cell (human females go through it too) with more than 1 X-chromosome. The X-chromosomes produce a substance that inactivates them. Which gets inactivated is somewhat mysterious -- it is semi-random. That is, it is basically a random choice in which gets inactivated in each cell, but if one is damaged or has some mutations, then it will be 'randomly' inactivated more often.
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