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Genetics problem

  1. Oct 15, 2007 #1
    The question:

    In dogs, dark coat colour is dominant over albino, and short hair is dominant over long hair. If these effects are caused by two independently segregating gene pairs, write the most probable genotypes for the parents of each of the following crosses: (Use the symbols D and d for dark and albino coat colour alleles, and S and s for the short- and long-hair alleles, respectively.)

    Parental Phenotypes Phenotypes of the Offspring
    Dark, Short Dark, Long Albino, Short Albino, Long
    a) dark short x dark short 89 31 29 11
    b) dark short x dark long 18 19 0 0
    c) albino short x albino short 0 0 28 9
    d) dark long x dark long 0 32 0 10
    e) dark short x dark short 46 16 0 0
    f) dark short x dark long 29 31 9 11


    Attempt at solution:

    I'm trying to write out the genotypes but there are so many possibilities. I know I can narrow it down using the data provided but I don't know what to look for. For example for letters (a) and (e):

    dark short: DDSS DDSs DdSS DdSs
    dark short x dark short (crossing possibilities):
    DDSS x DDSS DDSs x DDSs DdSS x DdSS DdSs x DdSs
    DDSS x DDSs DDSs x DdSS DdSS x DdSs
    DDSS x DdSS DDSs x DdSs
    DDSS x DdSs

    Is this correct? If so, how would I go about deciding which ones would best fit the outcomes given?

    I know how to find the genotypes given the crossing but I don't think I need to find the genotypes of all possible crosses to find the answer. Any suggestions?

    *Sorry about the table... it looks fine when I write it here but in the post it's all scrunched up together. Don't know what to do about it.
     
  2. jcsd
  3. Oct 18, 2007 #2

    Moonbear

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    Okay, the numbers of each phenotype in the offspring is the key to answering this question. If you recall something about the probability of each outcome with different genotypes crossed, you can start working backward to determine the genotypes of the parents.

    The easiest ones to start with are the ones with some zeros. These help tell you something about alleles absent in one or both parents. For example, in (b), there are no albino offspring. What does this tell you about the genotypes of the phenotypically dark-haired parents?

    (In the meantime, I'm going to see if I can find someone to help us both learn how to format tables here! At least I can read it correctly when quoting.)
     
  4. Oct 18, 2007 #3

    Moonbear

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    Okay, I goofed around the formatting, so now it'll be easier to refer to the table when working on this. I hope there's an easier way than I used, but it at least works for now. :smile:

    Parental Phenotype....................................Phenotypes of the Offspring
    ......................................Dark, Short.........Dark, Long ......... Albino, Short......... Albino, Long
    a) dark short x dark short......... 89 ..................31....................... 29....................... 11
    b) dark short x dark long.......... 18.................. 19..........................0.........................0
    c) albino short x albino short ..... 0.................... 0........................28........................ 9
    d) dark long x dark long ............ 0.................. 32..........................0........................10
    e) dark short x dark short ........ 46.................. 16..........................0.........................0
    f) dark short x dark long .......... 29.................. 31......................... 9....................... 11
     
  5. Oct 18, 2007 #4

    Gokul43201

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    Looking for something like this, perhaps?

    Code (Text):
         Parental Phenotype                 Phenotypes of the Offspring

                                  Dark,Short  Dark,Long  Albino,Short  Albino,Long

    a) dark short x dark short        89           31          29           11
    b) dark short x dark long         18           19          0            0
    c) albino short x albino short    0            0           28           9
    d) dark long x dark long          0            32          0            10
    e) dark short x dark short        46           16          0            0
    f) dark short x dark long         29           31          9            11
    Too late, eh?
     
  6. Oct 19, 2007 #5

    Moonbear

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    Dang! Too easy. :biggrin: Now, after I put in all those dots, where did the OP disappear to?
     
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