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Thread: Genetic exercise with two traits

  1. #1 Genetic exercise with two traits 
    Forum Freshman
    Join Date
    Jul 2009
    Posts
    63
    Could someone please take a look at this exercise and shed some light?

    Chromosome 1 has Rr trait
    Chromosome 2 has Yy trait
    Generation F1 has RY from Father, ry from Mother.
    Code:
    CELL
    1F	R	
    2F		Y
    1M	r	
    2M		y
    Starting meiosis, there are duplicate chromosomes (a and b)
    At Metaphase I, they're separated and matched by type (e.g. 1F at top with homologue 1M at bottom):
    Code:
    CELL
    1F.a	R	
    1F.b	R	
    2F.a		Y
    2F.b		Y
    -------------------------
    1M.a	r	
    1M.b	r	
    2M.a		y
    2M.b		y
    At Telophase I, the cell divides:
    Code:
    CELL 1
    1F.a	R	
    1F.b	R	
    2F.a		Y
    2F.b		Y
    
    CELL 2
    1M.a	r	
    1M.b	r	
    2M.a		y
    2M.b		y
    At Metaphase II, they're aligned at midpoint; then chromatids separate (a from b) in Anaphase II:
    Code:
    CELL 1
    1F.a	R	
    2F.a		Y
    -------------------------
    1F.b	R	
    2F.b		Y
    
    CELL 2		
    1M.a	r	
    2M.a		y
    -------------------------
    1M.b	r	
    2M.b		y
    At Telophase II, the cells divide:
    Code:
    CELL 1
    1F.a	R	
    2F.a		Y
    
    CELL 2
    1F.b	R	
    2F.b		Y
    
    CELL 3		
    1M.a	r	
    2M.a		y
    
    CELL 4
    1M.b	r	
    2M.b		y
    Each cell brings those traits (RY, RY, ry or ry) to another germ cell.

    However, the "RY, Ry, rY, ry" distribution is needed for the 9-3-3-1 pattern.
    See Dihybrid Crosses at:
    http://www.biologie.uni-hamburg.de/b.../u4aos1p4.html

    What's wrong here?

    (edit 11/21: add url and other details)


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  3. #2  
    Forum Professor Zwirko's Avatar
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    You've forgotten Mendel’s second law - independent assortment.


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  4. #3  
    Forum Freshman
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    Thanks. I reviewed the situation and think I understand it better.
    I made an Excel spreadsheet to provide a model.

    Following is a revision of my exercise of two traits.
    This is a relatively simple example and the genes are on separate chromosomes.

    Chromosome 1 has the R gene.
    Chromosome 2 has the Y gene.
    Generation F1 has RY from Father, ry from Mother.
    1F-R
    2F-Y
    1M-r
    2M-y

    Starting meiosis, there are duplicate chromosomes (a and b chromatids)

    At Metaphase I, matching homologs are found and aligned together (e.g. 1 to 1, 2 to 2).
    A random directive is given to enable about half the homologs face one pole, the others face the other pole. So "Father" chromosomes are not always on one side.
    That's a neat trick, and it enables the independent assortment.

    Code:
    About half with this alignment:   The other half with this alignment:
    1F.a-R    2F.a-Y                  1F.a-R    2M.a-y         
    1F.b-R    2F.b-Y                  1F.b-R    2M.b-y
    ----------------                  ----------------  
    1M.a-r    2M.a-y                  1M.a-r    2F.a-Y
    1M.b-r    2M.b-y                  1M.b-r    2F.b-Y
    At Telophase I, the cell divides:
    Code:
    CELL 1 
    1F.a-R    2F.a-Y                  1F.a-R    2M.a-y         
    1F.b-R    2F.b-Y                  1F.b-R    2M.b-y
    
    CELL 2  
    1M.a-r    2M.a-y                  1M.a-r    2F.a-Y
    1M.b-r    2M.b-y                  1M.b-r    2F.b-Y

    At Metaphase II, they're aligned at midpoint; then chromatids separate (a from b) in Anaphase II:
    Code:
    CELL 1 
    1F.a-R    2F.a-Y                  1F.a-R    2M.a-y         
    ----------------                  ----------------  
    1F.b-R    2F.b-Y                  1F.b-R    2M.b-y
    
    
    CELL 2  
    1M.a-r    2M.a-y                  1M.a-r    2F.a-Y
    ----------------                  ----------------  
    1M.b-r    2M.b-y                  1M.b-r    2F.b-Y

    At Telophase II, the cells divide:
    Code:
    CELL 1 
    1F-R    2F-Y                      1F-R    2M-y         
    
    CELL 2
    1F-R    2F-Y                      1F-R    2M-y
    
    CELL 3  
    1M-r    2M-y                      1M-r    2F-Y
    
    CELL 4
    1M-r    2M-y                      1M-r    2F-Y
    So, if only two cells did meiosis, with each having a different alignment in Metaphase I, it results in:
    Four germ cells (RY, RY, ry, and ry) from the first alignment; four germ cells (Ry, Ry, rY, and rY) from the second.
    With my sample lot of 1000, each set (e.g. RY) comes out about the same - the independent assortment.
    One run had 238, 260, 245, and 257.

    Then I mated these randomly and generated the 9-3-3-1 pattern.
    One run had RY:569, rY:185, Ry:205, ry:62.

    Excel note: "RY" equals "ry" (it's not case sensitive), so I added a number to make the values unique (e.g. RY1, rY4).

    I hope this information may help others understand as it did for me.
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