Read the pages indicated by G-12 and  study behavioral objective 26. Crossing over is a process where homologous pair of chromosomes can swap pieces of DNA (genes) with one another before meiosis  is completed. This allows for gene variability of linked genes that can be passed on to the offspring. Genes on the same chromosome would be permanently linked if it were not for crossing over. This phenomena  occurs only during prophase I of meiosis during synapsis and tetrad formation. You may want to go back to the previous lesson and review synapsis and tetrads. Click here to review synapsis.  click 

Figure represent  synapsis of one  homologous pair. 

Study the figure above.  "A" genes are linked to "b"  genes and "a" are linked to "B" genes.  Since these genes are linked during the transfer of hereditary material the phenotype corresponding to "A" will always be associated with the phenotype corresponding with "b".  Example: In humans the gene which controls facial freckles is linked to the gene for red hair.  Therefore most individuals with red hair have facial freckles.  Crossing over allows a means for the gene expressing the red hair phenotype to be on the same chromosome as the gene which does not produce facial freckles. 
 

1. During which phase of the cell cycle does crossing over occur? 

      a) prophase of mitosis                           d) prophase II of meiosis 
      b) anaphase I of meiosis                        e) both a and c 
      c) prophase I of meiosis 

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2. In the figure below, which genes would be linked together in the male or female gametes if their was no crossing over? 
        a) "A" and "b"        d)  "a" and "b" 
        b) "A" and "B"        e) both answer a and c 
        c) "a" and "B" 
  
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Read in the text  the section G-12  concerning crossing over and study the figure below. This figure shows the consequence of crossing over of the homologous chromosome pair above. Write in your notes a description of crossing and identify a chiasma which is indicated by "*"  on  the figure below. 
 
 
 
 
 
 

Examine the figure below and and answer the following question. 

3. In the figure above which  genes would be linked after crossing over. 

        a) "A" and "b"         d)  "a" and "b" 
        b) "A" and "B"        e) both answer b and d 
        c) "a"  and "B" 
 
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Note that only the inside chromatids crossed over; therefore, the outside chromatids could form haploid cells with no crossovers.  The four types of  haploid cells that may occur during meiosis are: (A linked to b, a linked to B) non crossovers and (A linked to B, and a linked to b) crossovers. In you notes draw these figures to show how these four chromosomes are formed. Identify the chiasma of the figure in your notes. 
 

Read pages indicated by G-13  Draw in your notes a figure  comparing  the prophase, metaphase, and anaphase stages of mitosis and meiosis. Study behavioral objective 24  and answer the following questions. 

 
4. Which of the  following  statements  is true when comparing the anaphase stage of mitosis, meiosis I, and meiosis II? 

a) During mitosis and meiosis I sister chromatids split and are pulled to opposite poles; whereas, during meiosis II homologous chromosomes are pulled to opposite poles. 

b) During mitosis and meiosis II sister chromatids split and are pulled to opposite poles; whereas, during meiosis I homologous chromosomes are pulled to opposite poles. 

c) During meiosis II sister chromatids split and are pulled to opposite poles; whereas, during mitosis and meiosis I homologous chromosomes are pulled to opposite poles. 

d) During mitosis sister chromatids split and are pulled to opposite poles; whereas, during meiosis I and meiosis II homologous chromosomes are pulled to opposite poles. 

e) During meiosis I  and meiosis II sister chromatids split and are pulled to opposite poles; whereas, during mitosis  homologous chromosomes are pulled to opposite poles. 
 
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5. Can a cell with a diploid complement of 24 chromosomes undergo mitosis?  If the answer is yes how many chromosomes will be in the daughter cells? 

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6. Can a cell with a haploid  complement of 12 chromosomes undergo mitosis?  If the answer is yes how many chromosomes will be in the daughter cells? 

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7. Can a cell with a diploid complement of 24 chromosomes undergo meiosis?  If the answer is yes how many chromosomes will be in the daughter cells? 

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8. Can a cell with a haploid complement of 12 chromosomes undergo meiosis?  If the answer is yes how many chromosomes will be in the daughter cells? 
 
 
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Questions 9 and 10.  A diploid somatic cell contains a total of  six chromosomes. 
 
9. How many homologous pair of  chromosomes are present in this cell ? 

         a) 0      b).   3      c) 6      d). 12 

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10.   How many "different"  sets of chromosomes  in each cell may be produced when this cell (diploid somatic cell with six chromosomes or three pair) undergoes  meiosis? (Hint:  Draw three pair of chromosomes with different genes. The homologous pair should have the same letter with one containing  an upper case and the other a lower case. (ex. Aa, Bb, and Cc). How many ways can you arrange these into haploid cells so that each cell contains one of each letter? (ex. ABC,ABc,etc.). 

         a) 2      b)  4      c)  8   d) 16 
 
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11. Review question: 

If  the dominate alleles R and S are linked on on chromosome and the alleles r and s are linked on its homologure, determine the types of gametes that may be fomrred. Include the cross overs and noncross overs. 

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Click here to relate meiosis to Sexual Reproduction Strategies  in  Humans.
...click here
 
     
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

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