Meiosis and Orossing over


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Meiosis and Crossing over

Video: Refresh yourself on the process of meiosis and crossing over by watching Professor Anderson’s Meiosis Video

Video: Watch this video if you want to understand Linked Genes

Animation/Problem: Work through the recombination problems at DNA from the Beginning.

Textbook:  Read Chapter 7 (Principles of Genetics, Snustad & Simmons 2012)

Important Formulas

Problems

  1. Ebony (black body, b) and apterous (vestigial wings, v) are recessive traits, present on different chromosomes. A cross is performed between an ebony female with normal wings and an apterous male with normal body color.
  1. What are the genotypes of the parental flies? (10 points)
  1. List the genotype(s) and phenotype(s) you expect to see in the F1 offspring. Explain how you arrived at your answer. (10 points)

 

  1. A cross is performed between an ebony female fly and a sepia (brown eyes; s) male fly. Ebony and sepia are recessive traits. All of the F1 offspring are wild-type.
  1. What are the genotypes of the parental and F1 flies? (10 points)

The F1 flies are crossed, and the following results are obtained:

48 wild-type body color and eyes

26 ebony, wild-type eyes

24 wild-type body color, sepia

2 ebony, sepia

  1. Is this result expected for traits that are not linked? Explain. (10 points)
  1. Assuming that these traits are linked – and therefore inherited together – draw the arrangement and inheritance of the genes in the F1 and F2 flies by filling in the boxes in the figure below. The parental arrangement has been drawn for you. The lines represent individual chromosomes (remember that flies are diploid organisms). (15 points)
  2. Looking at your genotypes in the figure above, list the phenotypes of the F2 offspring (from left to right). (5 points)

 

  1. What is the recombinant genotype? Explain. (10 points)
  1. What is the frequency of recombination? You must show your work! (10 points)
  1. In maize, three recessive genes (z, xt, and cm) are linked on chromosome 3. A homozygous plant for the recessives is crossed with a wild-type plant. The F1 is crossed to get an F2 generation with the following results:

 

Genotype Number Observed
z xt cm 444
z+ xt cm 59
z+ xt cm+ 74
z+ xt+ cm 5
z  xt+ cm   60
z xt cm+ 6
z xt+ cm+ 65
z+ xt+ cm+ 440

What are the parental, single recombinant, and double recombinant genotypes? (10 points)

What is the order of the three genes on chromosome 3? Explain your answer. (10 points)

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