Tag Archives: crossing over

Practice Problem


Crossover events occurring between two homologous chromosomes 2003 Pearson Education Inc.

Students in my General Biology class can work through this problem in place of one of your extra credit essays. The catch is that this work must be finished completely and presented clearly by Tuesday Nov. 27 no later than 12pm (hard deadline). I can answer questions either here online (post as comments) or in class on Tuesday Nov. 20. However, I will not take more than a couple minutes of class time to discuss this. If you need more time, please schedule that with me. Also, feel free to cooperate with other students.

You are a student examining four genes in humans about which you have collected a large amount of data that you need to analyze. Some of the work has been done for you by previous students in the lab, some of it you will need to work out yourself.

The genes and alleles are described below:

A- Club foot      /        a- normal foot

B- Baldness      /        b- normal hair

C- Small Ears   /         c- Large ears

D- Polydactyl  /         d- five fingered

You have a pool of data from the following crosses:

Data Set #1


P          AABB (Club foot & Bald)  x aabb (Normal foot & Normal hair)

F1        100% AaBb (Club foot& Bald)

F1        AaBb  x  AaBb

F2        1040 Club foot & Bald

100 Club food & Normal hair

160 Normal foot & Bald

1100 Normal Foot & Normal hair

Data Set #2

P          AACC (Club Foot & Large Ears)  x aacc (Normal foot & Small Ears)

F1        100% AaCc (Club foot& Large Ears)

F1        AaCc  x  AaCc

F2        900 Club foot & Small Ears

90 Club food & Large Ears

110 Normal foot & Small Ears

900 Normal Foot & Large Ears

A previous student already computed the recombination frequency between the D locus  (Polydactl) and the B locus (for Baldness) as 18%.

A second student computed the recombination frequency between the C locus (for ear size) and the B locus (for Baldness) as 20.8%

1. Compute the recombination frequencies for the two data sets you have (showing your work)

2. Draw a gene map for the four loci to the best approximation. Be sure to include distances in centi-Morgans between each locus.

3. You will not be able to pinpoint one gene’s locus exactly. Tell me what experiment(s) you would have to do and provide sample data that would enable you to pinpoint this last locus.

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Posted by on November 15, 2012 in Uncategorized


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Crossover events in Prophase I of Meiosis

There are two mechanisms for establishing diversity amongst progeny that are employed in meiosis. The first is independent assortment, which is the random distribution of chromosomes into sex cells, the second is crossover events that randomize DNA between paired homologous chromosomes.

Recall that in Prophase I pairs of chromosomes come together. This will ensure that when the chromosomes are distributed between the two daughter cells produced in Meiosis I each cell gets one of each pair. The pairing forms a structure known as a tetrad (referring to the four chromatids of the paired chromosomes). When the chromosomes are joined in this way it is possible (indeed likely) that there will be breaks that occur and swapping of genetic material from one chromatid to another.

Here’s a good (but dry) animation of this process presented by McGraw Hill:

Thomas Hunt Morgan ran one of the first labs studying crossover events (he was looking at fruit flies). We’ll be looking at his work in my class later in the semester, but here’s a preview that might help visualize what is happening using Morgan’s own sketches. This is the same process as described above, however Morgan only drew one chromatid for each chromosome – this does make the illustration simpler, but keep in mind that there are two chromatids present in the chromosomes of organisms during this stage (Prophase I)

Top – paired chromosomes prior to crossover; Middle – crossover occurring; Bottom – Chromosomes following crossover

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Posted by on October 16, 2012 in Education


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