Why are three-point crosses used for mapping genes? What are their advantages ov

Question

Why are three-point crosses used for mapping genes? What are their advantages over two-point crosses? Using example counts of progeny, describe in detail all the steps required to generate and analyze a three-factored cross, and explain the logic connecting them. Underline the following terms in your answer: interference, trihybrid testcross, double recombinant, and centimorgan. Draw one figure supporting your answer. You are required to include a table of phased progeny genotypes and the final genetic map.

Explanation / Answer

The three point test cross are mostly use for mapping genes, because by this one can determine the distance between three genes more accurately and say these genes whether linked or unlinked based on their recombination frequency. For example, consider following data for three point cross:

The cross between AABB with aabb, give F1 AaBb. Now this F1 progeny is crossed with fully homozygous recessive parent, so the cross is: AaBb× aabb gives following progeny:

Form the above data, if we see the Double recombinants we can say that B/b locus lies between the A/a locus and the C/c locus.

Recombination frequency = (recombinants)/total progeny

The gene distance between the A and B genes is: 23 + 152 + 148 + 27 = (350/1000)*100 = 35 centimorgan (c.m)

The gene distance between the B and C genes is: 81 + 23 + 27 + 89 = 220 = (220/1000)*100 = 22 centimorgan (c.m)

The genetic map is:

----A-------B------C--

     35 c.m+22 c.m  

So, the total gene distance is: 57 c.m

For the double recombinant data, the expected rate of double recombination is (350 / 1000) * (220 / 1000) = 0.077 or 77. The actual double recombinants are 50, so 50/70 = 0.65 is called coefficient of coincidence (c.o.c.).

The interference is: 1 c.o.c.

So, 1-0.65 = 0.35

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