Question

Paragraph Styles UTUCI.larse 4. The number of expected double crossing over events is always equal to the number of observed double cross overs.False Long Answers (32 marks)-Please include ALL your work for the problems you solve. 1. Prove that crossing overdoes not happen before the S phase but happens at the four- chromatid stage? Draw a diagram (4 marks) How do you calculate interference? What does it mean? (3 marks) 3 Mantharo lagi anda ll

Answer 1

3)False.

The expected double cross over and observed double cross over are not same.

Observed double cross over are the double cross over progenies obtained in the cross where as expectd double cross over is calculated by ( SCO1× SCO2) × total progenies.

Here SCO refers to the single cross over progenies.

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1)

The crossing over event does not occur before S phase. Because there is required two homologous chromosome event and recombination nodule, chiasmata should be there. We know that S phase leads to replication of the DNA molecule.

Then condensation ocuurs in the chromatin material. At prophase 1 of meiosis, there is found synapsis of the homologous chromsomes. After that the X shaped chiasmata is formed. Then there is found recombination between the non sister chromatids of homologus chromosmes. All this events occurs by special protein which is present in prophase 1 of meiosis 1. That's why the crossing over event does not take place before S phase.

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Answer 3)

The interference is calculated by:

1- coefficient of Coincidence

Coefficient of coincidence = Oberved double cross over/ expected double cross over.

And expected double cross over = (SCO1×SCO2)×total progenies.

That's why

Interference= 1- { observed DCO/ ( SCO1×SCO2× total progenies)}

Interference telle us about the interfernce to one single cross over because of second cross over occurs on the adjacent segment on the chromosomes. Simply, we can say that interfernce is the hinderance in the single cross over due to the two simultaneous cross over occurs on the chromosomes.

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