Question

1. Prove that crossing overdoes not happen before the Sphase but happens at the four- chromatid stage? Draw a diagram (4 marks) 2. How do you calculate interference? What does it mean? (3 marks)

Answer 1

ANSWERS ") Crossine over happens after chromosomal duplication during 's' phase ie, it happens during the Preophase - I of the meiotic division . . Let us consider the inheritance of a genes with alleles "A-a" and "Bob" respectively. Case - 1: Had the crossing over occurred before s-phase Let the maternal and paternal chromosome be represented in reed and blue olowe respectively, with alleles fore two genes beated at the specific locus . I crossing avere creassover products. case, only 2 types of crossover In this formed . products are

Case - I (Crossing over taking place after 's phase). During s phase, duplication of the chromosomal al takes place. Hence, a pair of chromosomes hos '4 chromatids - known as te tread The crossing over in a telread can be represented as follows i X X - I crossing over HA < CROSS OVer producto In this case, 4 types of crossover products ie a recombinanti & p parental fapes acé ablained, which has been observed and proved to be true through various experiments.

*) Interference : Cressing over does not occur uniformly along a chromosome and also the foremation of chiasma reduces the probability of another chiasma formation in an immediately adjacent region of the chemesome. This U lack of . independance is known as interference The net result of interference is the observation of fewer double crossover types. Interference varies in different sections of the cremesome and is measweed by the efficient of coincidence ... Interference = 1 - Coufficiant of coincidence. - If the coefficient of coincidence es, o then interference is complete with no double crossover between 0 and 1 → indicates partial interference, • 1 then there is no interference & expected double crossovers are observed. Wheree, coefficient the number of the number of of coincidence is the reatio between observed double crossovers and expected double crossoveres.