Question

Why is the correct answer b? I thought it would be a and c. To me, this looks like crossing over. Why is this a heteroduplex region? This is all the information that was given for the question.

What undoubtedly happened in the Neurospora octad to the right? a. Crossing over between gene a and the centromere b. Heteroduplex DNA formed at gene a c. Gene conversion d. a and c e. b and c EEEEEEEE IP

Answer 1

This is an example of a cross in which the spores are formed with the genes a⁺ and a^- in the end after the stages of the cell division.

In the first case, the octet is not getting formed and the crossing over is not taking place in this example of Neurospora crassa. There is formation of the spores derived from the parental spores after the stages of first and second meiotic division independently in the two different spores. The meiotic division, zygote formation and the process of mitosis leading to the formation of the spores is taking place separately in the two strains of Neurospora crassa. The diagram shows the location of the genes a⁺ and a^- entirely without crossing over in the derived daughter spores in the Neurospora crassa. The ratio of the progeny spores formed will be the same as given in the question diagram as as explained in the diagram below :

conjugation 8-centromere mutant normal (a ) oogote a Ula alla 1st meiotic division 2nd meiotic division mitosis OOOOOOOO

If the crossing over takes place at different junctions then it can be called as a single crossover and the double cross over leading to the generation of the recombinants. In this mechanism, the genes of the two strains of the Neurospora crassa undergo the process of crossing over along with the first and second segregation division leading to the formation of the daughter spores in the progeny. The genes are rearranged in the progeny cells due to the process of crossing over leading to the generation of the parental forms, non parental ditypes and the tetratypes in the progeny spores. The spores formed will be different in the progeny.

It is given below in the form of a diagram as follows:

conjugation normal (a ) zygotes mutant (a) Ugala ist meiotic division 2nd meiotic division au mitosis @ @@ @

Considering the diagram in the question the progeny formed are a⁺a⁺, a⁺a^-, a⁺a^-, a^-a^-, a^-a^-. Therefore the process of crossing over is taking place along with the first and second meiotic division as evident by the new recombinant spores formed in the Neurospora octet. The crossing over of the genes takes place due to the formation of the heteroduplex DNA with the recombination of the parental genes to form the new progeny strains consisting of new characteristic traits. Therefore initially the recombination and heteroduplex formation did not take place but it took place later on as evident by the formation of new spores from the parental strain of Neurospora crassa.