A Cross Was Made Between Two Neurospora Strains with Genotypes $$+ b ; + b ; + + ; + + ; a b ; a b ; a + ; a +$$

question 47

Essay

A cross was made between two Neurospora strains with genotypes + b and a +.The genes are linked.The genotypes of the spores in one ascus from this cross are listed in order,from left to right:
$$+ b ; + b ; + + ; + + ; a b ; a b ; a + ; a +$$
a)Is the ascus a parental ditype (PD),a tetratype (T),or a nonparental ditype (NPD)?
b)Do the alleles of the a gene show first or second division segregation?
c)Do the alleles of the b gene show first or second division segregation?
d)Show how the above tetrad of spore pairs was derived by diagramming the chromosomes at pachytene of meiosis I.Indicate with a line between appropriate chromatids,the simplest crossover that can yield this tetrad.
e)If recombination between a and b is 10% and 2/100 asci are NPD,how many tetratype asci would you expect to find? How many PD?
f)Diagram how a tetrad with the same spore pairs,as shown above,would arise if the genes showed independent assortment.

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Definitions:

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