Question

You decide to cross the reciprocal translocation strain to a pure (black, sepia) line to generate female F₁ flies that are both translocation heterozygotes and BbSs dihybrids.

You then backcross these F₁ females to males from the pure (black, sepia) line. This diagram shows synapsis in the F₁ females. (In the diagram, NII = normal chromosome II; TII = translocated chromosome with the chromosome II centromere; NIII = normal chromosome III; TIII = translocated chromosome with the chromosome III centromere.)


You wish to recover several categories of F₂ flies. What event must occur to produce each category of F₂ flies listed in the table below?

Complete the table by dragging the labels to the correct locations. Labels can be used once, more than once, or not at all. (Note: CO = crossover; NCO = no crossover.)

B S TIII NIII s b a 특 b T

Answer 1

Here Pure Black is represented by BB, Pure Sepia by SS:

F2 Progeney to be black has to be either BB or Bb or bb or bB;

and to be sepia it has to be with allele SS or Ss or ss or sS

1. So as per case I in 1st row of the table

the F2 Progeny is black, semi-sterile: So it will be Gamete from Dihybrid:

B allele on TIII with region "a" NCo

s allele on TIII with region "b" Co

ie. 3 rd choice from list.

Reason as translocation alelles are semifertile and black being dominant as "B". So the above choice will give F2 progeny as black, semi-fertile.

2. For 2nd row:

the F2 progeny is sepia, fully fertile, so it will give gamete from hybrid:

b allele on NII with region "a" NCo and

S allele on NIII with region "b" Co

i.e 6th choice from list.

Reason as normal alleel are fully fertile and "S" being dominant for sepia trait. So the above choice will give F2 progeny as spia fully fertile.

3. for 3rd row:

the F2 progeny coulb be (black, sepia), semifertile

So the choice will be as;

b allele on TIII and "a" as Co

s allele on TIII and "b" as NCo

i.e 1st choic from the list.

Reason, as both being non dominant and semifertile so translocation alleles will suffice and both are "b" and "s" are recessive traits. so that will give heterozygous black sepia with semi-fertile trait.

NOTE: Co and NCo are dependent upon the figure positions of the alleles hence could be interpretated from the regions as mentioned whether to crosover or not.