Question

Two different female Drosophila were isolated, each heterozygous for the autosomally linked genes b (black body), d (dachs tarsus), and c (curved wings). These genes are in the order d–b–c, with b being closer to d than to c. Shown here is the genotypic arrangement for each female along with the various gametes formed by both: Female A Female B db+++c d+++bc ? Gamete formation ? (1) d b c (5) d + + (1) d b + (5) d b c (2) + + + (6) + b c (2) + + c (6) + + + (3) + + c (7) d + c (3) d + c (7) d + + (4) d b + (8) + b + (4) + b + (8) + b c

Now rank the frequency of each class of gametes from least frequent to most frequent.

noncrossover

double crossover

single crossover from b to c

single crossover from d to b

Answer 1

Female A: db+/++c
The cross over can take place between any of these genes. Hence we can get recombinants and non recombinants (parental).
Noncross over = db+ and ++c
double cross over = d+c and   +b+
single cross from b to c = dbc and +++
single crossfrom d to b = d+c and   +b+

Female B: d++ / +bc
Noncross over = d++ and +bc
double cross over = db+ and ++c
single cross from b to c = d+c and   +b+
single crossfrom d to b = dbc and +++

since the genes are linked, we get Noncrosover most of the time. The least occurance is double cross over. The gene b and d are closer than d and c. Hence cross between b and d occur less compared to d and c.
So order :
Double cross over < single cross b to d < single cross b to c < non recombinant.