In Drosophila, trisomics and monosomics for chromosome 4 are viable. However, nullisomics and tetrasomics are not viable. The b locus is on this chromosome. The phenotypic ratios in the progeny of the following crosses are given:
a) Parents:
b + /b/b x b/b
Gametes:
1/6 b+/b
1/3 b/b
1/3 b+/b
1/6 b/b
The progeny of this cross will have a phenotypic ratio of 1:1 which can be written as 1 wild type: 1 b.
b) Parents:
b + /b +/b x b/b
Gametes:
1/6 b/b
1/3 b+
1/3 b+/b
1/6 b+/b+
The progeny of this cross will have a phenotypic ratio of 5:1 which can be written as 5 wild type: 1 b.
c) Parents:
b + /b + /b x b+/b
Gametes:
1/6 b
1/3 b+
1/3 b+/b
1/6 b+/b+
1/2 b
1/2 b+
The progeny of this cross will have a phenotypic ratio of 11:1 which can be written as 11 wild type: 1 b.