We start have a strain that is homozygous for a recessive mutation (m) that give a phenotype M and that the heterozygote of m and a deletion (âˆ†) is also M.

Genotype Phenotype
m/m M
m/âˆ† M

The second line above shows that the absence of the gene (a null) with m will give the M phenotype.

Thus, even if other mutations in this gene (m1, m2, m3, etc.) give lethality if homozygous, they will only produce the M phenotype with the original m mutation (m1/m, m2/m, m3/m, etc.).

So to get these additional mutations, we need obtain additional noncomple-menting alleles. We can do this by mutating â™‚â™‚ (with EMS) and mating them to âš¥âš¥ that have the original m mutation. Because we will want to keep track of the chromosomes, the m-containing chromosome in the âš¥âš¥ also carries recessive mutations in flanking genes (e.g., unc m dpy).

If a mutageneized male has a new mutation (e.g., m1), the cross progeny will have the M phenotype and the unc m dpy/+ m1 + genotype.

Self-progeny from this unc m dpy/+ m1 + will be:

Genotype Phenotype

unc m dpy/unc m dpy M
unc m dpy/+ m1 + M
+ m1 +/+ m1 + ?

If the m1/m1 + animals (and similarly derived m2/m2, m3/m3 etc.) have the M phenotype, then the null phenotype is M.

If the m1/m1 + animals (and m2/m2, m3/m3 etc.) have a phenotype that is not M, then that is the null phenotype.

We start have a strain that is homozygous for a recessive mutation...