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Answer:
a. 3:1 (normal bristles to hairless)
b. 2:1 (normal bristles to hairless)
Explanation:
a. If H allele is present, the fly will be hairless. But H allele is lethal at homozygous condition, so all hairless flies will be heterozygous for H locus (Hh). S allele has no effect on bristle number, but in presence of H allele, S produces a "normal" phenotype and is also lethal at the homozygous condition. So genotypes and phenotypes will be like this:
- SSHH: No viable- Lethal
- SSHh: No viable-Lethal
- SShh: No viable-Lehtal
- SsHH: No viable-Lethal
- SsHh: Normal
- Sshh: Hairless
- ssHH: Normal
- ssHh: Normal
- sshh: Normal
A cross between two normal flies carrying the two alleles (SsHh) will produce (according to the attached Punnett Square):
6 Normal (SsHh, ssHh, and sshh)
2 Hairless (Sshh)
Therefore, the ratio of flies with normal bristles to hairless individuals will be 3:1.
b. When the hairless progeny of the previous cross (Sshh) are crossed with one of the parental normal flies from part a (SsHh), it will be produced (according to the second Punnett Square):
8 Normal (SsHh, ssHh, and sshh)
4 Hairless (Sshh)
So, the ratio of flies with normal bristles to hairless individuals will be 2:1.
