A Drosophila stock that is heterozygous null for a unique nuclear target gene was sib-mated. The target gene is essential for the development of Drosophila. The embryos from the cross were analyzed and the following results were obtained:

• PCR analysis of the genomic DNA isolated from embryos showed that 25% of the embryos did not have the target gene.
• Northern analysis of the RNA isolated from the above embryos showed the presence of transcript corresponding to the target gene.
• No lethality was observed in the progeny.

Which one of the following options can best explain the above observations?

Correct option is B

The key observation in the problem is that 25% of the embryos did not have the target gene in their genomic DNA, but Northern analysis showed the presence of the transcript corresponding to the target gene. This suggests that even though the gene is absent in the embryos' genomic DNA, the embryos are still receiving the gene’s transcripts.

This points to a case where the mRNA of the target gene is maternally contributed to the embryo. In many species, including Drosophila, certain genes are maternally deposited in the egg before fertilization, and these maternal transcripts are used by the embryo before its own genome is activated.

Information Booster:

1. Maternal inheritance is a form of gene expression where the maternal parent provides mRNA, proteins, and other factors that influence early embryonic development.
2. In Drosophila, maternal transcripts are critical for the early stages of development before the zygotic genome becomes transcriptionally active.
3. The absence of the target gene in 25% of embryos suggests homozygosity for the null allele, but the mRNA provided maternally allows development to proceed normally in these embryos.
4. Northern blot analysis helps detect RNA molecules, confirming that maternal mRNA is available in the embryos despite the absence of the gene in their DNA.
5. 25% non-viable embryos indicate Mendelian inheritance of the null allele, showing the recessive nature of the null allele and no lethality from the maternal supply of the gene's transcript.

Explanation of Other Options:

• (a) Transcripts of the target gene are paternally contributed: Incorrect — This is unlikely since paternal contribution would require the presence of the gene in the paternal genome, which in this case, is absent in 25% of the embryos.
• (c) The transcripts are observed due to mitochondrial inheritance: Incorrect — Mitochondrial inheritance typically involves the transmission of mitochondrial DNA and does not explain the presence of nuclear gene transcripts.
• (d) The transcripts are being detected from yeast that larvae eat: Incorrect — While external contamination could theoretically explain the presence of transcripts, this is highly unlikely in controlled lab conditions, where such contamination is avoided.