Chapter 18, Problem 1NST

Some mutations in the tra gene of Drosophila cause XX individuals to appear as males. In contrast, other mutations in tra cause XY flies to appear as females. Which of these phenotypes would you expect if a mutation in tra resulted in a null allele? Which would you expect if the mutation produced a constitutively active tra gene product?

HINT: This problem involves an understanding of the Sxl regulatory pathway in Drosophila. The key to its solution is to review the functions of the normal TRA protein in Drosophila males and females.

Summary Introduction

To describe: The phenotype that would be expected if a mutation in tra gene results into null allele.

Introduction: The sex of Drosophila melanogaster is determined by the ratio of sex chromosomes to autosomes. The male Drosophila melanogaster is produced when the ratio of sex chromosomes to autosomes is 0.5. However, the female Drosophila melanogaster is produced when the ratio is 1.0.

Explanation of Solution

The tra gene is present in both male and female Drosophila melanogaster. The activity of tra gene is controlled by Sxl gene. In the absence of Sxl gene, the tra gene is spliced in a way that stop codon “UGA” remains present in the mature mRNA. This is observed in male Drosophila melanogaster as they lack the Sxl gene. The translation of mRNA with stop codon leads to the formation of a non-functional protein. However, in female Drosophila melanogaster, the Sxl gene promotes the removal of “UGA” from mRNA, and this forms a functional tra protein.

The mutation in the null allele leads to the formation of male Drosophila melanogaster. This is because the null gene has no effect on the Sxl gene. It does not activate the Sxl gene. The inactivated Sxl gene cannot remove the UGA gene from mRNA, and this leads to the formation of male Drosophila.

Conclusion

Thus, male Drosophila melanogaster would be expected if a mutation in the tra gene occurs in null allele.

Summary Introduction

To determine: The phenotype that would be expected if mutation produces a constitutively active tra gene.

Introduction: The ratio of sex chromosomes to autosomes is termed as chromosomal ratio. The chromosomal ratio is analyzed by three genes. These are sex-lethal (Sxl), transformer (tra), and doublesex (dsx). The activity of tra gene plays a crucial role in determining the sex of Drosophila.

Explanation of Solution

The activity of tra gene depends upon the Sxl gene. The active tra gene indicates that there is Sxl gene. The tra gene in the presence of the Sxl gene is able to remove the stop codon “UGA” from the mRNA. It produces a function tra protein. The active tra protein is found in female Drosophila. However, the male Drosophila has inactive tra protein. This indicates that female Drosophila would be expected if the mutation produces a constitutively active tra gene.

Conclusion

Thus, the phenotype that would be expected if mutation produces a constitutively active tra gene is a female Drosophila melanogaster.