A male with singed bristles (sn) appeared in a culture of Drosophila. How would you determine if this unusual phenotype was due to an X-linked mutation? Can you show the crosses that you would do to determine this?

To determine if the singed bristles phenotype in a male Drosophila is due to an X-linked mutation, you need to perform a series of controlled crosses. Here is how you can approach it:

1. Select a male with singed bristles (sn) as the test subject and cross it with a Drosophila female with wild-type bristles (Sn) carrying a recessive mutation on the X chromosome, such as curly wings (cy). Let's call this female SnCy. This cross is called a test cross.

Test Subject Male (sn) x SnCy Female (Sn Cy)

2. Collect and observe the offspring (progeny). If the singed bristles phenotype is due to a recessive X-linked mutation, you will see the following results:

a) All F1 progeny will have wild-type bristles (Singed is recessive to wild type). This is because the female parent contributes a wild-type allele (Sn) to the offspring, overriding the singed allele from the male parent (sn).

b) In the F2 generation, you will observe:

i) Males: Approximately half of the males will have singed bristles (sn) and the other half will have wild-type bristles (Sn).

ii) Females: All females will have wild-type bristles (Sn).

These results would indicate that the singed bristles phenotype is indeed due to an X-linked recessive mutation.

3. To confirm your findings further, you can perform a reciprocal cross between a male with singed bristles (sn) and a wild-type female (Sn). If the singed bristles phenotype is due to an autosomal recessive mutation, you would expect to see singed bristles in both male and female F1 offspring.

Here is an overview of the crosses:

Cross 1:
Test Subject Male (sn) x SnCy Female (Sn Cy)
- F1 Progeny: All wild-type (Sn)
- F2 Progeny: 50% Male: Singed (sn), 50% Male: Wild type (Sn); 100% Female: Wild type (Sn)

Cross 2 (Reciprocal Cross):
Test Subject Male (sn) x Wild-type Female (Sn)
- F1 Progeny: 100% Singed (sn)

By analyzing the resulting progeny from these crosses, you can determine if the unusual singed bristles phenotype is due to an X-linked mutation.