Cytogenetics Multiple Choice Questions on “Sex Determination in Drosophila”.
1. Drosophila follows which of these modes of sex determination?
A. XX /XY
B. XX/ XO
C. ZZ/ ZW
D. MM/NN
Answer: A
Clarification: Drosophila follows the XY system of sex determination while other modes are seen in other animals. MM/NN or MN is a system of blood group and not a system of sex determination.
2. As Human and Drosophila both follow XY system of sex determination, a male in human is always a male in drosophila and same is for female.
A. True
B. False
Answer: B
Clarification: Even though Human and drosophila both have XY system it is named so just depending on the structural constitution of chromosomes of normal male and female. However, the mechanism of maleness and femaleness development in both cases in very different.
3. Which of the following is true for Drosophila sex determination?
A. Males are produced by presence of Y chromosome
B. Females are produced by presence of Y chromosome
C. Two X chromosome will always produce a female
D. Two Y chromosome will always produce a male
Answer: C
Clarification: In Drosophila the sex is determined by the sex chromosome: set of autosome ratio. If X:A is 1 as in case of XX then it’s a female, ratio of 0.5 in case of XY gives a male. The presence or even absence of Y chromosome makes little difference.
4. Which of these genes is responsible for the development of sex of Drosophila?
A. Xce
B. Xist
C. XIC
D. Sxl
Answer: D
Clarification: Sxl or sex lethal gene is responsible for the development of male or female characters in Drosophila. Other genes mentioned work in human beings.
5. The Sxl gene is turned ____________ by a ratio of _____________for X:A.
A. OFF, 1
B. OFF, 0.5
C. ON, 0.5
D. ON, 0.75
Answer: B
Clarification: The Sxl gene is turned ON only when the ratio of X:A is 1, other times it is OFF. An OFF state produces a male while on state produces a female.
6. Sxl directly affects _____________
A. SR
B. Dsx
C. Tra
D. Xce
Answer: C
Clarification: Sxl directly affects the splicing of tra pre-mRNA and produces a functional gene when Sxl is turned on. Downstream of this the female characters are produced. Xce is present in human and not in Drosophila.
7. Which of the following is a default pathway in Drosophila?
A. Development of male characters
B. Repression of male characters
C. Development of female characters
D. All pathways are induced
Answer: A
Clarification: Development of male characters is like a default pathway in Drosophila which is turned off by female specific genes.
8. Which of the following will give a normal female Drosophila?
A. AAA + XX
B. AA + XXY
C. AA + XY
D. AA + XXX
Answer: B
Clarification: In the following case the X: A ratio is 1 which is required for developing female characters. In all the other cases viable female is not produced.
9. Genetic combination of AA : XYY in drosophila gives a _________
A. Female
B. Non-viable female
C. Male
D. Non-viable male