Question

White eye is a sex linked recessive trait in drosophila. In cross of white eyed female with normal male, what are the phenotypes of F1 generation?
A. All normal
B. All males white eyed, all females normal
C. 50% females and all males white eyed
D. 50% males and 50% females white eyed

Answer 1

Hint: In drosophila, the gene for eye colour is present on the X chromosome. The female parent has white eyes which implies that both the chromosomes carry the recessive gene. A normal male parent will possess a dominant gene.

Complete Answer:
In a cross between white eyed female and red eyed male, the offsprings formed will be-
The cross will produce 2 females with red eyes and 2 males with white eyed males. This cross can be understood by knowing following facts-
- The gene for eye colour is linked to X-chromosome. X chromosome from the female parent will be sent to both the sons due to which they will have white eyes.
- The daughters produced by this cross will have recessive X chromosome from the mother but also an additional normal X chromosome from their father, and will therefore have red eyes. The recessive chromosome is able to express itself in the male offspring because of the presence of a single X-chromosome.
- The daughters on the other hand will carry the recessive gene which will be passed on to their offspring.
- These experiments were carried out by T. H. Morgan who proposed the concept of linkage through his experiments on drosophila.

Thus, the correct answer is- B. All males white eyed, all females normal.

Note: The cross between white-eyed female with red-eyed male will produce all the daughter flies having red-eyed and all the male flies with white-eyes. This result shows deviation in Mendelian ratio. This proves that inheritance of eye colour depends on X-chromosome.