3. The normal eye color of Drosophila is red, but mutant strains in which all fl

Question

3. The normal eye color of Drosophila is red, but mutant strains in which all flies have brown eyes are available. Similarly, wings are normally long, but there are mutant strains with short wings. A female from a pure breeding line (homozygous) with brown eyes and short wings is crossed with a pure breeding normal male. The F1 consists of normal females and short winged males. The F2 generation is produced by intercrossing the males and females in the F1. Both sexes of F2 flies show the following phenotypes: 3/8 red eyes, long wings 3/8 red eyes, short wings 1/8 brown eyes, long wings 1/8 brown eyes, short wings Outline the inheritance pattern of these two phenotypes using symbols that you have clearly defined. State the genotypes of all three generations State the genotypic proportions of the F1 and F2 P Brown Eyed, Short Winged FemalesX Normal Males F1 Normal Females x X Short Winged Males F2 3/8 red eyes, long wings (males and females) 3/8 red eyes, short wings (males and females) 1/8 brown eyes, long wings (males and females) 1/8 brown eyes, short wings (males and females)

Explanation / Answer

Red-eyed is dominant and the normal eye colour of Drosophila. Whereas the brown-eyed are caused due to mutation. Since males differ from females in their genotype with regard to wing length that is mutant strains, this trait is sex-linked.

Drosophila females are XX and males are XY,

the long-winged females tells us that long is dominant to short and that the gene is X-linked.

Let B = red, b =brown, XS = long and Xs = short.

The cross indicated in the question can be written as follows.
P: bb XsXs x BB XSY

(homogygous females with brown eyes and short wings) x ( pure breed males with red eyes and long wings)

F1: 50% female: Bb XSXs
50% male: Bb XsYF2

The genotypes of all three generations:
1/16 BB XSXs - red, long,
1/16 BB XsXs - red, short
1/8 Bb XSXs - red, long,
1/8 Bb XsXs - red, short
1/16 bb XSXs - brown, long
1/16 bb XsXs - brown, short
1/16 BB X SY - red, long,
1/16 BB X sY - red, short
1/8 Bb X SY - red, long,
1/8 Bb X sY - red, short,
1/16 bb X SY - brown, long
1/16 bb X sY - brown, short,

The genotypic proportions of the F1 and F2:

3/8 red, long
3/8 red, short
1/8 brown, long
1/8, brown, short

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