Question

Given a sepia fruit fly and a vermilion fruit fly parental cross, what would the phenotypes of the F1 and F2 generations expressed be?

Answer 1

This is the PUNNET SQUARE diagram of the F1 and F2 generations of the cross between the two fruit flies sepia and vermilion. Sepia has a kind of brown eyes, but there are also other species of fruit flies that have brown eyes so to avoid confusion, I have named it as ‘Sepia Eyed’ and Vermilion is the fruit fly variety that has red eye so I have named it as ‘Red Eyed’. This experiment was first conducted and observed by Thomas Hunt and he used red eyed females and  white eyed males.

Let me explain you what I have just drawn.

The parents are two fruit flies, SEPIA EYED FEMALE (with X^s+ gametes) and RED EYED MALE (with X^s gamete) When we cross these two we get the results of F1 GENERATION: Sepia eyed females and Sepia eyed males.. There was a red eyed male trait (X^s gamete) in one  of the parent but it was not expressed. So the phenotypic ratio of the F1 Generation is- 2:2 (because two phenotypic categories of offsprings are obtained: sepia eyed males and sepia eyed females which are found equally— see the Punnet square diagram)

Now for the F2 GENERATION, the offsprings  of the F1 generation are crossed. That is Sepia eyed females and Sepia eyed males are crossed. Here we get the results as: Sepia eyed females, Sepia eyed males and Red Eyed males. So the phenotypic ratio of F2 generation would be- 2:1:1 (because three phenotypic categories of offsprings are obtained: sepia eyed females, sepia eyed males and red eyed males which are found in the proportion of 2:1:1 respectively, meaning, every fourth one born will be a red eyed male — See the Punnet Square diagram)

And extra info: Note that every time  the Red Eye trait is expressed it is only expressed in Males, meaning that trait is SEX LINKED. This was what Thomas Hunt was trying to observe.

If in case you have further doubts or can’t understand anything please comment and I would explain.