A pure-breeding fruit fly with curled wings mates with a pure-breeding fruit fly with normal (straight) wings. The F1 mate with each other to produce an F2 generation that consists of 160 flies with curled wings and 80 with straight wings. What can you infer from this observation?

The dominant curled wing allele is also a recessive lethal.

Explanation:

If we look at the F2 ratio we see that :  

curled wings: straight wings= 160:80

                                               =2:1

Hence,  if curled wings is considered to be a dominant trait then Curled wings * straight wings

Dd x dd

Punnet square will be as follows :

           d                    d

D   |    Dd   |        Dd

d    |    Dd   |        dd

Hence in order to get F2

Dd x dd

Punnet square will be the same as above if the F1 cross is Dd* Dd

        D    d

DDDDd

dDddd

if DD is lethal then the ratio is

Dd:dd=2:1

that is curled wings: straight wings=2:1

Hence, The dominant curled wing allele is also a recessive lethal.