Drosophila

Introduction & Aim

Genetics is the study of inheritance. Many characteristics of an organism are controlled by its genes. Most genes have more than one form, and these different forms are called alleles. Some characteristics are controlled by a single gene, but many are influenced by two or more genes which interact with eachother. We are going to observe how these genes interact in Drosophila. Drosophila melanogaster is a fruit fly, a little insect about 3mm long, of the kind that accumulates around spoiled fruit. IN this experiment we will be crossing different races of drosophila melanogaster. We are going to predict and find out which alleles are dominant, which recessive and if there are any sex-linked alleles. Dominant means that an allele affects the characteristics of the offspring the most. Recessive means it only affects characteristics of the offspring if there is no dominant allele present. Sex-linked is when certain characteristics are only found in male flies, or female flies. The aim of this experiment is to find out how how dominancy, sex-linkage and recessive alleles work out in crosses and to learn to predict how it will work out.
My hypothesis about this is that long flies are dominant. This is because in nature more there are to be found more flies with long wings than with short wings. Flies should have long wings because then they can fly. I observe this characteristic in the same way as being colour blind in humans. It's a sort of handicap, and it's rare. Flies are called flies because they can fly, so if they don't fly, it aren't really flies actually. And about eye colour: red eyes are dominant. This is because when we started the experiment and we started crossing red eyed flies with white eyed flies, there were fewer flies with white eyes than with red eyes, so I assumed that red eyes must be dominant.

Theoretical background

The life cycle
There are four distinct stages in the life cycle of the fruit fly: egg, larva, pupa, and adult. At 25 C a fresh culture of Drosophila will produce new adults in 9-10 days. Five days in the egg and larval stages and four days in the pupal stage. The adult flies may live for several weeks. Drosophila cultures should not be exposed to high temperatures (e.g.. above 30 C) which result in sterilization or death of the flies no to low temperatures (e.g. below 10 C) which result in a prolonged life cycle (perhaps 57 days) and reduced viability. The adult Drosophila female starts to deposit eggs on the second day after emergence. Embryonic development of the egg takes about one day at 25 C.
The larva is white, segmented and worm like. The larval stage is a feeding stage and consists of three subdivisions called instars. The first and second instars stages end in moulting which allows the larva to grow. The third instar ends with pupation. Prior to pupation, the animal stops feeding and crawls to some relatively dry surface and the cuticle hardens and
darkens to form the puparium.
Metamorphosis occurs in the puparium and takes about four days. The pupa begins to darken just prior to the emergence of the adult fly. Most flies emerge from the pupa in the early morning hours.

The way of breeding
In our school (Anne van Rijn college) breeding is done in a sort of breeding oven. It is about 25 degrees in there. The flies are in tubes with in the bottom squeezed banana for them to eat.

Differences between the sexes.
Before starting an experiment with drosophila you better first learn to recognise the differences between the sexes. In drosophila melanogaster the differences is easy to see with a magnifying-glass. The female flies have stripes on their abdomen (their but) and it is rounded and ends up pointed and the males have a black abdomen which is more narrow and round.

The varieties we use
In our experiment we use three different races of drosophila. The wild ones, these have long wings and red eyes. The vestigial flies, these have short wings and red eyes. And we use the white ones, these have long wings and white eyes.

Materials & method

Materials
- dropper bottle with ether
- sorting plate
- flies in breeder tube
- etheriser
- brush
- stereo microscope

Method
We did two crosses and each time we had to sort the flies and put them together we did this:
We let the teacher or technical assistant add 10 drops of ether into the etheriser. We put the flies into the etheriser and after they fell asleep we sorted them out. We sorted them in sex and race. Then we had to put three male flies of one race and three female flies of another race into new tubes. The rest of the flies were but back in their tubes again, after which we were told they would go to elderly homes for flies. The first crosses we had to make were:

1. ? + ? ? vg
2. ? vg ? ? +
3. ? wh ? ? +
4. ? + ? ? wh

The second crosses we had to make were the same but now with the F1 flies.

Results

F1
Cross 1 wild male x vestigial female
numbers percentages Class percentage
Female - long 0 0 39
Male - long 0 0 33
Female- short 20 45,5 14
Male - short 22 54,5 14

Cross2 vestigial female x wild male
Numbers Percentages Class percentage
Female- long 14 54 50
Male - long 11 42 50
Female-short 0 0 0
Male-short 1 4 0

Cross3 white female x wild male
numbers Percentages Class percentage
Female-red 11 79 51
Male-red 0 0 7,3
Female-white 0 0 6,7
Male-white 3 21 35

Cross4 red female x white male
Numbers Percentages Class percentage
Female-red 24 47 49
Male-red 27 53 51
Female-white 0 0 0
Male-white 0 0 0

F2
Cross1
numbers percentages Class percentage
Female-long 17 36 40
Male-long 17 36 35
Female-short 5 10 14
Male-short 8 17 12

Cross2
numbers percentages Class percentage
Female-long 15 38 44
Male-long 15 38 37
Female-short 5 13 7
Male-short 4 10 12

Cross3
Numbers percentages Class percentage
Female-red 20 53 45
Male-red 7 18 32
Female-white 5 13 12
Male-white 6 16 11

Cross4
numbers Percentages Class percentage
Female-red 25 59 62
Male-red 13 31 30
Female-white 0 0 0
Male-white 4 10 8

Conclusions

My conclusion is that long wings are dominant, short wings recessive. Red eyes are dominant and white eyes recessive. There is some sex-linkage in this. Red eyes are sex-linked in some crosses and in some not.

Long wings are dominant to short wings, this can be seen in the F1 cross1:
male female
X a a
A AA AA
A AA AA
All the offspring will have long wings.

Red eyes are dominant to white eyes, this can be seen in F1 cross2
male female
X a a
A Aa AA
A Aa AA
All the offspring will have red eyes.

Red eyes are sex-linked, this can be seen in cross 4:
male female
X XB Xb
XB BB Bb
Y YB Yb
All females will have red eyes, half of the males will have white eyes.

The aim was to help us understand more about dominancy and sex linkage . By doing the Drosophila Experiment you learn a lot about dominancy and when thinking about this, you find out about sex linkage. In the F1 you see that long wings are dominant to short wings and that red eyes are dominant to white eyes. In the F2 you see that long wings are about 75% while short wings are about 25%. This is the same for the red eyes (75%) and white eyes (25%). Although the numbers are not exactly right, my conclusion is: my hypothesis "long wings are dominant to short wings" and "red eyes are dominant to white eyes" is correct! I didn't include the sex-linkage in my hypothesis, but in the experiment I found out about that.

Discussion

Some of our results didn't meet our conclusions. In The F1 in cross 1 we had some strange results. I think we put the wrong flies in the tube or something. We have another strange result, in cross 2 in the F1 we find one male fly with short wings. It is a stranger, and I don't understand how it got there. In the results of the class there were some other strange results, but I think most of them are because of mistaken flies in wrong tubes, I can't imagine a fly with long wings hided them when we sorted them out. I think overall the results met quite well with the conclusions.