Fruit Fly Genetics

Question 1

What species is used in this experiment? Give both common and latin names

Answer 1

Fruit fly

Drosophila melanogaster

Question 2

What 4 reasons have fruit flies become a model organism for genetics research?

Answer 2

1. easy to rear in laboratories
2. life cycle is short (less than 2 weeks)
3. a single mating can produce several hundred offspring
4. only 4 pairs of chromosomes = 3 pairs of autosomes and 1 pair of chromosomes

Question 3

Define wild type allele

Answer 3

the allele most commonly expressed in the natural population

Question 4

What is the wild type allele for fruit fly eye colour?

Answer 4

red eyes

Question 5

When one gene is nonfunctional, what eye colours can result instead of the wild type?

Answer 5

Either white or sepia, depending on which gene is nonfunctional

Question 6

For each gene, what was examined in this lab?

Answer 6

the results of reciprocal crosses to determine whether the gene is autosomal or sex linked

Question 7

What does reciprocal cross mean?

Answer 7

the same phenotypes are used for the two crosses but the opposite sexes are associated with the specific phenotypes

Question 8

What is an example of a reciprocal cross in this lab?

Answer 8

a red-eyed female crossed with a white-eyed male is one cross

its reciprocal cross: a white-eyed female crossed with a red-eyed male

Question 9

How does the fruit fly life cycle begin?

Answer 9

as an egg which hatches into a small larva after ~12 hours

Question 10

What covers the larva initially? What needs to happen to it in order for the larva to grow?

Answer 10

A stiff outer cuticle (exoskeleton) which needs to be shed for the larva to grow

Question 11

What does molting mean?

Answer 11

The shedding of the exoskeleton

Question 12

Which life stage feeds and for how long?

Answer 12

the larva stage feeds ‘voraciously’ for 4 days

Question 13

What does the small larva develop into and how long after hatching does this occur?

Answer 13

a larger larva ~24-48 hours after hatching

Question 14

What does the last larval stage form?

Answer 14

A pupa

Question 15

Is the pupa a feeding stage? How long does this stage last?

Answer 15

nonfeeding

lasts ~4 days

Question 16

What occurs during the pupa stage?

Answer 16

the larval tissues are rearranged into those of the adult fly

Question 17

The female fly is able to ___ and start ___ ___ after ~12 hours of emerging from its pupa

Answer 17

mate; laying eggs

Question 18

What is the best way to determine the sex of a fruit fly?

Answer 18

the presence or absence of sex combs on the front pair of legs

Question 19

Which sex will have the sex combs on their front legs?

Answer 19

the males

Question 20

What else can be looked at to determine the sex of fruit flies?

Answer 20

the shape and pigmentation of the abdomen

Question 21

What does the abdomen look like on males?

Answer 21

the tip of the abdomen is rounded and the last few segments are solid black

Question 22

What does the abdomen look like on females?

Answer 22

the tip of the abdomen is pointed and the dark pigment is distributed in bands throughout the abdomen

Question 23

In the lab, what did the lowercase ‘s’ symbolize?

Answer 23

the sepia eye colour allele

Question 24

How many feeding larval stages does a fruit fly have?

Answer 24

3

Question 25

the reciprocal cross of a white female x red male is a ___ female x ___ male

Answer 25

red female x white male

Question 26

Which eye colour alleles are recessive in fruit flies? Which is dominant?

Answer 26

Red is dominant

White and sepia are recessive

Question 27

T or F: In the lab, F1 generation fruit flies from a sepia female x brick red male cross were scored

Answer 27

True

Question 28

What was the product of the ‘W’ allele of the white gene?

Answer 28

red pigment

Question 29

How many genes were studied in this lab? What were they?

Answer 29

2

gene for white eyes (W gene)
gene for sepia eyes (S gene)

Question 30

What is the purpose of reciprocal crosses?

Answer 30

to determine whether a gene is autosomal or sex-linked

Question 31

What did ‘w’ represent in the lab?

Answer 31

white allele

Question 32

What did ‘S’ represent in the lab?

Answer 32

red allele

Question 33

Was the gene for white eyes located on an autosome or an X-chromosome? why?

Answer 33

Located on an X-chromosome

because the sex-linked cross between a white-eyed female and red-eyed male resulted in the male offspring expressing the white-eye allele but the female offspring did not

Question 34

Was the gene for sepia eyes located on an autosome or an X-chromosome? why?

Answer 34

located on an autosome

because in the cross with the sepia-eyed female did not result in the male offspring expressing the sepia eye colour

Question 35

Explain why the male offspring showed the mutant eye colour phenotype for the X-chromosome gene

Answer 35

male fruit flies only have one X chromosome and that is given to them by the mother

Y chromosomes do not carry the white-eye colour gene in fruit flies

So any allele passed from the female parent will be expressed because that is the only X chromosome received by the male offspring

Question 36

Explain why the female offspring did not show the mutant eye colour phenotype for the X-chromosome gene

Answer 36

Female fruit flies have 2 X chromosomes, one from each parent

So for the female to express the mutant allele type, the 2 inherited alleles must both be the mutant allele

Question 37

What were the expected ratio and experimental ratio of males to females for the white-eyed and sepia-eyed autosomal crosses?

Answer 37

expected: 100% heterozygous, showing red eyes

experimental: 100% heterozygous with red eyes except for the cross that had the white-eyed female parent
- that cross resulted in the male offspring expressing the white eyed allele because the gene for white eyes is X-linked

Question 38

What were the expected ratio and experimental ratio of males to females for the white-eyed gene X-chromosome crosses?

Answer 38

expected red eyed mother: 100% heterozygous, red eyes displayed

expected white eyed mother: 50% red eyed (females): 50% white eyed (males)

Experimental: same ratios because the white gene is X-linked

Question 39

What were the expected ratio and experimental ratio of males to females for the sepia-eyed gene X-chromosome crosses?

Answer 39

expected red eyed mother: 100% heterozygous, all red eyes

expected sepia eyed mother: 50% red eyes (female): 50% sepia eyed (male)

experimental: 100% sepia eyed because the sepia-eye colour gene is autosomal not sex-linked

Question 40

In a fly with RED (wild type) eyes, all 3 enzymes are functional. Which pigment(s) does this wild type fly produce?

Answer 40

Bright red pigment and sepia pigment will be produced, displaying the red (wild type) eye colour

Question 41

If a fly with WHITE eyes produces functional versions of all but one of the enzymes in this biochemical pathway, which enzyme would be non-functional? Explain

Answer 41

The first enzyme past purine riboside would be non-functional because if it is not working, the purine riboside (containing the white pigment) cannot continue along the pathway to express other pigments = white eyes

Question 42

If a fly with SEPIA eyes produces functional versions of all but one of the enzymes in this biochemical pathway, which enzyme would be non-functional? Explain

Answer 42

the second enzyme would be non-functional because the purine riboside would need to be converted into the intermediate compound which requires the first enzyme to be functioning

Then to block the pathway to the red pigment, the second enzyme would have to be nonfunctioning so that the third enzyme can produce the sepia pigment = sepia eye colour

Question 43

how many chemical reactions occur in the biochemical pathway responsible for eye colour pigments in fruit flies?

Answer 43

3, each controlled by a different enzyme

Question 44

What does the ability of an enzyme to do its job in a biochemical pathway depend on? What is that determined by?

Answer 44

Its physical structure which is determined by a gene

Question 45

How many enzymes are active in the biochemical pathway of a red eyed fruit fly?

Answer 45

all 3

Question 46

If a mutation exists in one of the genes in the biochemical pathway, what will happen?

Answer 46

the mutation will be copied into the RNA produced from the gene during transcription

if this RNA is translated, the resulting protein will also contain the mutation which will effect the enzyme’s ability to function

Question 47

If an enzyme in a biochemical pathway is not functional, what will occur to the fly?

Answer 47

the fly with the mutation will display the mutant phenotype

Question 48

Where are the genes for the 3 enzymes located?

Answer 48

on different chromosomes

Question 49

Each gene has a(n) ___ that results in a ____ ___ that successfully converts the raw material (substrate) into a product

Answer 49

allele; functional enzyme

Question 50

What does the functional version of enzyme 1 do?

Answer 50

converts purine riboside (a white substance) into the intermediate compound

Question 51

T or F: each of the 2 genes studied in this lab also has an allele that results in a nonfunctional enzyme

Answer 51

True