Eye Colour Genetics

Question 1

why are fruit flies models organisms for genetics research? what is the scientific name?

Answer 1

1. easy to rear in the lab
2. short life cycle (less than 2 weeks)
3. produce a lot of offspring
4. has just 4 pairs of chromosomes

-Drosophila melanogaster

Question 2

how many autosomes/sex chromosomes do fruit flies have?

Answer 2

3 pairs of autosomes

1 pair of sex chromsomes

Question 3

what colour eyes does a normal “wild type” fruit fly have? what causes a fruit fly to have different coloured eyes than the wild type? what colours might this cause?

Answer 3

• red
• if a fly has a mutation in one or more of the genes involved in producing the red eye colour, making it nonfunctional, then it will have a different coloured eyes
• colours may include: white, apricot, sepia, scarlet, or black

Question 4

what is a reciprocal cross? what is the purpose of a reciprocal cross?

Answer 4

a reciprocal cross is when the same phenotypes are used for two separate breeding crosses, but the opposite sexes are associated with the specific phenotypes
eg. red-eyed female X white-eyed male
and a white-eyed female X red-eyed male

-purpose is to determine whether a gene is autosomal or sex-linked

Question 5

what is the purpose of this experiment?

Answer 5

is to examine the results of reciprocal crosses to determine whether the gene for red eye colour is autocomal or sex-linked

Question 6

how many genes are involved in producding the red colour?

Answer 6

-2

Question 7

a stereomicroscope has a two sided plate, which side should you use for what?

Answer 7

• use the black side for transparent specimens

- use the white side when viewing opaque specimens

Question 8

at what magnification does a stereomicroscope have the greatest depth of focus?

Answer 8

at the lowest magnification

Question 9

what is the life cycle of a fruit fly (include approximate times in each stage)?

Answer 9

1. egg (12 hours)
2. 1st instar larva (approx 24 hours)
3. 2nd instar larva (approx 24 hours)
4. 3rd instar larva
5. pupa (approx 4 days)
6. Adult fly

Question 10

what are the larva covered with? what happens to this in order for the larva to grow?

Answer 10

• covered with stiff out cuticle called an exoskeleton

- the exoskeleton must shed in order for larva to grow, this is called molting

Question 11

which stage of the fly life cycle is a feeding stage? which is a non-\feeding stage?

Answer 11

feeding –> larva

non-feeding –> pupa

Question 12

what happens during the pupa stage? how long after emerging from its pupa is a female fly able to mate and start laying eggs?

Answer 12

• the larval tissues rearrange into those of an adult fly

- approx 12 hours

Question 13

how can you tell male and female fruit flies apart?

Answer 13

males:
-have sex combs (look like little hairs) on their front legs
-the tip of their abdomen is rounded
-last few segments of abdomen are solid black
females:
-do not have sex combs
-tip of abdomen more pointed
-have bands of dark pigment

Question 14

what are the hairs on a males leg used for?

Answer 14

the sex combs are used to grasp females during mating

Question 15

what are two things we have to assume in order for the reciprocal cross?

Answer 15

1. that the parents are homozygous

2. the allele for red eyes (wild type) is dominant to the allele for the mutant eye colour

Question 16

according to given data is the gene that controls for white eyes sex-linked or autosomal? what about for sepia eyes colour?

Answer 16

white eyes –> sex-linked
-located on sex chromosome because if it was autosomal all offspring would have red phenotype, but the results were that just males had white eyes and this is because having white eyes is a recessive gene, so can only be carried on by male offspring if female parent is homozygous recessive
sepia eyes –> autosomal
is autosomal because all offspring had red eyes. therefore not reliant on mother being homozygous recessive

Question 17

how many enzymes are responsible for the bright red eye colour in flies? what determines whether an enzyme is functional and does its job correctly? what is an enzyme is non-functional?

Answer 17

3 enzymes are responsible

• its function is determined by its physical structure, which is determined by a gene
• then the flies with this mutation will have a mutant phenotype

Question 18

where are the genes for the three different enzymes located?

Answer 18

-all on different chromosomes

Question 19

what is the expected ratio of males to females in this experiment? explain why you think this may or may not have happened?

Answer 19

• the expected ratio is 50/50
• this did not happen exactly but was pretty close to.
• like any population there is variation in why you may or may not have a fully 50/50 population

Question 20

in a fly with red (wild type) eyes, all 3 enzymes are functional. which pigments does this fly produce?

Answer 20

• this fly produces all three pigments
• but the white pigment will be converted into an intermediate compound and large amounts of the bright red pigment will be produced, which when mixed with the sepia colour will appear red

Question 21

in a fly with white eyes, which enzyme would be nonfunctional if the rest are functional?

Answer 21

• enzyme 1 would be non-functional
• as this enzyme would convert the purine riboside into the intermediate compound
• potentially this would mean the eyes would stay white because the next enzymes would not be able to act upon purine riboside as it is not those enzyme’s intended substrate (?this is a guess)

Question 22

in a fly with sepia eyes, which enzyme would be nonfunctional if the rest are functional?

Answer 22

• enzyme 2 would non functional.
• as both enzyme 2 and 3 would rely upon enzyme 1 working and producing the intermediate compound that they could work upon.
• if enzyme 2 was non-functional it means that the dominant red pigment would not be made and would allow for the recessive sepia colour to be the phenotype (? this is a guess)