Fruit Fly Genetics Flashcards
What species is used in this experiment? Give both common and latin names
Fruit fly
Drosophila melanogaster
What 4 reasons have fruit flies become a model organism for genetics research?
- easy to rear in laboratories
- life cycle is short (less than 2 weeks)
- a single mating can produce several hundred offspring
- only 4 pairs of chromosomes = 3 pairs of autosomes and 1 pair of chromosomes
Define wild type allele
the allele most commonly expressed in the natural population
What is the wild type allele for fruit fly eye colour?
red eyes
When one gene is nonfunctional, what eye colours can result instead of the wild type?
Either white or sepia, depending on which gene is nonfunctional
For each gene, what was examined in this lab?
the results of reciprocal crosses to determine whether the gene is autosomal or sex linked
What does reciprocal cross mean?
the same phenotypes are used for the two crosses but the opposite sexes are associated with the specific phenotypes
What is an example of a reciprocal cross in this lab?
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
How does the fruit fly life cycle begin?
as an egg which hatches into a small larva after ~12 hours
What covers the larva initially? What needs to happen to it in order for the larva to grow?
A stiff outer cuticle (exoskeleton) which needs to be shed for the larva to grow
What does molting mean?
The shedding of the exoskeleton
Which life stage feeds and for how long?
the larva stage feeds ‘voraciously’ for 4 days
What does the small larva develop into and how long after hatching does this occur?
a larger larva ~24-48 hours after hatching
What does the last larval stage form?
A pupa
Is the pupa a feeding stage? How long does this stage last?
nonfeeding
lasts ~4 days
What occurs during the pupa stage?
the larval tissues are rearranged into those of the adult fly
The female fly is able to ___ and start ___ ___ after ~12 hours of emerging from its pupa
mate; laying eggs
What is the best way to determine the sex of a fruit fly?
the presence or absence of sex combs on the front pair of legs
Which sex will have the sex combs on their front legs?
the males
What else can be looked at to determine the sex of fruit flies?
the shape and pigmentation of the abdomen
What does the abdomen look like on males?
the tip of the abdomen is rounded and the last few segments are solid black
What does the abdomen look like on females?
the tip of the abdomen is pointed and the dark pigment is distributed in bands throughout the abdomen
In the lab, what did the lowercase ‘s’ symbolize?
the sepia eye colour allele
How many feeding larval stages does a fruit fly have?
3
the reciprocal cross of a white female x red male is a ___ female x ___ male
red female x white male
Which eye colour alleles are recessive in fruit flies? Which is dominant?
Red is dominant
White and sepia are recessive
T or F: In the lab, F1 generation fruit flies from a sepia female x brick red male cross were scored
True
What was the product of the ‘W’ allele of the white gene?
red pigment
How many genes were studied in this lab? What were they?
2
gene for white eyes (W gene)
gene for sepia eyes (S gene)
What is the purpose of reciprocal crosses?
to determine whether a gene is autosomal or sex-linked
What did ‘w’ represent in the lab?
white allele
What did ‘S’ represent in the lab?
red allele
Was the gene for white eyes located on an autosome or an X-chromosome? why?
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
Was the gene for sepia eyes located on an autosome or an X-chromosome? why?
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
Explain why the male offspring showed the mutant eye colour phenotype for the X-chromosome gene
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
Explain why the female offspring did not show the mutant eye colour phenotype for the X-chromosome gene
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
What were the expected ratio and experimental ratio of males to females for the white-eyed and sepia-eyed autosomal crosses?
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
What were the expected ratio and experimental ratio of males to females for the white-eyed gene X-chromosome crosses?
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
What were the expected ratio and experimental ratio of males to females for the sepia-eyed gene X-chromosome crosses?
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
In a fly with RED (wild type) eyes, all 3 enzymes are functional. Which pigment(s) does this wild type fly produce?
Bright red pigment and sepia pigment will be produced, displaying the red (wild type) eye colour
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
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
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
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
how many chemical reactions occur in the biochemical pathway responsible for eye colour pigments in fruit flies?
3, each controlled by a different enzyme
What does the ability of an enzyme to do its job in a biochemical pathway depend on? What is that determined by?
Its physical structure which is determined by a gene
How many enzymes are active in the biochemical pathway of a red eyed fruit fly?
all 3
If a mutation exists in one of the genes in the biochemical pathway, what will happen?
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
If an enzyme in a biochemical pathway is not functional, what will occur to the fly?
the fly with the mutation will display the mutant phenotype
Where are the genes for the 3 enzymes located?
on different chromosomes
Each gene has a(n) ___ that results in a ____ ___ that successfully converts the raw material (substrate) into a product
allele; functional enzyme
What does the functional version of enzyme 1 do?
converts purine riboside (a white substance) into the intermediate compound
T or F: each of the 2 genes studied in this lab also has an allele that results in a nonfunctional enzyme
True
