GENE 314 Lab 4: Inheritance of 2 Genes and Introduction to Drosophila

Scientific name for fruit fly?

Drosophila melanogaster

Why is Drosophila a “favorite?”

• rapid generation time (9 days)

• ease of handling

• wealth of genetic information that has accumulated about it

What does independent assortment result in?

all progeny have an equal chance of all possible combinations

What does Mendel’s Laws not take into account?

1. linked genes

2. genes on the X chromosome (sex linkage)

Stages of Drosophila life cycle

1. embryonic stage (egg)

2. three larval stages

3. pupal stage

4. adult stage

How long is the Drosophila life cycle?

10-14 days

What is a “wild type?”

the standard phenotype of an organism

• ex: elephants are grey

What are genes in Mendelian genetics arbitrarily named after?

one of the phenotypes observed

• ex: W- for round and ww for wrinkled

• ex: Y- for yellow and yy for green

Drosophila notation?

genes are given names that correlate with the phenotype of the mutant

Drosophila notation example

the wild type state of Drosophila eye color is brownish-red. if a mutant has white eyes, then the gene necessary for that trait is called white (w). the wild type would be designated with a superscripted plus (w⁺). the plus would be read as NOT; in this case, the eyes would be NOT white.

How else is Drosophila notation more informative?

the ability to distinguish genes on sex chromosomes and genes found linked on the same chromosome

What is Drosophila notation a representation of?

homologous chromosome pairing and alignment at Metaphase I of meiosis

Rule 1

Drosophila notation is to be used ONLY for Drosophila

Rule 2

homologous alleles are not written next to each other as in Mendelian notation (aa), but one over the other separated by a line to represent homologous pairing (a/a)

Rule 3

if there is no other information about a wild type trait, assume the fly is homozygous

Rule 4

lowercase letters designate recessive mutations (pr), whereas uppercase letters designate dominant mutations (B)

Rule 5

if two genes are on the same chromosome, rhey are shown linked on the same line, without interruption (ab/ab)

Rule 6

if two genes are on different chromosomes, they should be separated by a semicolon (a/a;b/b)

Rule 7

Drosophila have four chromosomes: X, 2, 3, 4. genes should be written in order of the chromosome they are found on, if known

Rule 8

male flies only, have a single Y chromosome that pairs with the X. it should only be written if the mutation is found on the X chromosome (w/↳)

Rule 9

a wild type female may sometimes be represented with an X/X for the wild type X chromosomes and any wild type X chromosome may be represented as X in the notation. this is particularly important in signifying the fly’s sex if the sex chromosomes are not part of the genes of interest in a cross

How many pigment pathways are responsible for Drosophila WT eye?

two; ommochrome and pteridine pathways

Ommochrome pathway

produces a brown pigment through a series of biochemical steps of tryptophan metabolism

Pteridine pathway

metabolizes purines into a series of pterins, resulting in a red pigment

What does WT eye require?

production of both brown (ommochrome) and red (pteridine) pigments, along with the function of a pigment-binding enzyme in the final step

What happens when there is a mutation in the ommochrome pathway?

alteration or loss of the brown pigment

What happens when there is a mutation in the pteridine pathway?

alteration or loss of the red pigment

Cinnabar

mutation in ommochrome pathway results in scarlet eye

Sepia

mutation in pteridine pathway results in brown eye

How does white eye occur?

disruption of both pathways simultaneously

Pigment binding protein

responsible for binding the pigments to grandules in the pug

Disruption of the pigment binding enzyme

non-functional enzyme results in the inability to place pigments in the eye, producing white eye

Mutations in the pigment binding enzyme are epistatic to any other mutations in the pigment pathway

a defect in this enzyme masks the effects of other mutations

Dihybrid cross

AaBb x AaBb

9:3:3:1

unlinked

How does linkage alter the standard 9:3:3:1 ratio?

yield less recessive offspring and an increase in double recessive offspring

When are single mutants observed?

only when a crossover event occurs in prophase I

What is the frequency of crossover related to?

“distance” between genes

% recombination = map distance

• ex: 50 map units → 50% recombination

What can be determined from a dihybrid cross?

1. number of loci controlling a phenotypic trait

2. the dominance relationship between alleles

3. the types of gene interactions taking place

Unlinked autosomal

mutations are present on 2 different chromosomes, neither of which being the X chromosome

9:3:3:1

Unlinked sex/autosomal

mutations are present on 2 different chromosomes, one of which being the X chromosome

9:3:3:1

Linked autosomal

2 mutations are present on the same chromosome, but not the X chromosome

• there is cross over between loci only in females

Linked on the X chromosome

2 mutations are present on the X chromosome

• there is cross over between loci only in females

How many flies will be anesthetized with CO₂?

10-15 wild type

What wild type traits must be paid attention?

• eyes

• size and shape of wing

• body color

Differences between male and female flies

1. female abdomen is pointed (ending in ovipositor), male is round

2. the ventral tip of the female abdomen has a few bristles, while males have numerous and other structures that compose external genitalia

3. females are banded evenly along the entire abdomen; the dorsal side of the male abdomen is black

4. female has 5 clumps of bristles along abdomen; male only has 3-4

5. on the fourth segment of the leg, males have sex combs and females do not

   1. only visible at the highest setting on dissecting microscope

How long can flies be examined under CO2 anesthesia?

about 15 minutes without killing or sterilizing them

Why must the vial remain on its side until flies regain consciousness?

if upright, they’d fall into the sticky food and die

Crossveinless (cv)

wing crossveins missing; longitudinal veins only

X chromosome

Forked (f)

bristles (especially scutellars) are forked and jagged

X chromosome

Dumpy (dp)

wings are short and posteriorly scalloped

chromosome 2

Vestigial (vg)

wings greatly reduced in length and width, temperature sensitive such that higher temperature produces larger wing

chromosome 2

Apterous (ap)

wings and haltera reduced to traces, second legs bent

chromosome 2