Important piece of information for Monday:

Last names beginning A-L: 101 Barker

Last names beginning M-Z: 240 Mulford

Testing site:

(1) Deletion mapping of genes (Benzer)using the segregational test for allelism:

(2) Deletion mapping of genes (Bridges)using the functional test for allelism:

If you can, the point mutant is outside the region deleted.

If it is, the point mutant is outside the region deleted.

Can you recover a wildtype allele from the mutant hybrid?

(a recombination test)

Is the mutant hybrid phenotype wildtype?(a complementation test)

Deletion mapping based on a complementation test

Fig. 14.8

w- rst- fa-

Df(1)256-45)

= w- (mutant) phenotype (failure to complement) hence white is within region deleted.

phenotype

Bulge in the synapsedpolytene chromosomesshows what is deleted.

Inversions also have helped locate genes on the Drosophila polytene chromosome map via complementation tests

Inversion breakpoints

may knock out bw+ …cause loss of ability to complement bw-

bw+ In(3R)bw- In(3R)bw-/bw- = brown eye phenotype

The complementation testas an operational definition of the gene

is not quite as straightforwardas it may sound.

genes can have verycomplex complementation patterns

because of all the various kinds of information they containthat work only in cis

(and all the various ways in which that informationcan be changed by mutation)

p227 Heading: “rII regon has two genes”

Is this statement compatible with the statement thatcomplementation groups are what we want to call genes?

(starting bottom of p291):A gene is not simply the DNA that is transcribed into the mRNA codons specifying the amino acids of a particular polypeptide. Rather, a gene is all the DNA sequences needed (IN CIS) for expression of the gene into a polypeptide product. A gene therefore includes the promoter sequences that govern where transcription begins and, at the opposite end, signals for the termination of transcription. A gene also includes sequences dictating where translation starts and stops. In addition to all these features, eukaryotic genes contain introns that are spliced out of the primary transcript to make the mature mRNA. Because of introns, most eukaryotic genes are much larger than prokaryotic genes. Are promoter mutations part of complementation groups?

From your text:

How about intron mutations?

So promoters are part of genes.

Fig. 8.12

A bacterial promoter(cis-acting information for

transcription start)

51 bp transcript--->

1,612 INDEPENDENT mutants mapped for Fig. 7.21 (and ultimately >3000)

B -- A

the fine-structure map of rIIA & rIIB generated byrecombination between

mutants in the same genes (as complementation groups)?

There are 12 bp betweenrIIA & rIIB “genes”(T4 has 168,903 total bp)

…hence no room for a standard promoter

rII-A rII-B

rII makes a polycistronic mRNA

P

Figure 17.5: The Lactose Operon in E. coli

classic example ofpolycistronic mRNA

rII-A35

6671

rII-B2658799

723

rIIA&B mutants in the promotor for a polycistronic mRNA

rII “complementation map” of point mutants(a very different kind of “map”)

three complementation groups?

No, one complex complementation group(one “gene”) provided 7 and 23 don't meiotically map as large deletions

Alternative pre-mRNA splicingis what allows Drosophila

DSCAM gene to make30,000 different proteins

Fig. 8.18

introns

exons

6 vs. 7-8 alternativeexons

Eukaryotic genesare even messier:

…and the regulatory regions

(non-protein-coding)can extend enormous

distances on both sides

(NATURE 184:1927-29, 1959)

(r+ encodes a single polypeptide with three sequential enzymatic activitiesrequired for purine biosynthesis)

largest class

These arethe mutantsthat argue forone gene witha complexcomplementation pattern(14/31)

This “map” of rudamentary allelesdoes not imply anything about where

the various mutations might lie on a meiotic map.

It is simply a schematic representation of a collection of data froma series of complementation tests

designed to determine functional allelism

did they complement (Y or N)?

a -/b - : Yes

they don’t overlapon the map

Let’s consider three r mutant alleles:

…and the hybrid fly looked: wildtype

a -/h - :

did they complement (Y or N)?

No

they do overlapon the map

Consider a new r mutant allele, rz3 z3 / a

z3 / h

z3 / i

Which heteroallelic combination (hybrid) is most likely to have a mutant phenotype?

z3 / i

Which heteroallelic combination is most likely to generate a wildtype allele during meiosis?

? no basis fora determination

This “map” tells us next to nothing about thepossible molecular basis

for the complex complementation pattern

…but so long as we have reason to believethat group i contains at least some point mutants,all the mutants on this “map” are likely to be in the

same (thing that we want to call a) gene.

…and i is the mostfrequent class

(1) What kinds can we make? (categories)

(2) How do we make them? (mutagenesis)

(3) How do we find them? (mutant screens & selections)

Mutations (changes in DNA):

the lifeblood of genetic analysis

(4) Why bother?Central Dogma:information flowN.A. Protein

?

The young S. Benzer: (1950s)phage T4

as his genetic workhorse(while fly work was in its

"eclipse period")•founded many of the most interesting areas of modern behavioral genetics (clocks & learning, etc.)•set up the experimental system most effective for studying fly development: the compound eye

Benzer's question for the fly:how do genes encode

complex nervous systems?

An older Benzer: (1967-2007)fruit fly (D.melanogaster)as his genetic workhorse

the returnof the fly

Fig. 20.10

Can mutations affecting the fly eye tell us anything about our eyes?

hypomorphic or null mutations of the eyeless gene in an adultfruit fly

hypomorphic or null mutations of the Pax-6 genIn a fetal mouse

Fig. 20.4(homozygote would have

died as an embryo) (human aniridiadominant “genetic disease”)

Fig. 20.10

Induced “ectopic” expressionof mouse Pax-6 gene product

A neomorphic dominant mutation in the fly Antennapedia gene causes ectopic expression of a leg-determining gene

in structures than normally produce antennae

(actually its wildtype function is to force cells that would otherwise make an antenna to make a leg instead)

Fig. 8.31, panel d:

and abx

Fig. 20.23

…but the most informative mutations may not be compatable with survival to the adult stage(a somewhat revolutionary idea, remarkably enough)

wildtype

Fig. 20.2 a fruit fly 9h after fertilization

ftz amorph (null mutant) homozygote

(ftz+)

(it’s recessive lethal)

head tail

denticle belt pattern on the larval cuticle(cuticle: “skin,” tho. actually “skeleton”)

(denticles: maggot tire treads)reveals where fly cells think they are in space

something Thomas Hunt Morgan never guessed (among other things):you can get a huge amount of phenotypic information out of

the skin of a dead maggot

Wieschaus and Nüsslein-Volhard, Nobel Prize 1995

by 16h after fertilization, even if doomed

Fig. 20.19

wildtype

Krüppel hunchback knirps

(ftz is in the “pair rule” family, not the “gap” family of mutant phenotypes)

maximally informative mutant phenotypesfor understanding metazoan pattern formation