Genetics can be used to characterizebiological pathways

Epistasis tells which gene productsare involved in common pathways

and which act earlier or later in a process.

Complementation tells us if variation is due to mutations in one gene or several genes.

What are the relationships between color types?

Purple is dominant to white A

XpurpleRR

white Arr

purpleRr

Relationship between2 chosen color variants

F2 1 RR, 2Rr and 1rr

X

Purple is dominant to White1

purpleRR

white Arr

F1X

purpleRr

Punnet square

r

r rr

Female gametesMale gametes

R

R RR Rr

Rr

What are the relationships between color types?

Purple is dominant to red

XpurpleRRPP

PurpleRrPP orRRPp

RedrrPP or RRpp

Complementation test

Red and white A are caused by mutations in different genes

Xwhite ArrPP

PurpleRrPp

redrrPP orRRpp

Cross two recessive mutants to determine if the mutations are in one gene or more than one.

Epistasis

Two genes for flower color

Are they two steps in the same pathway to make pigment?

Where are the two genes in the pathway?

1. Purple is either a mixture of blue and red pigments each made in a separatebiochemical pathway.

or

2. Purple results from modification of the same precursor from a white precursor to a red intermediate and finally a purple pigment.

We can use genetics to distinguish the two possibilities.The effect of variant alleles in multiple genes that affect pigment in combination will answer the question.

Precursor 1 Precursor 2

Blue Red

Precursor 1

RP

Red

R

P

Purple

Pathway 1 Pathway 2

Coexpression of blue and red pigmentderived from different precursorsmakes purple.

Modification of the sameprecursor leads to first a red pigment and then a purple pigment

Epistasis test

XWhite Arr

PurpleRr Pp

Redpp

Start with complementation test:Cross two recessive mutants to determine if the mutations are in one gene or more than one.

Epistasis test part 2

X

Purple F1Rr Pp

Cross F1 plants from the complementation testAnd follow how the different alleles segregate in the F2 generation.

Purple F1Rr Pp

?

Punnet Square: two genes with randomly segregating alleles

Male gametes

Female gametes

RP

Rp

rP

rp

rprPRpRP

RRPP

RRPp

RRPp RrPP RrPp

RRpp RrPp Rrpp

RrPP RrPp rrPP

rrppRrPp Rrpp rrPp

rrPp

9R_P_3R_pp3rrP_ 1rrpp

RrPp X RrPp

Precursor 1 Precursor 2

Blue Red

RP

If Pathway 1

Coexpression of Blue and red pigmentderived from different precursorsMakes purple

9R_P_ 3R_pp 3rrP_ 1rrpp

Phenotypes:

purple whitered blue

Recessive allelesLead to lack of eitherRed or blue pigment

Relationship between white a and red

X

X

white ArrPP

redRRpp

F1 is all purpleRrPp

F2

9 43

F2: 9R_P_ 3R_pp 3rrP_ 1rrpp

Phenotypes: purple whitered white

rr - get no red precursorneither purple nor red pigment can be made

pp – can get red pigmentif correct R alleles are present but not purple

Precursor 1

Red

R

P

Purple

Pathway 2 Modification of the sameprecursor leads to first a red pigment and then a purple pigment

R is epistatic to P

Mutations in the R gene cover the effect of mutations in the P gene.

This is because R is upstream of P in a biological pathway

The P protein requires the wild type function of theR protein.

R can be a regulator required to activate expression of P or R can be an enzyme upstream in a biochemical pathway

Using multiple allelism tests with diverse recessive mutants,

We can identify all the genes specificallyinvolved in making the purple pigment

Genetics can be used to determine the order of steps in a biological pathway

Epistasis tells which gene productsare involved in common pathways

and which act earlier or later in a process.

Mouse as a model for mammalian genetics

Origins of Mouse Genetics

Early domestication by Greeks and Romans

Chinese and Japanese fondness for unusual-looking mice

Early 19th century-popular objects of fancy in Europe

Early 20th century-English and American mouse fanciers

Early pioneers included LC Dunn, Clarence Little, Sewall Wright, and George Snell

Why Mice As an Experimental Organism?

Short life cycle

Easily bred

High fecundity

Hardy

Requires little space

Large amount of phenotypic variation

Easy to genetically engineer

Mammalian species

Evolutionary Relationships

0 myr bp

1002003004005006007008009001000

C. elegans

D. melanogaster

Xenopus

Mice

Humans

A mouse is not a mouse is not a mouse

Hundreds of strains

Great phenotypic diversity

Variation exceeds that in

the human population

Why is there biological concordancefor human and mouse

Evolutionary conservation!!

genome (gene content, arrangement and sequence)

structure (gross and molecular anatomy)

function (physiology and molecular circuits)

regulatory systems

Why is there biological concordancefor human and mouse

Evolutionary conservation!!

Important loci represent a finite set of key regulatory genes

“Key” means location in the regulatory network (nodes)

Engineered Models

Allows controlled experimental testing of

• specific genes• specific environmental

conditions or exposures

Ideally suited to test specific hypothesis generated from human population studies or other laboratory findings

Engineered Models

Transgenics• usually used to over-express

genes• can be global or tissue-specific• can be temporally regulated

Knockouts/knockins• usually used in inactivate genes• can be global or tissue-specific• can be temporally regulated• can introduce genes into a

foreign locus• can make amino acid

modifications

UV Mutagenesis in Yeast

Geneticists need variation to study the function of gene products.

We create variation in the laboratory by mutagenesis

Fig. 7.2

Fig. 7.6

Fig. 7.12b1

Fig. 7.12b2

By choosing the correct mutagens, we can control the type of mutations we make

Fig. 7.7

Photoreactivation requires photolyase enzyme

Mutagenesis of yeast

haploid

Irradiate with UV. Calculate survival curveSelect optimal dose for isolation of mutations.

Select on appropriate selective media:

Replica plating to identify nutrient deficiencies.