lecture 9 and 10 - allelism and prelude to lab Flashcards
what is albinism and what are their children’s phenotypes?
Albinism: Recessive - failure to make melanin
Two albino parents can have EITHER
i) ALL albino children
ii) NO albino children
connection between genotype and phenotype is not necessarily obvious
another example is RECESSIVE Mutations in OCA2 gene can result in being
BLUE-EYED or in ALBINISM
what are the phenotypes of bald / non bald mutants?
A+ OR B+ = hair
whereas
bald mutants could be a-/a- B+/B+or A+/A+ b-/b- = bald
lower case a- means that the precursor is crossed as the mutant is at gene A and a lower case b- at gene B could cross out the intermediate
Mutations in DIFFERENT genes can result in the
SAME PHENOTYPE which means
RECESSIVE mutations affecting different
enzymes that act in the same
Metabolic Pathway
e.g., MAKING or DEGRADING
amino acids, histidine, methionine, leucine, tryptophan…nucleosides ,adenine, cytosine
what can we assume that individuals with…
the SAME phenotype are defective in the SAME gene
DIFFERENT phenotypes are defective in DIFFERENT genes
NEED independent, rigorous tests of ‘ALLELISM’
(i.e., are mutations alleles of each other = same gene
or
Are mutations on different genes?)
what can we cannot simply assume that mutations that result in…
the SAME phenotype affect the SAME gene
DIFFERENT phenotypes affect DIFFERENT genes
NEED independent, rigorous tests of ALLELISM
a) the Complementation test
b) Segregation pattern (non examinable)
what is the allelism test?
the complementation test also known as the cis-trans test
its Easy Definitive But ONLY works if both mutations being tested are RECESSIVE
by Ed Lewis, Caltech (1918-2004)
what are the steps to perform the complementation test in diploid organisms?
Step 1: determine if mutations are RECESSIVE
Proceed ONLY if they are recessive (see later)
Step 2: cross the HOMOZYGOUS mutants
(ALL parents have same phenotype in this example but might have different genotype)
Step 3: phenotype (‘look at’) F1 progeny (= the children)
if behave as MUTANTthe mutations do NOT complement Parents MUST be mutant in the SAME gene
if behave as WILD TYPE the mutations COMPLEMENT Parents MUST be mutant in DIFFERENT genes
Example: the crossover of 2 bald = more bald offspring = do not ‘complement’
the crossover of 2 bald = non bald offspring = complementation
If presented with bald individuals. What is their genotype?
Could sequence genome etc. (but need to know genes already and fully understand the mutations)
Or [more simply, cheaply and universally]
Just mate them to each other (in an ethically appropriate way of course)
Lets assume that all mutations here are recessive
And we will consider the 3 possible cases of..
Parents defective in the same gene: both a-/a-
Parents defective in the same gene: both b-/b-
Parents defective in different genes: a-/a- X b-/b-
what is case 1- Parents defective in the same gene: both a-/a-…
ALL children have the SAME genotype
as each other
as their parents
All children are homozygous mutant: a-/a-
or more correctly: a-/a- B+/B+
precursor is scored out in this example
If parents are mutant in the SAME gene (here, gene A)then their children also show MUTANT PHENOTYPE
what is case 2? Parents defective in the same gene: both b-/b-…
ALL children have the SAME genotype
as each other
as their parents
All children are homozygous mutant: b-/b-
or more correctly: A+/A+ b-/b-
the precursor is normal but the intermediate is crossed out
If parents are mutant in the SAME gene (here gene B)then their children also show MUTANT PHENOTYPE
what is the generalised rule?
If parents are mutant in the SAME gene as each other then their children also show MUTANT PHENOTYPE
what is case 3? Parents defective in different genes: a-/a- X b-/b-…
ALL children have the SAME genotype
as each other BUT different from their parents
All children are double heterozygous:a-/A+ b-/B+
the precursor is normal and so too is the intermediate which results in hair
When parents are mutant in DIFFERENT genes:
All children behave the same as each other
All different from their parents
All behave as wild-type
i.e., The mutations COMPLEMENT each other
what is complementation in albinism?
Four Oculocutaneous Albinism Genes:increasing melanin and reduction of tyrosine as you go down the genes
OCA1: tyrosinase enzyme
mutants have severe albinism
OCA2: P Protein (tyrosinase ‘helper’)
mutants have mild albinism
OCA3: tyrosine-related gene (very rare)
mutants have weak albinism
OCA4: SLC45A2 Protein (tyrosinase helper)
mutants have mild albinism
Several other albinism-like syndromes are described (Hermansky-Pudlak Syndrome)
why must mutants be recessive?
If ONE mutation is DOMINANT: then the children will ALWAYS behave as MUTANT, whether parents mutant in same gene or not
TEST FAILS:
cannot deduce anything
If parents have DOMINANT mutations then test comes out as follows-
- If parents mutant in same gene: 2 crossover of balds = bald
- If parents mutant in different genes 2 crossover of balls = bald offspring
The same outcome = uninformative
= cannot deduce anything about the parents
BUT If parents have RECESSIVE mutations then test comes out as follows
- If parents mutant in same gene: 2 bald crossover = bald offspring
- If parents mutant in different genes : 2 bald offspring crossover = offspring with hair
DIFFERENT outcomes = Informative = Can deduce something about the parents
what are genetic model organisms?
Prokaryote: (phage) E coli
Eukaryotes:
yeast Saccharomyces cerevisiae
worm Caenorhabditis elegans
fly Drosophila melanogaster
fish Zebrafish
plant Arabidopsis thaliana
mammal Mus Musculus
what are the characteristics of genetic model organisms?
sex (controllable)
cheap to maintain (small….)
lots of phenotypes (actually universally true – just think harder…)
reproduces quickly
lots of offspring
others working on it
standardized backgrounds - everyone works with same strains
MONEY: industrial relevance, research grant, path to impact..
minimal ethical concerns
(tasty? Probably not.. Cannot eat them if genetically-modified.
eating experiments is never recommended)
what is the purpose of the model organism
The organism is not (usually) the topic of study
A model organism is mostly a TOOL (equivalent to a microscope)
A window on specific areas of biology
Different model ~ different aspects of biology are in sharp focus
what are the types of model organisms?
Mouse physiology, neurobiology..
Fly/worm multicellular development (simple organ systems)
Yeast basic cell biology
The process you want to study dictates the best system
look at the screenshot for the life cycle of yeast but what are the 3 cell types of yeast?
2 HAPLOID (a, alpha) and 1 DIPLOID (a/alpha)
how do haploids make genetics?
very very easy as WT = wild type phenotype and
MUT = mutant phenotype Recessive AND dominant mutations affect phenotype
in haploids
what is the dominance test?
mututant a cell crossed over with an alpha wild type cell
= a / alpha cell
what is the Complementation test: recessive mutations ONLY?
mut1 a cell crossed over with an mut2 alpha cell
= a / alpha cell = phenotype - Mutant: alleles of SAME gene, WT: alleles of DIFFERENT genes
what are wild type yeasts? (phototrophs)
need a source of C, N, P and trace metals and vitamins.
Can manufacture all its own complex building blocks such as amino acids e..g.,histidine, leucine, methionine, nucleosides e.g.,adenine
Can grow on Minimal Medium (MM)
what are auxotrophs?
need a source of C, N, P and trace metals and vitamins
AND
the end product of the metabolic pathway they are unable to perform.
They CANNOT grow on Minimal Medium (MM).
However, they CAN grown on appropriately supplemented
medium:
Histidine (His-) auxotrophs can grow on MM + histidine
Adenine (Ade-) auxotrophs can grow on MM + adenine
One of the first mutants isolated in yeast (and fungi: see Tatum and Beadle’s work on Neurospora crasa in textbook) are unable to make one (or more) of the metabolites they need
what are the Tools for plating Micro-organisms on test media (yeast, bread mold, bacteria, etc)
Petri dishes for growing up colonies
Tooth picks for sampling colonies
Minimal medium (MM)
Supplemented MM (MM plus one additive)
Complete medium (MM with every known
useful additive)
how do we plate out culture?
1- Organisms are grown in liquid nutrient medium
2- Diluted And spread on solid agar Medium in a petri dish
3- Each organism grows into a visible colony.
A colony therefore contains progeny derived from a single original cell.
how do you check for the phenotype using if a strain is able to grow on medium?
if the strain is able to grow on certain medium, then the phenotype is present in the strain
if able to grow on all it is a prototroph
how can you check a strains genotype from its genotype?
if its phenotype is present then we add on 1 example Ade- = ade1 bt for examples like Ade?his? we do…
Pairwise complementation tests:
BXD (are ade- mutations same or not: ade1 or ade2?)
and for His- mutations need CXD, CXE, DXE
what do we use this to work out?
Until now, we looked at
MM
MM supplemented e.g., + HIS
MM Complete
PROBLEM: for an auxotroph to grow on Supplemented MM, it needs ALL of its auxotrophys addressed e.g.,
ade- his- ura- will only grow in
MM + Adenine, Histidine AND Uracil
Hence, difficult to work with mutants with multiple auxotrophys
but what if we
MM Complete
MM
MM Complete but lacking Histidine (HIS dropout)
MM Complete but lacking Adenine (ADE dropout)
MM Complete but lacking Uracil (URA dropout)
etc.
Should allow clean phenotyping for EACH auxotrophy
AND examine ONE PHENOTYPE at a time
How do we use dropout medium growth score? = look at the screenshot
