Overview
Outline Course Week 1 2 3 4 5 6 7 8 9 10 11 12 Topic Course objectives and Introduction to genetics Human Pedigrees Patterns of Inheritance: sex-linkage Chromosomal basis of inheritance Changes in chromosome number Gene Mapping Gene to Phenotype Modified Mendelian ratios Model organisms and mutants Genetics of Plant Development (Arabidopsis) Genetics of Animal Development (Drosophila) Behaviour Genetics/Quantitative genetics Chapter Ch. 1 & Ch. 2 Ch. 2 Ch. 2 Ch. 3 Ch. 15 Ch. 4 Ch. 6 Ch. 6 Ch. 6 &Ch. 16 Ch. 18 Ch. 18 Ch. 16 + papers
Model organisms and mutants
http://www.erin.utoronto.ca/~w3bio/bio207/index.htm
March 6
Gene mutation
Gene mutation: a gene can mutate from one allelic form to another
Forward mutation:
A+ to a D+ to D
Model organisms
Selected by researchers based on some feature that renders it particularly useful for studying the genetic process of interest to that researcher. Each model organism usually has a database and a community of researchers
Reverse mutation:
a to A+ D to D+
Mutations can be used for two purposes:
To study the process of mutation itself To permit the dissection of a biological function
Text p. 14-15 & 707-23
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Drosophila melanogaster
Database: FlyBase < http://flybase.org> Mutations in fruit flies
E.g. Vestigial wings (vg)
Drosophila melanogaster
Life cycle of a fruit fly
Drosophila melanogaster
Lethal genes: some alleles of a gene are lethal in the homozygous form. This could be the homozygous recessive or the homozygous dominant genotypes that are lethal. This changes the ratios.
For example with the Cy (curly) gene in Drosophila it is lethal as a homozygous dominant Cy/Cy so the phenotypic ratio you get from a hybrid cross is 2:1. When calculating the ratio you do it from the progeny that live (these are the only ones you can score!).
Lethality in Drosophila
Cy is a homozygous lethal mutation Cross two heterozygotes
Cy/+ X Cy/+
Cy /+ X Cy/+
Cy/Cy Cy/+ +/Cy +/+ dies lives lives lives Curly Curly wildtype
Get a 2:1 ratio of Curly to wildtype
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ClB chromosome
In 1928 Herman Muller devised a method of searching for any lethal mutation on the X-chromosome (chromosome 1 in Drosophila) He constructed a chromosome called ClB which carries an inversion a cross-over suppressor (C), a lethal (l) and a dominant mutation Bar-eye (B) marker.
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Balancers
Since Muller constructed the ClB chromosome many other chromosomes bearing inversions have been constructed Chromosomes bearing multiple inversions are used to suppress crossing over and are often marked with recessive lethal mutations Only offspring that are heterozygous for the inversion AND the lethal mutation survive Balancers e.g.:
FM7 First chromosome balancer SM6 Second chromosome balancer marked with Cy TM3 Third chromosome balancer marked with Stubble
Deletion mapping
Deletions change dominance relationships Pseudodominance: recessive alleles appear dominant over a deletion
Eg pn shows pseudodominance with deletion 264-38
Mutations and screens
Making mutations in Drosophila melanogaster Enhancer trap screens Transposable elements are used as mutagens because as they hop they disrupt the integrity of a gene. The gene is identified by transposon tagging (see chapter 13 p438) Reverse genetics also uses P-elements to insert DNA into the fly
Recessive lethal mutations are maintained over a balancer chromosome
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Mutations and screens
Making mutations in Drosophila melanogaster Enhancer trap screens Transposable elements are used as mutagens because as they hop they disrupt the integrity of a gene. The gene is identified by transposon tagging (see chapter 13 p438) Reverse genetics also uses P-elements to insert DNA into the fly
Mus musculus
Database: <http://www.informatics.jax.org> MGI Mouse coat colour genetics
5
Lethal genes
Lethal genes: mutant alleles capable of causing the death of the organism Mouse coat colour mutation yellow is homozygous lethal allele of Agouti (A)
yellow X yellow gave
2/3 yellow, 1/3 wildtype progeny
Lethal genes
AY / A
x
AY / AY AY / A A/A
AY / A
lethal yellow wildtype (Agouti, Black)
Describe with symbols what you think is happening
Mice
Mus Musculus Specific fragments of DNA can be inserted into the genome of mice
E.g. the jellyfish gene for green fluorescent protein
Text ch. 11 p.374
6
Neurospora crassa
N. crassa Growth mutants Database: http://www.mips.biochem.mpg.de/proj/neurospora/) Genome sequenced in 2003
Text ch. 21 p. 700
Neurospora sex
Mating type is a simple form of sex Mating types are determined by two alleles of a single gene: MAT-A and MAT-a. Crosses between the two haploid mating types produces diploid meiocytes then ascospores
Forward genetics
Forward genetics often uses genetic selection protocols in which only the desired mutant phenotype survives
E.g. nutritional mutations in N. crassa.
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Testing strains
20 progeny of a cross ad leu+ X ad+ leu What is the genotype of culture 8? What is the genotype of culture 9?
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Testing strains
20 progeny of a cross ad leu+ X ad+ leu What is the genotype of culture 8?
ad leu+
Neurospora crassa
Forward genetics often uses genetic screens protocols in which the desired mutant phenotype is selected from among a large number of individuals
E.g. growth mutants in Neurospora
What is the genotype of culture 9?
ad leu
Reverse genetics
In Neurospora a useful mechanism for targeting mutations to a specific gene is Repeat-Induced Point mutation (RIP) A strain with an ectopically induced wildtype transgene is crossed In the resulting strains the transgene has many mutations (GC AT transitions)
9
Caenorhabditis elegans
C. elegans
Database: WormBase http://www.wormbase.org Nematode, a soil-dwelling roundworm Hermaphrodite Transparency makes it easy to for screen for mutations
Synthetic lethals
Synthetic lethals are formed when two viable single mutants are intercrossed and the resulting double mutant is lethal
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Reverse genetics: RNA...