Lecture 20: Developmental Genetics Flashcards
(58 cards)
How are all living forms related?
- evolution has conserved basic strategies of development across multicellular eukaryotes
- many homologous proteins have highly conserved functions
give examples of homologous genes in different organisms
- eyeless (Drosophila), Pax-6 (mouse), and Aniridia (humans) genes are homologous
- Pax-6 and Aniridia wild-type genes can direct eye development in Drosophila
distinguish between forward and reverse genetics
Forward:
Phenotype → Find gene
- Mutagenize, screen for trait, map gene
- Use when you know the trait but not the gene
Reverse:
Gene → Find phenotype
- Knockout/mutate gene, observe effect
- Use when you know the gene but not its function
what is an example of mutagenesis screens in organisms?
- motor axon guidance mechanisms in Drosophila.
- scientists mutagenise C. elegans, observe the phenotype, then try and identify the gene responsible for each phenotype
what is a modifier mutagenesis screen?
type of genetic screen used to find genes that influence the severity or expression of a known phenotype, rather than causing the phenotype on their own.
define redundancy
two or more genes perform overlapping or similar functions, so that loss of one gene has little or no effect on phenotype
why is it not possible to just mutagenise the wild-type organism in a modifier mutagenesis screen?
if you just mutagenise the wild type, you may have a whole bunch of different mutations, but if there’s redundancy you won’t see the phenotype you are studying
what is the function of the sev gene in Drosophila?
sev is a receptor tyrosine kinase that activates Ras signaling to specify the R7 photoreceptor.
what is the function of the Ras gene in Drosophila?
When sev is activated by its ligand, it triggers a cascade that activates Ras, which in turn promotes cell differentiation (the specification of the R7 photoreceptor neuron).
what is Ras^G12V?
gain of function mutation - this mimics constant “on” signaling, so R7 cells develop even without the proper upstream signal, or develop abnormally, causing things like a rough eye phenotype.
What does the “modifier mutagenesis screen” using sev::Ras^G12V in Drosophila eyes reveal about genetic interactions?
- Wild type: Normal eye structure.
- sev::Ras^G12V: Rough eye due to hyperactive Ras signaling.
- Enhanced phenotype (E⁻/E⁺; sev::Ras^G12V): worsened mutant phenotype → E is a negative regulator of Ras.
- Suppressed phenotype (S⁻/S⁺; sev::Ras^G12V): More normal eye → S is a positive regulator of Ras.
Use of Genome wide screens
- we can design RNAs that can target specific genes and inactivate them, similar to a knockout
- this produces a phenocopy of a loss of function mutation
define a phenocopy
a phenotype that mimics the effects of a gene knockout, caused by methods like RNAi or CRISPR.
4 ways in which we can analyse developmental pathways
Nature of the encoded protein:
- we can infer amino acid sequence from nucleotide sequence and use computer searches to identify known motifs
Location and timing of gene expression:
- during development, where and when is the mRNA found?
Location of the protein product:
- during development, where and when is the protein found?
Developmental phenotypes:
- what cells or tissues are affected by loss-of-function?
two methods to follow the localisation of proteins
- attach antibodies to the protein
- tag the proteins with GFP
why must the interactions of genes in a developmental pathway be determined?
genes don’t work in isolation, and many biological processes are complicated and require the coordinated action of many genes
two key steps to ordering genes in a pathway
analysis of effects of one gene on expression of another gene:
- does a mutation in one gene affect the level or distribution of mRNA or protein from another gene?
analysis of double mutants - epistatic interactions:
- do mutations in two different genes define successive steps in a pathway?
give an example of the impacts that gene products can have on the expression of other genes
- staining of Drosophila wing imaginal disks for wingless protein (Wg, green) and vestigial protein (Vg, red)
- wild-type produces a different staining pattern than wingless mutant
- this suggests that the wingless gene product is required for the expression of the vestigial gene
2 requirements for an epistasis analysis:
- phenotypes of the two mutants must differ
- alleles must either be null or constitutive
null allele
A mutant allele that results in complete loss of function of the gene.
constitutive allele
A mutant allele that causes the gene to be continuously active, regardless of normal regulatory signals.
use the sevenless signalling pathway as an example of epistasis analysis
- the phenotype of the double mutants reveals epistatic interactions
- in a double mutant, the phenotypes the same as the Ras^G12V mutant, therefore seven less is upstream of Ras
- the downstream gene is the one that is epistatic
- thus, Ras is downstream and epistatic to sevenless (‘masks’ sevenless)
what is the programmed cell death pathway and how was it understood?
- eliminates unwanted or damaged cells
- was understood by epistatic analysis screens
relationship between embryonic and adult body axes in drosophila
each embryonic segment defines a specific structure in the adult: 3 head segments, 3 thoracic segments, 8 abdominal segments
