Hunting the gene L4 and 5 Flashcards
What is a problem with genetic mapping?
The assumption that crossing over occurs randomly along a chromosome is an oversimplification as:
1) frequency of chiasmata increases further away from the centromere
2) chiasmata formation anywhere reduces chance of second crossover in adjacent region (interference)
3) Females in many species have mose crossovers than males (at the extreme in Drosophila there is no male recombination)
What are better methods than genetic mapping?
Restriction mapping or sequencing
What is an example of an incomplete dominance relationship?
In Antirrhinum flower colour (red, pink, white)
What is an example of codominance?
MN blood group system in humans, MN children express both phenotypes simultaneously
Why do dominant and recessive genes not strictly speaking exist?
Dominance relationships are defined at the level of the phenotype
How is sickle cell anaemia a complex dominance relationship?
Hb(A) Homozygous- normal
Hb(S) Homozygous- sickle cell rbc shape and severe anaemia
Hb(A)Hb(S) - not anaemic but some sickle red blood cells show themselves in low oxygen conditions (incomplete dominance for cell morphology)
2 haemoglobin variants can be detected by electrophoresis in blood- codominance at level of protein expression
What is an example of sex-linkage in drosophila?
Red eye and white eye see notes (for drosophila XY is male same as humans)
What are some-other sex linked genes and what pattern is observed with all of these?
Red-green colour blindness, haemophilia, DMD
Many more affected males than females
What is a lower eukaryotic fungus that many genetic experiments were performed on and what extended phase does it have?
Sacharomyces Cerevisiae, has extended haploid phase
Draw out the altering haploid-diploid phases in the fungi
see notes
What is a meiocyte?
A cell that differentiates into a gamete by meiosis
Why are fungi suited to genetic analysis?
When analysing progeny there are no dominance relationships to worry about (progeny of yeast meiosis is a tetrad of 4 spores that germinate to give haploid vegetative cells)
Isolating 4 products of 1 meiosis enables sophisticated genetic analysis
Draw out fungi cross of yellow-spored and biotin-requiring fungi
see notes
What is epistasy?
Effect of one gene is masked by the effect of another eg. eye absent in Drosophila
Describe the 4 genetic loci involved in determining mouse coat colour
see notes
Compare mitochondrial and human genome size
Human 3x10^9 bp
Mitochondrial 2x10^4 bp
When was non-mendelian cytoplasmic inheritance first observed?
Carl Correns 1910 breeding variegated 4 o’clock plants
Phenotype of progeny matched type that contributed egg and source of pollen was irrelevant
If egg-bearing parent has variegated phenotype what is phenotype of offspring?
Variegated, green or white
Because chloroplasts are randomly distributed to daughter cells and now and then (rarely) you get an all-green or all-white daughter cell, called Mitotic Segregation
Explain differences in variegated/green/white leaf colour
White chloroplasts result from mutation to chloroplast genome
Chloroplasts transmitted to progeny via egg cytoplasm, none from pollen
How can you study cytoplasmic inheritance in yeast?
Petite mutants grow slowly on a medium with glucose as a carbon source but can’t grow when glucose is replaced by glycerol
shows defect in mitochondrial function
What are the two types of yeast petite mutant?
1) segregational petites, mutation in nuclear genome, 1:1 WT:petite ratio in haploid progeny
2) Cytoplasmic petites, diploids with only petite mutant mitochondria fail to go through meiosis and sporulate
Absence of all petite tetrads
Why is genetic mapping hard in prokaryotes?
Absence of conventional sexual cycle
Draw T4 phage and its components
see notes
How many genes does the chromosome of a typical bacterial phage have eg. lambda, T4?
~50
What is a phage life cycle?
1) phage attaches to outside of bacterial host, injects chromosome into victim
2) genes on phage chromosome direct the replication of the chromosome inside the cell and makes the proteins that comprise the phage coat
3) New phage particles self-assemble in the bacterium
4) New phage particles are released when the cell is lysed
How can phage particles be detected?
Spread dilute phage suspension onto lawn of sensitive bacteria, presence of individual phage is indicated by presence of clear regions called plaques that form due to repeated infection cycles
What are advantages of using a phage
1) Short generation time 2) simplicity 3) high number of progeny, one yields up to 10^10 phage
What did benzer do in 1955?
Used a t4 phage looking at the r11 locus (r stands for Rapid lysis), phages with mutation at this locus infect and lyse E Coli B, leading to large clear plaques on bacterial lawn
However they can’t lyse E Coli K
If 2 r11 mutants are crossed by double infection of E Coli B wild type recombinants can infect E Coli K
(Reason you get recombination is that partial diploids are generated during infection)
How many mutants did Benzer analyse and what minimum recombination frequency did he observe?
Over 2000, observed minimum recombination frequency of 0.01% between adjacent mutations
What does min recombination frequency show?
There is a basic unit of recombination, a genetic counterpart to the base pair
