Hunting the gene L4 and 5

Question 1

What is a problem with genetic mapping?

Answer 1

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)

Question 2

What are better methods than genetic mapping?

Answer 2

Restriction mapping or sequencing

Question 3

What is an example of an incomplete dominance relationship?

Answer 3

In Antirrhinum flower colour (red, pink, white)

Question 4

What is an example of codominance?

Answer 4

MN blood group system in humans, MN children express both phenotypes simultaneously

Question 5

Why do dominant and recessive genes not strictly speaking exist?

Answer 5

Dominance relationships are defined at the level of the phenotype

Question 6

How is sickle cell anaemia a complex dominance relationship?

Answer 6

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

Question 7

What is an example of sex-linkage in drosophila?

Answer 7

Red eye and white eye
see notes (for drosophila XY is male same as humans)

Question 8

What are some-other sex linked genes and what pattern is observed with all of these?

Answer 8

Red-green colour blindness, haemophilia, DMD

Many more affected males than females

Question 9

What is a lower eukaryotic fungus that many genetic experiments were performed on and what extended phase does it have?

Answer 9

Sacharomyces Cerevisiae, has extended haploid phase

Question 10

Draw out the altering haploid-diploid phases in the fungi

Answer 10

see notes

Question 11

What is a meiocyte?

Answer 11

A cell that differentiates into a gamete by meiosis

Question 12

Why are fungi suited to genetic analysis?

Answer 12

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

Question 13

Draw out fungi cross of yellow-spored and biotin-requiring fungi

Answer 13

see notes

Question 14

What is epistasy?

Answer 14

Effect of one gene is masked by the effect of another eg. eye absent in Drosophila

Question 15

Describe the 4 genetic loci involved in determining mouse coat colour

Answer 15

see notes

Question 16

Compare mitochondrial and human genome size

Answer 16

Human 3x10^9 bp

Mitochondrial 2x10^4 bp

Question 17

When was non-mendelian cytoplasmic inheritance first observed?

Answer 17

Carl Correns 1910 breeding variegated 4 o’clock plants

Phenotype of progeny matched type that contributed egg and source of pollen was irrelevant

Question 18

If egg-bearing parent has variegated phenotype what is phenotype of offspring?

Answer 18

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

Question 19

Explain differences in variegated/green/white leaf colour

Answer 19

White chloroplasts result from mutation to chloroplast genome
Chloroplasts transmitted to progeny via egg cytoplasm, none from pollen

Question 20

How can you study cytoplasmic inheritance in yeast?

Answer 20

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

Question 21

What are the two types of yeast petite mutant?

Answer 21

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

Question 22

Why is genetic mapping hard in prokaryotes?

Answer 22

Absence of conventional sexual cycle

Question 23

Draw T4 phage and its components

Answer 23

see notes

Question 24

How many genes does the chromosome of a typical bacterial phage have eg. lambda, T4?

Answer 24

~50

Question 25

What is a phage life cycle?

Answer 25

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

Question 26

How can phage particles be detected?

Answer 26

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

Question 27

What are advantages of using a phage

Answer 27

1) Short generation time 2) simplicity 3) high number of progeny, one yields up to 10^10 phage

Question 28

What did benzer do in 1955?

Answer 28

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)

Question 29

How many mutants did Benzer analyse and what minimum recombination frequency did he observe?

Answer 29

Over 2000, observed minimum recombination frequency of 0.01% between adjacent mutations

Question 30

What does min recombination frequency show?

Answer 30

There is a basic unit of recombination, a genetic counterpart to the base pair