Download Lecture 7: Tetrad analysis

Lecture 7: Tetrad analysis
1. Life cycle of Neurospora
2. Gene to centromere mapping
3. Mapping the distance between two genes
4. Mitotic segregation and recombination
Bread mold: fungus Neurospora crassa
A colony on a petri dish
Growing mycelium
Life cycle of Neurospora
a haploid organism
with 7 chromosomes
n=7
Generation of an ordered tetrad within an ascus in N. crassa
= doubled
tetrad
Segregation of alleles during
meiosis in Neurospora crassa:
“M I segregation pattern”
= doubled
tetrad
Two possible types of
M I segregation pattern
a
a
a
a
a
A
A
A
A
A
50%
50%
Cross-over between locus A and
centromere results in
“M II segregation pattern”
Four possible MII patterns
25%
A
A
a
a
A
A
a
a
25%
a
a
A
A
a
a
A
A
25%
A
A
a
a
a
a
A
A
25%
a
a
A
A
A
A
a
a
Each particular pattern is equally frequent as any other MII pattern
Frequency of asci with MII patterns is used to
determine map distance between the locus A
and the centromere
RF (A, CM) = ½ * MII asci / Total asci
Why to multiply by ½ ?
...because in any MII
ascus only ½ of the
spores are recombinant
Two simple formulas to remember
RF (A, CM) = ½ * MII asci / Total asci
RF max (A, CM) = 33%
Gene-to-gene mapping: only three types of tetrads are possible
for two pairs of alleles (regardless of linkage)
Case 1: No linkage between the loci a and b
PD=NPD
Case 1: No linkage between the loci a and b
in Tetratype: 50% parentals = 50% recombinants
A cross-over in each of the two chromosomes would
produce PD or NPD with equal frequency (again PD=NPD)
Case 1: No linkage between the loci a and b - summary
PD=NPD
total parentals =
total recombinants
(RF = 50%)
Case 2: the loci a and b are linked
RF (a,b) =
=
recombinants
total
NPD + ½ T
=
Total asci
PD>>NPD
additional types of DCO result in PD or T and, therefore, remain unnoticed
Mitotic non-disjunction: weird stuff
M dominant allele for
slender bristles in
Drosophila
M/M+, slender bristles
M+/M+, normal bristles
First described by C. Bridges in 1930-ies
Mitotic non-disjunction: weird stuff
M dominant allele for
slender bristles in
Drosophila
M/M+, slender bristles
M+/M+, normal bristles
a
a
A
A
A/A/a
a
mitotic non-disjunction may reveal recessive alleles
Mitotic recombination
Drosophila X-linked genes:
y, yellow body
sn, singed (short, curly
bristles)
Discovered by
Curt Stern in 1936
A heterozygote y+ sn / y sn+ looks mostly
normal, but shows strange spots on the
body with recessive phenotype
Homologous chromosomes pair up and
even cross-over during mitosis