lecture 7: tetrad analysis - jcu.czkmb.prf.jcu.cz/userfiles/media/file/slides_for_lecture_7.pdf ·...
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Lecture 7: Tetrad analysis
1. Life cycle of Neurospora2. Gene to centromere mapping3. Mapping the distance between two genes
4. Mitotic segregation and recombination
Bread mold: fungus Neurospora crassa
A colony on a petri dish Growing mycelium
a haploid organism with 7 chromosomes n = 7
Life cycle of Neurospora
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 ofM 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
Each particular pattern is equally frequent as any other MII pattern
25% 25% 25% 25%
AAaaAAaa
aaAAaaAA
AAaaaaAA
aaAAAAaa
Frequency of asci with MII patterns is used to determine map distance between the locus Aand 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 max (A, CM) = 33%
RF (A, CM) = ½ * MII asci / Total asci
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) =
recombinantstotal
NPD + ½ TTotal asci
=
=
additional types of DCO result in PD or T and, therefore, remain unnoticed
PD>>NPD
Mitotic non-disjunction: weird stuff
First described by C. Bridges in 1930-ies
M dominant allele for slender bristles in Drosophila
M/M+, slender bristles
M+/M+, normal bristles
Mitotic non-disjunction: weird stuff
M dominant allele for slender bristles in Drosophila
M/M+, slender bristles
M+/M+, normal bristles
a Aa A
mitotic non-disjunction may reveal recessive alleles
A/A/a
a
Mitotic recombination
Discovered by Curt Stern in 1936Drosophila X-linked genes:
y, yellow body sn, singed (short, curly bristles)
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