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