Crossing Over in the
Fungus “Sordaria”
AP Biology Lab 3
What you need to know!
 Important differences between mitosis
and meiosis.
 How germ cells use meiosis to become
gametes
 The impact of crossing over on genetic
variation
 How to map genes using crossing over
frequency
Sordaria Mycelium
 Sordaria is a
fungus of the
family
Ascomycetes
(molds)
 It forms a
spreading
microscopic
mycelium (see
below)
Sordaria Lifecycle
 Sordaria exists as an alternating diploid
(2n) and haploid (n) organism
 Alternation of generations
 Its main form is haploid:
 Mycelium (n)
 Ascospores (n)
 When two mycelia fuse (n+n), fertilization
occurs (2n = zygote and sporophyte)
 Followed by meiosis: forming haploid
spores
Sordaria Lifecycle
 Two haploid mycelia
fuse (n+n  2n)
 All chromosomes
then have a
homologous partner
 During the
subsequent meiosis,
crossing over
happens and can be
observed using the
gene for spore color
Sordaria Color
 Sordaria can have
different colored
species, tan and wild
type (black). If grown
in the same Petri dish
they will form hybrid
(mixed) Sordaria where
both cultures meet
Asci
 Each asci (string) will
have 8 spores
 Each spore shows a
specific color (tan or
black)
 The 8 spores form
patterns which depend
on whether crossing
over happened or didn’t
happen
Spore Patterns
 4:4 = no Xing over
 2:4:2 = Xing over
 2:2:2:2 = Xing over
Frequencies and Gene
Maps
 DNA at the centromere do not exchange
during crossing over
 The farther from the centromere a gene is,
the more likely it will cross over
 Calculating crossing over frequency for a
gene will determine how far from the
centromere the gene is
 We use map units as an arbitrary unit for
chromosome measurement