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