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 Lifecycle Sordaria is an alternating diploid (2n) and haploid (n) organism Alternation of generations Haploid Mycelium (n) Ascospores (n) Diploid Sporophyte

Sordaria Color Sordaria are either tan or wild type (dark brown) If grown together they will form hybrid Sordaria Hybrids are diploid sordaria with both the tan and wild type alleles Meiosis followed by mitosis create spore pods with 8 spores, called asci Each spore shows the 1 color it inherited

Asci Each spore shows a specific color The 8 spores form patterns which depend on whether crossing over occured

Spore Patterns 4:4 = no Xing over 2:4:2 = 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