The Sordaria Crossing Over simulation is based on a classic AP Biology Lab (part of Investigation 7 in AP Biology Investigative Labs). The primary objective of this lab is to calculate the distance in map units between a gene (the spore color gene) and the chromosome centromere. An additional perk of the lab is that it introduces students to an alternate life cycle.
Intro to Sordaria
In a high school setting, we mostly deal with species that follow a diploid adults --> meiosis --> haploid gametes --> fertilization --> diploid offspring cycle. Sordaria fimicola, a fungus, follows a different cycle; haploid adult --> fusion/fertilization --> diploid zygote --> meiosis --> haploid spores. While memorizing the details of various types of life cycles is not necessary for the AP Biology exam, understanding that variations do exist is important.
In both life cycles, haploid cells fuse to form diploid zygotes. In the Sordaria life cycle, this zygote undergoes meiosis, forming 4 haploid cells. These haploid cells will develop into the ascopores (spores), which are released into the environment to develop into a new individual.
Map units
The lab studies crossing over using ascopore color. Ascopore color is determined by a single gene and can be either black (wild type) or tan. The lab determines the distance of the color gene from the centromere of the chromosome. The breaks that are necessary for crossing over are less likely to occur close to the centromere. Therefore, the more crossing over is observed, the further that gene is from the centromere. Recombinant chromosomes are those that were involved in crossing over (a recombination of the two starting chromosomes), while chromosomes that are unchanged are called parental.
The % crossing over determines the map units/centiMorgans (cM) from the centromere. A higher % of crossing over indicates a greater number of map units. This is a relative measure of distance on chromosomes. As the name suggests, map units are used to map the locations of genes on a chromosome. For additional discussion of map units and finding the distance between genes, see this post.
Identifying crossing over in Sordaria
In the Sordaria lab, crossing over is determined by observing the color pattern of the asci resulting from a cross between black and tan strains of the fungus. A 4:4 pattern indicates that crossing over did not impact the color gene (no crossing over), while any other pattern indicates that there was crossing over.
Why is the % of asci showing crossing over divided by 2?
The map units are determined by dividing the % of asci showing crossing over by 2. This is because, in each ascus that demonstrates crossing over, only half of the spores have a recombinant chromosome. The data is based on the asci (8 individuals), not each individual organism.
We do not divide by 2 when we are looking at animal data because each individual is identified as either being recombinant or not. In an animal back cross (again, see more here), the counted data includes only the gametes that are fertilized, while in Sordaria all four of the cells produced in meiosis will form spores.