Fungi from the class Ascomycetes produce an ascus sac that has all four products of meiosis. (Figure 8.1)
When crossing parents that differ for two genes that are located on different chromosomes, two obvious outcomes can occur. (Figure 8.2)
In one case, spores are like those found in the parents. These are called the parental ditypes.
Another is when they are not like their parents, but there are only two types of spores. These are called nonparental ditypes. These should be equal in frequency with parental ditypes.
A third group is sometimes found, depending on the frequency of crossovers between the genes and centromeres. These are called tetratypes.
When crossing parents with alleles for two genes that are on the same chromosome, linked genes, we observe four different types of spores. (Figure 8.3)
The number of parental and nonparental ditypes will likely be quite different.
The nonparental ditypes are generated by two, single crossovers.
We can calculate gene distances from collected data on numbers of different asci. (See Figure 8.4)
Centromere Mapping with Ordered Tetrads in Neurospora
In Neurospora, the products of meiosis, the spores, are arranged in the order that they were generated during meiosis.
Because of this, we can map relative to the centromere because the centromere can be followed. Remember, the homologous centromeres separate from each during the first meiotic division.
See Figure 8.7 and 8.9. The equation is: 1/2 X (second division segregation asci / total).
Mapping Using Deletions in Drosophila
Flies with recessive phenotypes are crossed to flies that are heterozygous for a known deletion. If the recessive trait appears, it must be within that deleted area. See Figure 8.11.
Linkage in Humans
The X-chromosome is easy to study because males have but one, and we can see what genes he has. We also know what genes the daughters got from their father.
Other genes require the analysis of numerous pedigrees.
Somatic Cell Fusions for Mapping Chromosomes
Fuse two cells together.
One of the cells is TK-, the other is HPRT- (hypoxanthine phosoribosyl transferase (minus))
Cells are placed into HAT medium (Hypoxanthine, aminopterin, thymidine containing medium).
Only fused cells survive.
Cells lose chromosomes. Relate presence or absence of a protein or enzyme to the presence or loss of a chromosome.
See Figures 8.16 and 8.17. Look at your Table 8.2.