Page 170 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

162 Chapter 5 Linkage, Recombination, and the Mapping of Genes on Chromosomes

Problem 44 at the end of the chapter helps you derive a cor- Figure 5.25 How ordered tetrads form. Spindles form
rected equation for RF. The corrected RF equation takes parallel to the long axis of the growing Neurospora ascus, and the
into account all of the DCO meioses that contribute to the cells cannot slide around each other. The order of ascospores thus
tetrads resulting from a cross where PD >> NPD, but NPD reflects meiotic spindle geometry. After meiosis, each haploid cell
undergoes mitosis, producing an eight-cell ascus (an octad). The
is greater than zero. octad consists of four pairs of cells; the two cells of each pair are
genetically identical.

Two genes far apart on a single chromosome: Meiosis I Meiosis II Mitosis Resulting
PDs = NPDs Metaphase Octad
In tetrad analysis, just as in en masse linkage analysis, two
genes may be so far apart on the same chromosome that
they will be indistinguishable from two genes on different
chromosomes: In both cases, PD = NPD. If two genes are Spindle
sufficiently far apart on the chromosome, at least one cross-
over occurs between them during every meiosis. Under
such circumstances, no meioses are NCOs, and therefore
all PD tetrads as well as all NPD tetrads come from equally
frequent kinds of DCOs (events c and f in Fig. 5.24). Thus, Genetically
whether two genes are assorting independently because identical cells
they are on different chromosomes or because they are far
apart on the same chromosome, the end result is the same:
PD = NPD and RF = 50%. To understand the genetic consequences of the geom-
etry of the ascospores, it is helpful to consider what kinds
of tetrads you would expect from the segregation of two
Ordered Tetrads Help Locate Genes alleles of a single gene. (In the following discussion, you
in Relation to the Centromere will see that Neurospora geneticists denote alleles with
symbols similar to those used for Drosophila, as detailed in
Analyses of ordered tetrads, such as those produced by the the Guidelines for Gene Nomenclature.) The mutant white-
bread mold Neurospora crassa, allow you to map the cen- spore allele (ws) alters ascospore color from wild-type
tromere of a chromosome relative to other genetic markers, black to white. In the absence of recombination, the two
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information that you cannot normally obtain from unor- alleles (ws and ws) separate from each other at the first
dered yeast tetrads. As described earlier, immediately after meiotic division because the centromeres separate at that
specialized haploid Neurospora cells of different mating stage. The second meiotic division and subsequent mitosis
types (A and a) fuse at fertilization, the diploid zygote un- create asci in which the top four ascospores are of one gen-
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dergoes meiosis within the confines of a narrow ascus (re- otype (for instance ws ) and the bottom four of the other
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view Fig. 5.20b). At the completion of meiosis, each of the (ws). Whether the top four are ws and the bottom four ws,
four haploid meiotic products divides once by mitosis, or vice versa, depends on the random metaphase I orienta-
yielding an octad of eight haploid ascospores. Dissection tion of the homologs that carry the gene relative to the long
of the ascus at this point allows one to determine the phe- axis of the developing ascus.
notype of each of the eight haploid cells. The segregation of two alleles of a single gene at the
The cross-sectional diameter of the ascus is so small first meiotic division is thus indicated by an ascus in which
that cells cannot slip past each other. Moreover, during an imaginary line drawn between the fourth and the fifth
each division after fertilization, the microtubule fibers of ascospores of the octad cleanly separates haploid products
the spindle extend outward from the centrosomes parallel bearing the two alleles. Such an ascus displays a first-
to the long axis of the ascus (Fig. 5.25). These facts have division (MI) segregation pattern (Fig. 5.26a).
two important repercussions. First, when each of the four Suppose now that during meiosis I, a crossover occurs
products of meiosis divides once by mitosis, the two ge- in a heterozygote between the white-spore gene and the
netically identical cells that result lie adjacent to each centromere. As Fig. 5.26b illustrates, this can lead to four
other. Because of this feature, starting from either end of equally possible ascospore arrangements, each one de-
the ascus, you can count the octad of ascospores as four pending on a particular orientation of the four chromatids
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cell pairs and analyze it as a tetrad. Second, from the pre- during the two meiotic divisions. In all four cases, both ws
cise positioning of the four ascospore pairs within the as- and ws spores are found on both sides of the imaginary line
cus, you can infer the arrangement of the four chromatids drawn between ascospores 4 and 5, because cells with only
of each homologous chromosome pair during the two one kind of allele do not arise until the end of the second
meiotic divisions. meiotic division. Octads carrying such configurations of