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Tamarin: Principles of Genetics, Seventh Edition
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II. Mendelism and the Chromosomal Theory
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6. Linkage and Mapping in Eukaryotes
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The McGraw Hill Companies, 2001
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Haploid Mapping (Tetrad Analysis)
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b b b+ b+
PD ab ab a+b+ a+b+ NPD
Nonrecombinant
Independent assortment
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b b b+ b+
Independent assortment
ab+ ab+ a+b a+b
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b+ b+ b b
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ab ab+ a+b a+b+
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Formation of parental ditype (PD), nonparental ditype (NPD), and tetratype (TT) asci in a dihybrid yeast by linkage or independent assortment at meiosis. Open circles are centromeres.
crossing over adds. A type A centromere is always a type A centromere, whereas, due to crossing over, a chromosome attached to that centromere may be partly from the type A parent and partly from the type a parent.) Before the ascospores mature in Neurospora, a mitosis takes place in each nucleus so that four pairs rather
than just four spores are formed. In the absence of phenomena such as mutation or gene conversion, to be discussed later in the book, pairs are always identical ( g. 6.21). As we will see in a moment, because of the ordered spores, we can map loci in Neurospora in relation to their centromeres.
Tamarin: Principles of Genetics, Seventh Edition
II. Mendelism and the Chromosomal Theory
6. Linkage and Mapping in Eukaryotes
The McGraw Hill Companies, 2001
Six
Linkage and Mapping in Eukaryotes
(n) (n)
Fertilization (2n) Aa
Vegetative hyphae (n)
Ascus (2n) (n)
Asexual spore
Vegetative hyphae
Meiosis
(n) (n)
a a A A
Ascospores (n)
Mitosis
a a A A
Life cycle of Neurospora. A and a are mating types; n is a haploid stage; 2n
is diploid.
First and Second Division Segregation
Recall that there is a 4:4 segregation of the centromeres in the ascus of Neurospora. Two kinds of patterns appear among the loci on these chromosomes. These patterns depend on whether there was a crossover between the locus and its centromere ( g. 6.22). If there was no crossover between the locus and its centromere, the allelic pattern is the same as the centromeric pattern, which is referred to as rst-division segregation (FDS), because the alleles separate from each other at meiosis I. If, however, a crossover has occurred between the locus and its centromere, patterns of a different type emerge (2:4:2 or 2:2:2:2), each of which is referred to as second-division segregation (SDS). Because the spores are ordered, the centromeres always follow a rstdivision segregation pattern. Hence, we should be able to map the distance of a locus to its centromere. Under the simplest circumstances ( g. 6.22), every second-division segregation con guration has four recombinant and four nonrecombinant chromatids (spores). Thus, half of the chromatids (spores) in a second-division segregation as-
cus are recombinant. Therefore, since 1% recombinant chromatids equal 1 map unit, map distance (1 2) the number of SDS asci total number of asci 100
An example using this calculation appears in table 6.5. Three-point crosses in Neurospora can also be examined. Let us map two loci and their centromere. For simplicity, we will use the a and b loci. Dihybrids are formed from fused mycelia (ab a b ), which then undergo meiosis. One thousand asci are analyzed, keeping the spore order intact. Before presenting the data, we should consider how to group them. Since each locus can show six different patterns ( g. 6.22), two loci scored together should give thirty-six possible spore arrangements (6 6). Some thought, however, tells us that many of these patterns are really random variants of each other. The tetrad in meiosis is a three-dimensional entity rather than a at, four-rod object, as it is usually drawn. At the rst meiotic division, either centromere can go to the left or the right, and when centromeres split at the second mei-
Tamarin: Principles of Genetics, Seventh Edition
II. Mendelism and the Chromosomal Theory
6. Linkage and Mapping in Eukaryotes
The McGraw Hill Companies, 2001
Haploid Mapping (Tetrad Analysis)
Meiosis I
Meiosis II
Mitosis
Eight spores formed
Figure 6.21 Meiosis in Neurospora. Although Neurospora has seven pairs of chromosomes at meiosis, only one pair is shown. A and a, the two mating types, represent the two centromeres of the tetrad.
otic division, movement within the future half-ascus (the four spores to the left or the four spores to the right) is also random. Thus, one genetic event can produce up to eight different patterns. For example, consider the arrangements gure 6.23 shows, in which a crossover occurs between the a and b loci. All eight arrangements, producing the ascus patterns of table 6.6, are equally likely.The thirty-six possible patterns then reduce to only the seven unique patterns shown in table 6.7. Note also that these asci can be grouped into the three types of asci found in yeast with unordered spores: parental ditypes,
nonparental ditypes, and tetratypes. Had we not had the order of the spores from the asci, that would, in fact, be the only way we could score the asci (see the bottom of table 6.7).
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