Page 166 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

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

the normally unicellular baker’s yeast (Saccharomyces same things. Yeast make unordered tetrads; that is, the
cerevisiae), is sold in supermarkets and contributes to the four meiotic products (the spores) are arranged at random
texture, shape, and flavor of bread; it generates four asco- within the ascus. Neurospora crassa produce ordered
spores with each meiosis. The other, Neurospora crassa, is tetrads, with the four pairs, or eight haplospores, arranged
a mold that renders the bread on which it grows inedible; it in a line.
too generates four ascospores with each meiosis, but at the To analyze both unordered and ordered tetrads, re-
completion of meiosis, each of the four haploid ascospores searchers can release the spores of each ascus, induce the
immediately divides once by mitosis to yield four pairs, for haploid cells to germinate under appropriate conditions,
a total of eight haploid cells. The two cells in each pair of and then analyze the genetic makeup of the resulting hap-
Neurospora ascospores have the same genotype because loid cultures. The data they collect in this way enable them
they arose from mitosis. to identify the four products of a single meiosis and com-
Haploid cells of both yeast and Neurospora normally pare them with the four products of many other distinct
reproduce vegetatively (that is, asexually) by mitosis. How- meioses. Ordered tetrads offer another possibility. With the
ever, sexual reproduction is possible because the haploid aid of a dissecting microscope, investigators can recover
cells come in two mating types, and cells of opposite mat- the ascospores in the order in which they occur within the
ing types can fuse to form a diploid zygote (Fig. 5.20). In ascus and thereby obtain additional information that is use-
yeast, these diploid cells are stable and can reproduce ful for mapping. We look first at the analysis of randomly
through successive mitotic cycles. Stress, such as that arranged spores in yeast tetrads as an example. We then
caused by a scarcity of essential nutrients, induces the describe the additional information that can be gleaned
diploid cells of yeast to enter meiosis. In bread mold, the from the microanalysis of ordered tetrads, using Neuros-
diploid zygote instead immediately undergoes meiosis, so pora as our model organism.
the diploid state is only transient.
Mutations in haploid yeast and mold affect many dif-
ferent traits, including the appearance of the cells and their Tetrads Can Be Characterized as Parental
ability to grow under particular conditions. For instance, Ditypes (PDs), Nonparental Ditypes
yeast cells with the his4 mutation are unable to grow in the (NPDs), or Tetratypes (Ts)
absence of the amino acid histidine, while yeast with the
trp1 mutation cannot grow without an external source of When diploid yeast cells heterozygous for each of two
the amino acid tryptophan. Geneticists who specialize in genes are induced to undergo meiosis, three tetrad types
the study of yeast have devised a system of representing can be produced whether the two genes are on the same
genes that is slightly different from the ones for Drosophila chromosome or different chromosomes. Consider the cross
and mice. They use capital letters (HIS4) to designate dom- in Fig. 5.21, in which haploid cells of opposite mating
inant alleles and lowercase letters (his4) to represent reces- types (a versus α) and with alternate alleles of two genes
sives. For most of the yeast genes we will discuss, the
wild-type alleles are dominant and may also be represented
by the alternative shorthand +, while the symbol for the Figure 5.21 Three tetrad types produced by meiosis of
recessive alleles remains the lowercase abbreviation (his4). dihybrid yeast. All three types may be produced whether or not
(See Guidelines for Gene Nomenclature.) Remember, how- genes A and B are on the same chromosome and whether or not
ever, that dominance or recessiveness is relevant only they are linked. Parental spores are orange and recombinant spores
for diploid yeast cells, not for haploid cells that carry only are blue.
one allele. Haploid parents A B × ab
(a-mating type) ( -mating type)

An Ascus Contains All Four Products of a A a B b
Single Meiosis Diploid cell

The assemblage of four ascospores (or four pairs of asco- Meiosis
spores) in a single ascus is called a tetrad. Note that this is
a second meaning for the term tetrad. In Chapter 4, a tetrad
was the four homologous chromatids—two in each chro- ab a B A b
mosome of a bivalent—synapsed during the prophase and Possible AB a B a B ab
metaphase of meiosis I. Here, it is the four products of a unordered AB A b
tetrads
single meiosis held together in a sac. Because the four ab A b AB
chromatids of a bivalent give rise to the four products of PD NPD T
meiosis, the two meanings of tetrad refer to almost the Parental ditype Nonparental ditype Tetratype