Page 148 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

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

Figure 5.7 Recombination helps ensure proper arms, but it also is a key component of the centromeres
chromosome segregation during meiosis I. (a) Mouse themselves. We will discuss in detail how cohesin con-
chromosomes during mid-prophase of meiosis I in a primary nects sister chromatids during mitosis and meiosis in
spermatocyte. A protein component of the synaptonemal complex is Chapter 12.
red, a component of recombination nodules is green, and a
component of centromeres is blue. Each bivalent has at least one The importance of crossing-over to proper chromo-
recombination nodule, although some are hard to see. (b) Mouse some segregation is underlined by the fact that each biva-
chromosomes in late prophase of meiosis I (diakinesis). Note that each lent in Figs. 5.7a and b has at least one recombination
bivalent has at least one chiasma (the arrow points to one example), nodule or chiasma. In fact, a mechanism called interference
indicating that crossing-over occurred earlier. (c) Artist’s representation that occurs in almost all sexually reproducing organisms
of cohesin complexes (orange rings) along the chromosome arms of a
bivalent during metaphase of meiosis I; cohesin at the centromeres is helps ensure that each chromosome pair undergoes at least
not shown. Cohesin complexes distal to the crossover point keep one crossover, thus preventing nondisjunction of any chro-
sister chromatids together. Arrows pointing toward the poles indicate mosome except when rare mistakes occur. We discuss
the forces that try to pull the homologous chromosomes apart. interference in more detail later in this chapter.
a: © Dr. Paula Cohen & Dr. Miguel Angel Brieño-Enríquez, The Cohen Lab,
Center for Reproductive Genomics, Cornell University, Ithaca, NY; b: © Dr. Paula
Cohen & Dr. Kim Holloway, The Cohen Lab, Center for Reproductive Genomics,
Cornell University, Ithaca, NY Recombination Frequency Reflects
(a) Prophase I (Pachytene)
the Distance Between Two Genes
Thomas Hunt Morgan’s intuitions that chiasmata represent
sites of physical crossing-over between chromosomes and
that such crossing-over may result in recombination, led
him to the following logical deduction: Different gene pairs
exhibit different linkage frequencies because genes are ar-
ranged in a line along a chromosome. The closer together
two genes are on the chromosome, the smaller their chance
of being separated by an event that cuts and recombines the
line of genes. To look at it another way, if we assume for
the moment that chiasmata can form anywhere along a
chromosome with equal likelihood, then the probability of
a crossover occurring between two genes increases with the
(b) Prophase I (Diplotene)
distance separating them. If this is so, the frequency of ge-
netic recombination also must increase with the distance
between genes.
To illustrate the point, imagine pinning to a wall a
10-inch piece of ribbon containing tiny black dots along
its length and then repeatedly throwing a dart to see where
you will cut the ribbon. You would find that practically
every throw of the dart separates a dot at one end of the
ribbon from a dot at the other end, while few if any throws
separate any two particular dots positioned right next to
each other.
Alfred H. Sturtevant, one of Morgan’s students, took
this idea one step further. He proposed that the percentage
(c) Metaphase I of total progeny that were recombinant types, the recombi-
Spindle pole nation frequency (RF), could be used as a gauge of the
physical distance separating any two genes on the same
chromosome. Sturtevant arbitrarily defined one RF per-
Centromere
centage point as the unit of measure along a chromosome;
Cohesin complex later, another geneticist named the unit a centimorgan
(cM) after T. H. Morgan. Mappers often refer to a centim-
organ as a map unit (m.u.). Although the two terms are
interchangeable, researchers prefer one or the other, de-
Chiasma
pending on their experimental organism. Drosophila ge-
Centromere neticists, for example, use map units while human
geneticists use centimorgans. In Sturtevant’s system, 1%
RF = 1 cM = 1 m.u.
Spindle pole