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5.3 Mapping: Locating Genes Along a Chromosome 143

Figure 5.11 Meioses possible in a dihybrid for genes on
the same chromosome. Meioses with no crossovers (NCOs) essential concepts
between A and B result in all parental chromosomes. Meioses with
an SCO between A and B result in half parental and half • Recombination occurs when chromatids of homologous
recombinant (R) chromosomes. Four kinds of DCOs are equally chromosomes exchange parts during prophase of meiosis I.
likely, and collectively they result in half parental and half • Crossing-over helps establish physical linkages between
recombinant gametes (bottom). In a 2-strand DCO event, one pair of homologous chromosomes needed to prevent
nonsister chromatids undergoes two crossovers. In either of two nondisjunction.
types of 3-strand DCOs, a single chromatid of one homolog crosses
over with each of the two chromatids of the other homolog. In a • The recombination frequency (RF) indicates how often two
4-strand DCO, each pair of nonsister chromatids recombines. genes are transmitted together. For linked genes, RF < 50%.
• Two genes are linked if they are physically so close that
NCO SCO
A B A B some meioses occur with no crossovers between them.
AB AB
A B A B
AB Ab • The maximum RF of 50% (independent assortment)
a b 0% R a b 50% R
ab aB occurs when two genes are on different chromosomes, or
a b a b
ab ab when two syntenic genes are so far apart that there is at
least one crossover between them in every meiosis.
DCO (2-strand) DCO (3-strand)
A B A B
AB Ab
A B A B
AB AB
a b 0% R a b 50% R
ab aB
a b a b
5.3 Mapping: Locating Genes
ab ab
DCO (3-strand) DCO (4-strand) Along a Chromosome
A B A B
AB Ab
A B A B
Ab Ab
a b 50% R a b 100% R
ab aB
a b a b learning objectives
aB aB
DCO average = 0% + 50% + 50% + 100% = 50% R 1. Establish relative gene positions using two-point cross
4 data.
2. Refine genetic maps based on data from three-point
occurs between them in every meiosis, the pool of gametes testcrosses.
produced by a dihybrid would be only 50% recombinant 3. Explain how a genetic map (in map units) is related to
types. You can see now that if two genes on the same chro- actual physical distance (in base pairs of DNA).
mosome are so far apart that no meioses occur as NCOs, 4. Describe the relationship between linkage groups and
then the alleles of the genes assort independently, just as if chromosomes.
they were on different chromosomes.
Even though crosses between two genes lying very far
apart on a chromosome may show no linkage at all, you can Maps are images of the relative positions of objects in
demonstrate that they are on the same chromosome if you can space. Whether depicting the floor plan of New York’s
tie each of the widely separated genes to one or more common Metropolitan Museum of Art, the layout of the Roman
intermediaries. Table 5.1 summarizes the relationship be- Forum, or the location of cities served by the railways of
tween the relative locations of two genes and the presence or Europe, maps turn measurements into patterns of spatial
absence of linkage as measured by recombination frequencies. relationships that add a new level of meaning to the origi-
nal data of distances. Maps that assign genes to specific
locations on particular chromosomes called loci (singular
TABLE 5.1 Properties of Linked Versus locus) are no exception. By transforming genetic data into
Unlinked Genes spatial arrangements, maps sharpen our ability to predict
Linked Genes the inheritance patterns of specific traits.
Parentals > Recombinants (RF < 50%) Geneticists have been obsessed with mapping genes be-
cause knowing a gene’s location gives scientists the ability
Linked genes must be syntenic and sufficiently close together on to identify the segment of chromosomal DNA correspond-
the same chromosome so that they do not assort independently.
ing to a gene. In later chapters of this book, you will see how
Unlinked Genes knowledge of a gene’s location can be used to isolate its
Parentals = Recombinants (RF = 50%) DNA, and furthermore, how molecular geneticists can use a
Occurs either when two genes are on different chromosomes or gene’s DNA to understand the gene’s function.
when they are sufficiently far apart on the same chromosome that We have seen that recombination frequency (RF) is a
at least one crossover occurs between them in every meiosis. measure of the distance separating two genes along a