Page 144 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

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

Figure 5.3 Designations of parental and recombinant relate to past history. Figure 5.2 has been redrawn here as Cross
Series A for easier comparison with Cross Series B, in which the dihybrid F 1 females received different allelic combinations of the white and
yellow genes. Note that the parental and recombinant classes in the two cross series are the opposite of each other. The percentages of
recombinant and parental types are nonetheless similar in both experiments, showing that the frequency of recombination is independent of
the arrangement of alleles.
Cross Series A Cross Series B
P P
w y + w + y w + y + w y

w y + w + y +
F 1 F 1
w y + w y + w + y + w + y +

w + y w y
F males F males
2
2
w y + w + y w y w + y + w + y + w y w + y w y +

Parental Parental Recombinant Recombinant Parental Parental Recombinant Recombinant
~ 99% ~ 1% ~ 99% ~ 1%

find out what kinds and ratios of gametes these F 1 females illustration of the linkage concept. The two genes are so
produce, we need to look at the telltale F 2 males. tightly coupled that the parental combinations of alleles—
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This time, as Cross Series B in Fig. 5.3 shows, w y / Y w y and w y (in Cross Series A of Fig. 5.3) or w y and
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and w y / Y are the recombinants that account for little w y (in Cross Series B)—are reshuffled to form recombi-
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more than 1% of the total, while w y / Y and w y / Y are nants in only 1 out of every 100 gametes formed. In other
the parental combinations, which again add up to almost words, the two parental allele combinations of these tightly
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99%. You can see that no preferred association of w and y linked genes are inherited together 99 times out of 100.
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or of y and w exists in this cross. Instead, a comparison of
the two experiments with these particular X chromosome Gene-pair-specific variation in the
genes demonstrates that the observed frequencies of the degree of linkage
various types of progeny depend on how the arrangement
of alleles in the F 1 females originated. We have redrawn Linkage is not always this tight. In Drosophila, a mutation for
Fig. 5.2 as Cross Series A in Fig. 5.3 so that you can make miniature wings (m) is also found on the X chromosome. A
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this comparison more directly. Note that in both experi- cross of red-eyed females with normal wings (w m / w m )
ments, it is the parental classes—the combinations origi- and white-eyed males with miniature wings (w m / Y) yields
nally present in the P generation—that show up most an F 1 generation containing all red-eyed, normal-winged flies.
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frequently in the F 2 generation. The reshuffled recombi- The genotype of the dihybrid F 1 females is w m / w m. Of the
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nant classes occur less often. It is important to appreciate F 2 males, 67.2% are parental types (w m and w m), while the
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that the designation of parental and recombinant gametes remaining 32.8% are recombinants (w m and w m).
or progeny of a doubly heterozygous F 1 female is opera- This preponderance of parental combinations among
tional, that is, determined by the particular set of alleles she the F 2 genotypes reveals that the two genes are linked: The
receives from each of her parents. parental combinations of alleles travel together more often
When genes assort independently, the numbers of pa- than not. But compared to the 99% linkage between the w
rental and recombinant F 2 progeny are equal because a and y genes for eye color and body color, the linkage of w
doubly heterozygous F 1 individual produces an equal num- to m is not that tight. The parental combinations for color
ber of all four types of gametes. By comparison, two genes and wing size are reshuffled in roughly 33 (instead of 1)
are considered linked when the number of F 2 progeny with out of every 100 gametes.
parental genotypes exceeds the number of F 2 progeny with
recombinant genotypes. Instead of assorting independently, Linkage of autosomal traits
the genes behave as if they are connected to each other Linked autosomal genes are not inherited according to the
much of the time. The genes for eye and body color that 9:3:3:1 Mendelian ratio expected for two independently
reside on the X chromosome in Drosophila are an extreme assorting, noninteracting genes, each with one completely