Page 146 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

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

two pure-breeding strains: black-bodied females with together and thus show genetic linkage. It is not as obvious
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straight wings (b c / b c ) and brown-bodied males with why all linked genes always show some recombination in a
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curved wings (b c / b c). All the F 1 progeny are double sample population of sufficient size. Do the chromosomes
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heterozygotes (b c / b c) and are phenotypically wild type. participate in a physical process that gives rise to the re-
In a testcross of the F 1 females with b c / b c males, all shuffling of linked genes that we call recombination? The
the offspring receive the recessive b and c alleles from their answer to this question is of more than passing interest as it
father. The phenotypes of the offspring thus indicate the provides a basis for gauging relative distances between
kinds of gametes received from the mother. For example, a pairs of genes on a chromosome.
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black fly with normal wings would be genotype b c / b c; In 1909, the Belgian cytologist Frans Janssens de-
because we know it received the b c combination from its scribed structures he had observed in the light microscope
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father, it must have received b c from its mother. As Fig. 5.5 during prophase of the first meiotic division. He called
shows, roughly 77% of the testcross progeny in one experi- these structures chiasmata; as described in Chapter 4, they
ment received parental gene combinations (that is, allelic seemed to represent regions in which nonsister chromatids
combinations transmitted to the F 1 females by the gametes of of homologous chromosomes cross over each other (review
each of her parents), while the remaining 23% were recom- Fig. 4.16). Making inferences from a combination of ge-
binants. Because the parental classes outnumbered the re- netic and cytological data, Thomas Hunt Morgan suggested
combinant classes, we can conclude that the autosomal that the chiasmata observed through the light microscope
genes for black body and curved wings are linked. were sites of chromosome breakage and exchange resulting
in genetic recombination.

essential concepts
Reciprocal Exchanges Between Homologs
• Genes on the same chromosome that do not assort
independently are said to be linked. Are the Physical Basis of Recombination
• Parental gametes contain alleles inherited together from Morgan’s idea that the physical breaking and rejoining of
a single grandparent; recombinant gametes contain chromosomes during meiosis was the basis of genetic re-
alleles inherited from different grandparents. combination seemed reasonable. But although Janssens’s
• The hallmark of linkage is that a dihybrid produces more chiasmata could be interpreted as signs of the process, before
parental gametes than recombinant gametes; as a result, 1930 no one had produced visible evidence that crossing-
the progeny ratio in a dihybrid cross involving linked over between homologous chromosomes actually occurs.
genes is not 9:3:3:1. The identification of physical markers, or cytologically vis-
• Testcrosses clarify linkage because each phenotypic ible abnormalities that make it possible to keep track of spe-
class of progeny corresponds to each gamete type cific chromosome parts from one generation to the next,
produced by the dihybrid parent.
enabled researchers to turn the logical deductions about re-
combination into facts derived from experimental evidence.
In 1931, Harriet Creighton and Barbara McClintock,
5.2 Recombination: A Result who studied corn, and Curt Stern, who worked with
of Crossing-Over During Meiosis Drosophila, published the results of experiments showing
that genetic recombination indeed depends on the recipro-
cal exchange of parts between maternal and paternal chro-
learning objectives mosomes. Stern, for example, bred female flies with two
different X chromosomes, each containing a distinct
1. Explain the physical process by which recombination physical marker near one of the ends. These same females
takes place. were also doubly heterozygous for two X-linked genetic
2. Describe the role of chiasmata in chromosome markers—alleles of genes that could serve as points of
segregation during meiosis. reference in determining whether particular progeny were
3. Discuss the relationship between the recombination the result of recombination.
frequency and the map distance separating two loci on Figure 5.6 diagrams the chromosomes of these hetero-
a chromosome. zygous females. One X chromosome carried mutations
4. Explain why the value of the recombination frequency producing carnation eyes (a dark ruby color, abbreviated
between any two genes is limited to 50%. car) that were kidney-shaped (Bar); in addition, this chro-
mosome was marked physically by a visible discontinuity,
which resulted when the end of the X chromosome was
It is easy to understand how genes that are physically broken off and attached to an autosome. The other X chro-
connected on the same chromosome can be transmitted mosome had wild-type alleles (+) for both the car and the