Page 141 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

PART I Basic Principles: How Traits Are Transmitted
5

chapter

Linkage,

Recombination, and

the Mapping of

Genes on

Maps illustrate the spatial relationships of objects, such as the
Chromosomes locations of subway stations along subway lines. Genetic maps
portray the positions of genes along chromosomes.
© Rudy Von Briel/PhotoEdit

chapter outline

• 5.1 Gene Linkage and Recombination
• 5.2 Recombination: A Result of Crossing-Over
During Meiosis
IN 1928, DOCTORS completed a four-generation pedi- • 5.3 Mapping: Locating Genes Along a
gree tracing two known X-linked traits: red-green color Chromosome
blindness and hemophilia A (the more serious X-linked • 5.4 The Chi-Square Test and Linkage Analysis
form of bleeders disease). The maternal grandfather of • 5.5 Tetrad Analysis in Fungi
the family exhibited both traits, which means that his • 5.6 Mitotic Recombination and Genetic Mosaics
single X chromosome carried mutant alleles of the two
corresponding genes. As expected, neither color blind-
ness nor hemophilia showed up in his sons and daugh-
ters, but two grandsons and one great-grandson inherited both of the X-linked
conditions (Fig. 5.1a). The fact that none of the descendants manifested one of
the traits without the other suggests that the mutant alleles did not assort inde-
pendently during meiosis. Instead they traveled together in the gametes forming
one generation and then into the gametes forming the next generation, producing
grandsons and great-grandsons with an X chromosome specifying both color
blindness and hemophilia. Genes that travel together more often than not exhibit
genetic linkage.
In contrast, another pedigree following color blindness and the slightly different B
form of hemophilia, which also arises from a mutation on the X chromosome, revealed
a different inheritance pattern. A grandfather with hemophilia B and color blindness
had four grandsons, but only one of them exhibited both conditions. In this family, the
genes for color blindness and hemophilia appeared to assort independently, producing
in the male progeny all four possible combinations of the two traits—normal vision
and normal blood clotting, color blindness and hemophilia, color blindness and normal
clotting, and normal vision and hemophilia—in approximately equal frequencies
(Fig. 5.1b). Thus, even though the mutant alleles of the two genes were on the same
X chromosome in the grandfather, they had to separate to give rise to grandsons III-2
and III-3. This separation of genes on the same chromosome is the result of recombi-
nation, the occurrence in progeny of new gene combinations not seen in previous
generations. (Note that recombinant progeny can result in either of two ways: from the
recombination of genes on the same chromosome during gamete formation, discussed

133