genmapp - Linkage and Genetic Mapping in Eukaryotes Linkage...

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1 Linkage and Genetic Mapping in Eukaryotes Linkage and Crossing Over General information Chromosomes consist of more than one gene, typically many hundreds to a few thousand. The term linkage is use to indicate that 1) two genes that are located on the same chromosome, and 2) genes that are close together tend to be transmitted from parent to offspring as a unit, which has an influence on inheritance patterns. Chromosomes are often called linkage groups, because the genes on a chromosome are physically connected to one another. Genes that are far apart on a chromosome may assort independently due to crossing over. In humans there are 22 autosomal linkage groups, an X linkage group, and a Y linkage group. When geneticists follow traits they rely upon dihybrid (two-factor) and trihybrid (three- factor) crosses. Crossing over may produce recombinant phenotypes. Crossing over may alter linkage of genes. The bivalent chromosomes consist of two homologous chromosomes with a pair of sister-chromatids each. Genetic recombination by crossing over can produce new combinations of alleles on chromosomes (Figure 5.1). The cells that contain the new allelic combinations are called nonparental or recombinant cells. The cells that contain the original combination of alleles are called parental or nonrecombinant cells. Bateson and Punnett discovered two traits that did not assort independently. Bateson and Punnett were the first researchers (1905) to demonstrate that not all traits assort independently. In crosses involving flower color and pollen length in the sweet pea, the researchers noticed that the offspring did not display a 9:3:3:1 phenotypic ratio as expected from a dihybrid cross (Figure 5.2). They suggested that their traits were coupled to one another in some manner.
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2 Morgan provided evidence for the linkage of several X-linked genes and proposed that crossing over between X chromosomes can occur. The first direct evidence that different genes are physically located on the same chromosome was provided by Thomas Hunt Morgan (Figure 5.3). Morgan studied X-linked patterns of inheritance. His experimental proof used three traits: body color, eye color, and wing shape. With three traits assorting independently, it was expected that there would be eight phenotypic classes, each in equal proportion. Morgan’s results (Figure 5.3) indicated that the parental combinations were the most prevalent in the offspring. Morgan correctly suggested that these three traits are found on the X chromosome and are inherited together as a unit. For offspring that did not contain the parental combinations, Morgan suggested that there was crossing over between the homologous X chromosomes. Morgan suggested three hypotheses to explain his results: All three genes are located on the same chromosome (the X chromosome).
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