Genetic Patterns

Morgan correctly hypothesized that X and Y chromosomes tare sex chromosomes, chromosomes that determine an individual's sex. All the other chromosomes, those not included in sex determination, are called autosomes. Although most fruit flies have red eyes, Morgan once observed a male fruit fly with white eyes.When he crossed a females with this male, the results followed Mendel's prediction: the F1 generation all had red eyes. Morgan next crossed members of the F1 generation and produced the expected ratio of three red-eyed flies were male. Why weren't any females white-eyed? Morgan hypothesized that the allele for red eyes was carried on the X chromosome. Assume the X chromosome carries the gene for eye color, either Xr or Xr. If an XRXR female is crossed with an XrY male, all of the F1 females will be XRXr, and all of the F1 males XRY. In the F2 generation half other female swill have the XRXRR genotype, and half the XRXr genotype. All will have red eyes. Half the F2 males will be XRY, or red eyed; half will be XrY, or white-eyed Morgan concluded that eye color in Drosophila was a sex-linked trait-one determined by alleles on the sex chromosomes. The group of genes located on one chromosome is a linkage group. These genes are usually inherited together. Morgan inferred that his rearrangement had taken place through crossing-over, the exchange of alleles between homologous chromosomes. Alfred H. Sturtevant, Morgan's student used crossing-over data to construct a chromosome map - a diagram of allele positions on a particular chromosome. Two alleles that are separated by crossing-over 1 percent of the time are considered to be 1 map

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