Wy

Box 1-2 FIGURE 1 The inheritance of a sex-linked gene in Drosophila. Genes located on sex chromosomes can express themselves differently irt male and female progeny, because if there is only one X chromosome present, recessive genes on this chromosome are always expressed Here are two crosses, both involving a recesstve gene (w, for white eye) tocated on the X chromosome, (a) The male parent is a white-eyed (wV) fly. and the female is homozygous for red eye (WW) (b) The male has red eyes (WY) and the female white eyes (ww). The letter Y stands here not for an allele, hut for the Y chromosome, present in male Drosophila in place of a homologous X chromosome. There is no gene on the V chromosome corresponding to the w or W gene on the X chromosome.

Gene Linkage uncí Crossing Over 11

examples of nonrandnm assortment were found as soon as a large number of mutant genes became available for breeding analysis. In every well-studied case, the number of linked groups was identical with the haploid chromosome number. For example, there are four groups of linked genes in Drosophila and four morphologically distinct chromosomes in a haploid cell,

Linkage, however, is in effect never complete. The probability that two genes on the same chromosome will remain together during meio-sis ranges from just less than 100% to nearly 50%, This variation in linkage suggests that there must be a mechanism for exchanging genes on homologous chromosomes. This mechanism is called crossing over. Its cytological basis was first described by Belgian cytologist F. A. Janssens, At the start of rneiosis, through the process of synapsis, the homologous chromosomes form pairs with their long axes parallel. At this stage, each chromosome has duplicated to form two chromatids. Thus, synapsis brings together four chromatids [a tetrad), which coil about one another. Janssens postulated thai, possihly because of tension resulting from this coiling, two of the chromatids might sometimes break at a corresponding place on each. These events could create four broken ends, which might rejoin crossways, so that a section of each of the two chromatids would be joined to a section of the other (Figure 1-4). In this manner, recombinant chromatids might be produced that contain a segment derived from each of the original homologous chromosomes. Formal proof of Janssens's hypothesis that chromosomes physically interchange material during synapsis came more than 20 years later, when in 1931, Barbara McCliriiock and Harriet B. Creighton, working at Cornell University with the corn plant Zea mays, devised an elegant cytological demonstration of chromosome breakage and rejoining (Figure 1-5).

synapsis of duplicated chromosomes to form tetrads two chromatids bend across each other eacb chromaStí fcreeks at point oí conlact and tuses wilti a por Son af thedher

FIGURE 1 -4 Janssens's hypothesis of crossing over.

synapsis of duplicated chromosomes to form tetrads two chromatids bend across each other eacb chromaStí fcreeks at point oí conlact and tuses wilti a por Son af thedher

FIGURE 1 -4 Janssens's hypothesis of crossing over.

parental genotypes exirachromosoma! c Wx material C

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