Genetic organization : a comprehensive treatise / edited by Ernst W. Caspari, Arnold W. Ravin.

Date:
1969-
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Genetic organization : a comprehensive treatise / edited by Ernst W. Caspari, Arnold W. Ravin. Source: Wellcome Collection.

    45/552 page 27
    i. genetics in iiistoiucai, i'ehspecnive 27 hiiNing entered a single egg wliieh was then fertilized by a Y-bearing sperm. All the exeeptional faets eould be aeeounted for by assuming that sex-linked genes ^^'ere ph) sieally loeated in the X ehromosome. The exact correspondence of e)'tologieal and genetic behavior provided Proof of the Chromosome Theory of Heredity the title of Bridges' definitive paper of 1916. Similarlv, anomalous genetic behavior of groups of linked genes was explained in some cases by translocation of a piece of chro- mosonic \\'ith its contained genes to a nonhomologous chromosome with which the translocated genes thereafter showed linkage; in others the order of genes in a segment of chromosome had become inverted, such as the order a cl с h e f derived from a h с cl e f. This changed the pairing pattern determined by affinity between homologous genes, so that gam¬ etes containing chromosome segments derived from crossing-over in the inverted region were incomplete or otherwise abnormal and failed to give rise to offspring. This loss of recombinant offspring had the appear¬ ance of crossover suppression associated with inverted regions. In these and other anomalies, such as duplication and deficiency, the breeding results were found to be accompanied by the chromosomal anomalies predicted by the theory that each chromosome consisted of a linear sequence of physical sites occupied by discrete genes which could change separately by mutation. By 1915 the evidence provided by breeding experiments and cyto- logical study of Drosophila was such that the group (Morgan, Sturtevant, Muller, and Bridges) chose as the title of the book they then published The Mechanism of Mendelian Heredity. Their claim to have worked out the formal mechanism of the genes in the chromosomes was justified by subsequent work both with Drosophila and with other diploid animals and plants. F. The Theory of the Gene Morgan, in his final book on the transmission mechanism of heredity, published in 1926 as The Theory of the Gene, summarized as follows the new principles added to those derived from Mendel (p. 25): We are now in a position to formulate the theory of the gene. The theory states that the characters of the individual are referable to paired elements [genes] in the ger¬ minal material that are held together in a definite number of linkage groups; it states that the members of each pair of genes separate when the germ-cells mature in ac¬ cordance with Mendel's first law, and in consequence each germ-cell comes to contain one set only; it states that the members belonging to difl^erent linkage groups assort independently in accordance with Mendel's second law; it states that an orderly in¬ terchange—crossing-over—also takes place, at times, between elements in correspond-
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