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Mendel 1865

[1] Einleitende Bemerkungen

Künstliche Befruchtungen, welche an Zierpflanzen deshalb vorgenommen wurden, um neue Farben-Varianten zu erzielen, waren die Veranlassung zu den Versuchen, die hier besprochen werden sollen. Die auffallende Regelmässigkeit, mit welcher dieselben Hybridformen immer wiederkehrten, so oft die Befruchtung zwischen gleichen Arten geschah, gab die Anregung zu weiteren Experimenten, deren Aufgabe es war, die Entwicklung der Hybriden in ihren Nachkommen zu verfolgen.

Dieser Aufgabe haben sorgfältige Beobachter, wie Kölreuter, Gärtner, Herbert, Lecocq, Wichura u. a. einen Theil ihres Lebens mit unermüdlicher Ausdauer geopfert. Namentlich hat Gärtner in seinem Werke

Die Bastarderzeugung im Pflanzenreiche

sehr schätzbare Beobachtungen niedergelegt, und in neuester Zeit wurden von Wichura gründliche Untersuchungen über die Bastarde der Weiden veröffentlicht. Wenn es noch nicht gelungen ist, ein allgemein giltiges Gesetz für die Bildung und Entwicklung der Hybriden aufzustellen, so kann das Niemanden Wunder nehmen, der den Umfang der Aufgabe kennt und die Schwierigkeiten zu würdigen weiss, mit denen Versuche dieser Art zu kämpfen haben. Eine endgiltige Entscheidung kann erst dann erfolgen, bis

Detail-Versuche

aus den verschiedensten Pflanzen-Familien vorliegen. Wer die Arbeiten auf diesem Gebiete überblickt, wird zu der Überzeugung gelangen, dass unter den zahlreichen Versuchen keiner in dem Umfange und in der Weise durchgeführt ist, dass es möglich wäre, die Anzahl der verschiedenen Formen zu bestimmen, unter welchen die Nachkommen der Hybriden auftreten, dass man diese Formen mit Sicherheit in den einzelnen Generationen ordnen und die gegenseitigen numerischen Verhältnisse feststellen könnte. Es gehört allerdings einiger Muth dazu, sich einer so weit reichenden Arbeit zu unterziehen; indessen scheint es der einzig, richtige Weg zu sein, auf dem endlich die Lösung einer Frage erreicht werden kann, welche für die Entwicklungs Geschichte der organischen Formen von nicht zu unterschätzender Bedeutung ist.

Die vorliegende Abhandlung bespricht die Probe eines solchen Detail-Versuches. Derselbe wurde sachgemäss auf eine kleinere Pflanzengruppe beschränkt und ist nur nach Verlauf von acht Jahren im Wesentlichen abgeschlossen. Ob der Plan, nach welchem die einzelnen Experimente geordnet und durchgeführt wurden, der gestellten Aufgabe entspricht, darüber möge eine wohlwollende Beurtheilung entscheiden.

Mendel 1865

[1] Introductory Remarks

Experience of artificial fertilization, such as is effected with ornamental plants in order to obtain new variations in color, has led to the experiments which will here be discussed. The striking regularity with which the same hybrid forms always reappeared whenever fertilization took place between the same species induced further experiments to be undertaken, the object of which was to follow up the developments of the hybrids in their progeny.

To this object numerous careful observers, such as Kölreuter, Gärtner, Herbert, Lecoq, Wichura and others, have devoted a part of their lives with inexhaustible perseverance. Gärtner especially in his work

Die Bastarderzeugung im Pflanzenreiche

, has recorded very valuable observations; and quite recently Wichura published the results of some profound investigations into the hybrids of the Willow. That, so far, no generally applicable law governing the formation and development of hybrids has been successfully formulated can hardly be wondered at by anyone who is acquainted with the extent of the task, and can appreciate the difficulties with which experiments of this class have to contend. A final decision can only be arrived at when we shall have before us the results of detailed experiments make on plants belonging to the most diverse orders.

Those who survey the work done in this department will arrive at the conviction that among all the numerous experiments made, not one has been carried out to such an extent and in such a way as to make it possible to determine the number of different forms under which the offspring of the hybrids appear, or to arrange these forms with certainty according to their separate generations, or definitely to ascertain their statistical relations.

It requires indeed some courage to undertake a labor of such far-reaching extent; this appears, however, to be the only right way by which we can finally reach the solution of a question the importance of which cannot be overestimated in connection with the history of the evolution of organic forms.

The paper now presented records the results of such a detailed experiment. This experiment was practically confined to a small plant group, and is now, after eight years' pursuit, concluded in all essentials. Whether the plan upon which the separate experiments were conducted and carried out was the best suited to attain the desired end is left to the friendly decision of the reader.

Incomplete dominance

• Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a

chromosome theory of inheritance

began to take form.

Fig. 15.1

• • Morgan spent a year looking for variant individuals among the flies he was breeding.

– He discovered a single male fly with white eyes instead of the usual red.

• The normal character phenotype is the

wild type

Alternative traits are

mutant phenotypes

Fig. 15.2

• This X-Y system of mammals is not the only chromosomal mechanism of determining sex.

• Other options include the X-0 system, the Z-W system, and the haplo diploid system.

Fig. 15.8

2. Sex-linked genes have unique patterns of inheritance

• In addition to their role in determining sex, the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex.

• These

sex-linked

genes follow the same pattern of inheritance as the white-eye locus in

Drosophila

Fig. 15.9

• Similarly, the orange and black pattern on tortoiseshell cats is due to patches of cells expressing an orange allele while others have a nonorange allele.

Fig. 15.10

• Morgan deduced that the gene with the white-eyed mutation is on the X chromosome alone, a sex-linked gene.

– Females (XX) may have two red-eyed alleles and have red eyes or may be heterozygous and have red eyes.

– Males (XY) have only a single allele and will be red eyed if they have a red-eyed allele or white-eyed if they have a white eyed allele.

Fig. 15.3

Fig. 15.4

• Morgan reasoned that body color and wing shape are usually inherited together because their genes are on the same chromosome.

• Morgan proposed that some mechanism occasionally exchanged segments between homologous chromosomes.

– This switched alleles between homologous chromosomes.

• The actual mechanism, crossing over during prophase I, results in the production of more types of gametes than one would predict by Mendelian rules alone.

Fig. 15.5a

• The occasional production of recombinant gametes during prophase I accounts for the occurrence of recombinant phenotypes in Morgan’s testcross.

Fig. 15.5b

• Sturtevant used the test cross design to map the relative position of three fruit fly genes, body color (b), wing size (vg), and eye color (cn).

– The recombination frequency between cn and b is 9%.

– The recombination frequency between cn and vg is 9.5%.

– The recombination frequency between b and vg is 17%.

– The only possible arrangement of these three genes places the eye color gene between the other two.

Fig. 15.6

• • Genes located far apart on a chromosome are mapped by adding the recombination frequencies between the distant genes and intervening genes.

Sturtevant and his colleagues were able to map the linear positions of genes in

Drosophila

into four groups, one for each chromosome.

Fig. 15.7

•

Nondisjunction

occurs when problems with the meiotic spindle cause errors in daughter cells.

– This may occur if tetrad chromosomes do not separate properly during meiosis I.

– Alternatively, sister chromatids may fail to separate during meiosis II.

Fig. 15.11

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2. Sex-linked genes have unique patterns of inheritance

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