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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 EntwicklungsGeschichte 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
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• 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 haplodiploid system.
+ environmental!
Fig. 15.8
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
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
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• 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
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• 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 whiteeyed allele.
Fig. 15.3
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 15.4
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• Morgan reasoned that body color and wing
shape are usually inherited together because
their genes are on the same chromosome.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• 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
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• The occasional production of recombinant
gametes during prophase I accounts for the
occurrence of recombinant phenotypes in
Morgan’s testcross.
Fig. 15.5b
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• 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
Fig. 15.6
between the other two.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• 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
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
• 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
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings