HOW DO THE MODIFIERS OF LINKAGE VALUES AFFECT
DETACHED PORTIONS OF THE CHROMOSOME?
BY
WELLINGTON SHANG HSU
THESIS
FOR THE
DEGREE OF BACHELOR OF SCIENCE
IN
AGRICULTURE
COLLEGE OF AGRICULTURE
UNIVERSITY OF ILLINOIS
1922
Digitized by the Internet Archive
in 2015
https://archive.org/details/howdomodifiersofOOhsuw
UNIVERSITY OF ILLINOIS
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CONTENTS
PAGE
I IJaTRODUCTION 1
II MATERIALS AITO METHODS 4
III THB DATA AND DISCUSSION
A- REGION l<5b2 8
B- REGION 4 13
IV CONCLUSIONS 13
V ACKNO’IPLEDGIMENT 17
VI literature cited 18
VII TABLES
A- TABLE I 3
B- TABLE II 9
C- TABLE III AND IV 14
D - TABLE V 19
E- TABLE VI 21
F- TABLE VII
23
iV
I
HOW DO THE MODIFIERS OF LIHKAGE VALUES AFFECT DETACHED PORTIONS
OF THE CHROMOSOME?
I
nJTBODUCTION
The axnooint of llteratixre that has been published recently bearing on
the phenomenon of linkage of hereditary factors makes it unnecessary to give any
description of the same at tills time. To eiQ)lain the vest accumulation of data
obtained from intelligently planned, and carefully controlled experiments, several
theories have been advanced; but as time goes on it is becoming more and more
obvious that the chiasmatype theory is the most plausible. According to the
chiasmtype theory, Mendelian factors, or genes, reside in a linear series on the
chromosomes, and each has a definite locus with its allelomorph occupying the
corresponding locus on the other member of the homologous pair of chromosomes.
The immediate cause of crossing over is attributed to the breaking of the
homologous pair of chromosomes, which twist arour.d each other during synapsis,
and the subsequent reunion of them, each taking a piece that formerly belonged to
the other.
Taking this theory as a working basis, geneticists had accismulated
sufficient data to convince themselves, until quite recently, that the crossover
value between two Mendelian factors, is constant and invariable* It is the
amount of such data and the accepted concept of the linear arrangement of genes
on the chromosome that led Morgan to believe that crossover valtie between two
linked factors is a fhncticn of distance. Attractive and suggestive as Morgan's
postulate may be, it has not been wholly successfhl, how'ever, in piecing itself
beyond question. New searches were conducted by different investigators and
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th«y have showi that cxx»saover values are variable, being Influenced by age
(Bridges *16), t^s^ierature (Plou^ *17), and also certain specific genetic factors
(Sturtevant *17),
The array of evidence against Morgan's view was amplified by the work
of Detlefsen and Boberts (Datlefsen and Hoberts *21) tnho worked on Drosophila,
and through selection have been able to reduce a crossover value of 32^ (from
white to miniature), to practically zero, in one of tl^e series at the end of the
12th generation. Instead of accepting Morgan's hypothesis of distance, Detlefsen
C 2 I) propounds that the linkage value is at least to a large extent "determined
by the different possible combinations of uultiple modifying factors." In order to
test out this view, he actually performed a hybridization experiment between the
low crossover stock and normal flies, A portion of his data is reproduced in
Table I. In the table, there ere to be found in the first and second lines the
distributions of crossover values of normal populations and generation of
series B, In ^42 generation of series B low stock, he mated fifty red long females
to normal white miniature males, and the crossover val'ues of these females are
shown in the third line. To secure a representative distribution of crossover
values of the Fp hybrid females thus derived, he chose four such females fTom
each of the forty- four pairs and crossed them to their brothers. The
distribution of such a population is given in the fourth line. It is stated that
since it is impossible to test out the crossover capacity of the male parents, we
mist accept for them the value of series B Fp generation, since they came from a
stock which had been used for class work for many years and always found to give
a value approximating 33^, The Fg distribution shown here is the total of three
distinct and separate F 2 distributions which came from P^^ pairs No. 2,5 and 6.
It is almost needless to point out that the F^ distribution lies in between those
of the two parents and so do both the value of the mean variate and mean total
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crossover value. The T 2 has a wider range of distribution than that of the
but the values of the mean variates and the mean total crossover values is about
the same as the Fx, Such features characterising a hybridization eij>eriment vdiere
multiple factors with incoB^lete dominance are involved are further confirmed by
the increased standard deviation of the Fg distribution. It is this and other
genetic analyses that led Detlef sen ( *21) to wonder "Just sdiat part distance
between two genes on a chromosome map may play in determining linkage values. "
The present work, vdiich is really a minor part of his, was ^undertaken
with the hope of throwing further light on the whole problem by attempting to
find out how the modifiers of linkage values affect detached portions of the
chromosome. Would the separated pieces still give low values, or would they be-
have in some other fashion?
II
MATERIimS mB METHODS.
To cope with the coiqplexity of such an experiment, Professor Detlef sen
developed a special system of designation Which I shall use in the following
description. According to this system, the piece of chromosome between vdaite and
miniature is divided into three different regions as shown In Figure 1. Individu-
als carrying the different pieces of chromosomes are named accordingly. For
instance, individuals carrying the piece of chromosome from red to cross-veined
end the rest of the chromosome, comes from the other member of the homologous
pair is considered as Region 1. Those carrying the pieces from cross-veined to
not-cut is called Region 2, etc. The shaded portion in the figure represents
the region outside of selection. With reference to this region a more detailed
llscusslon will appear in the latter part of this paper.
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portion represents the region that is outside of selection influence.
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The origin of our experiiTiental materials was as follows: Mr. L. S,
CleiQentd crossed normal homozygous recessive females of the zygotic constitution,
WjCjChiinif fdiite (eye) cross veinless (wing), cut (wing), and forked (bristle)
to a dominant male of the genetic formula, W,C,C'b)^*^ (red, cross-veined, not cut,
long, and not forked) which he obtained from Prof. Detlefsen’s low crossover
stock F 7 g, As usual he secured homozygous recessive males and dominant females
heterozygous for the factors involved:-
wcchmf 9 wccbmf <f
Mating the sibs together, crossovers in vario\u» regions weze
obtained, Mr. Feldman took over the females of Begion 1 (ib 2, and Region 4 was run
yss myself. Among these various region crossovers, we secured those of Begion 1 2
and those of Begion 4. The process in which they are supposed to have evolved
is shown in the following diagram;-
Begion 1 (!b 2
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A list of the classes of offspring obtained
and Heglon 4 females.
Parental —
Crossover — —
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Region 1
Double Crossovers
Region 2
-is-
Theee eroseovers made up the material of Experiment 1.
Experiment 2 had a somvvdiet elmllar origin. In Experiment 2, the male
that originated the population was from Eg^ generation of Dr, Detlefsen's low
crossover stock and the females to which it was mated came from a quadruple
recessive stock v^lch had been subjected to six generations of inbreeding. This
quadruple recessive stock was a white, cut, cross veinless, miniature.
Our methods of procedure with the Eg segregates of the various regions
are the same, for females carrying any given region were mated to the ultimate
recessive males. The different classes of offspring resulting from these two
crosses are shown in the accompanying chart. Each female was kept with three
or four males of the right kind in 8-drachm homeopathic vials. The common method
of culture for Drosophila was followed. Elies were r^sooved to new vials every
other day and a new supply of males was introduced to take the place of the dead
ones, should deaths occur. Nine days, from the day when the first offspring
emerged, were allowed for the last one to be hatched out in each vial, and three
countings were made during the nine days. All calculations were done twice on
one of the best calculating machines on the market, end it is hoped that
arithmetical errors have been reduced to a minimum.
Ill
DATA AND DISCUSSION.
A - Region 1 and 2.
The distributions of cross over values for Region 1 and 2 in each
experiment separately and combined into a total are given. They are plotted in
class intervals of 1.5^. The distribution for Experiment 1 ranges from 0 to 18.5^
and for Experiment 2 from 0 to 14.25^. The total of the two combined is shown
in the last line in the table, and the curves plotted from them are shown in
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Experiment II
Experiment I ft II
- 11 -
Figare 2, The most striking feature of the curves is the large numher of
individuals in the first class, making them look rather unique for the experi-
ment. However, if we turn to the hack and look thrcu^ Tables 5 and 6, we will
find that there are quite a few of the F£ segregates giving such a small number
of offspring that their crossover values reached zero. It is true that zero
percent crossover value should be expected to occur in this case, if we ejq)ect
modifiers of lixkage values to segregate at all; but still we should not over-
look the fact that over half of the individuals that give zero percent are those
that have exceptionally small number of offspring. Therefore due allowance must
be made on the first class in Table 2, and thus the medal classes of the curves
could be shifted to 5.25^ for Es^eriment 2 and 3.75^ for E:q>eriment 1 and
Experiment 1 and 2 combined.
From the description given in the early part of this paper, it is
very obvious that we have also performed a hybridization es^eriment on two strain:
of Drosophila differing from each other in their capacity for crossing over. In
fact, as has been shown, this has been admirably done by Prof. Detlefsen him>
self. The only point in which his work differs from mine lies in the fact that
he \ised the Tdiole region between white and miniature; while I dealt with se^ents
of it. Due to lack of experience in handling the material, and in the method of
culture, the results obtained could be much better. There are, however,
reflected in the data a few features that are characteristic of a hybridizing
experiment vhere multiple factor Inheritance with Incomplete dominance is
involved. These features stand out so strikingly that it is but Justifiable to
arrange the material and present it in its logical shape. Owing to the fact that
Prof. Detlefsen is dealing with the whole piece of chromosome, it is impossible
for me to find out the crossover values of Beglon 1A2 for the respective
generations to which the two originel males belong. However, we know the value
for the whole region, and in order to present the data in the shape it is now.
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• 12 -
It is Isperatlve that we assign to Beglon 1 (ib 2 a yalua which we think most
nearly true. This value is indicated hy a cross in the first line of Table 2.
The females used were normal, being considered capable of giving 18.5^ of cross-
overs within Beglon 1 cSb 2. From a cross of this description, we naturally e:^ect
an Fx population giving an intermediate value of crossover between the values of
the parents. This opportunity for testing the F^ generation, however, was not
given to the writer but throu^ the kindness of Mr, Clemente, the F^ crossover
values for both Experiments were obtained which fare shown in the table.
It is readily seen that the F^ crossover values of both experiments are
not hi enou^ to be called intermediate between the parents. Those of the F£
are still lower, A clearer idea of the data is better grasped by glancing
over the curves in Figure 2. If due allowance were made in the first claases, we
can but conclude that a partial dominance of the ”low modifiers" must be existing,
since the mean total crossover value of the F^ and Fg is lower than an arithmetical^
mean between the original parents.
Now, if we compare the F£ crossover values of detached piece with those
of v^nole chromosome, we will find that they show up in the same general magnitude,
being 5.44^ in Experiment 1 and 15,37;^ in Experiment 2 as compared to 5,817^ when
the vdxole chromsoma was involved. This direct comparison of Fj and F2 data brings
out clearly the fact that modifiers of linkage values affect detached portions of
chromosome same as they do the vdiole piece.
Since, as has been mentioned, the low male used in Experiment 2 was
from F34 generation and the females 0B5>loyed had been subjected to six generations
of inbreeding, vhile those used in Experiment 1 were just nonnal individuals and
the male was from Fyg^ ^3 8ho^lld expect the standard deviations of the two F
2
posalatlon, to Show soma diffaranoa. A glanoa at that oolwm In the table will
'4
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13-
tell that they do differ, and also in such a wai' as we eaqpected. The standard
deviation of Experiment 1 being bigger than that of Ej^) 0 riment 2.
B - Eegion 4.
As mentioned at the beginning of this paper the purpose of this work
is more of the nature to try to throw farther light on the whole problem of
modifiers of linkage values than merely to secure more data to expand the theme
stated in the subject, so I ran a test on Region 4 vdxich is the space between
miniature and forked, and it is outside the original region of selection. As it
appears in Table 3, this region does not seem to have been influenced by
selection. Forty-one faaales were tested and with an average of 127 offspring
per female, the crossover value obtained for the region is 19,75^, vdiich is very
close to the recognized valuet- 20.4^. Owing to the extremely small number of
offspring obtained from some of the females, the distribution table drawn up is
rather puzzling. Since selection does not seem to have any effect on this
region, the Fg population should show a normal curve varying arounii the recogniz-
ed value. Too much emphasis, however, shoxild not be laid on the distribution we
obtained, since gi^ite a number of the females give so few offspring that the
degree of reliability of their crossover capacity as reflected in the data is
doubtless deceiving. To eliminate this difficulty, females giving less than
fifty offspring are left out of consideration, and the resulting distribution
is shown in Table 4, A curve plotted from this distribution is shown in Figure 3
with 17J^ as the j modal class.
IV
CONCLUSIONS
From the above discussions of data the following conclusions may be
drawn.
Plotted from Table IV
Individual
O ' to 1/1 O'
o
at
3
ef
o
o
<0
«
o
<
(b
m
Figure 3.
-16-
1 Segnant# of chroaooome when separated from the vihola piece
of chromosome under selection still show low crossover value - a fact showing t’nat
modifiers of linkage values affect detached portions of chromosome in the seme
fashion as they do the vriiola piece.
2 In a hybridization experiment performed on "low" and "normal"
flies with only a segment of the idiole piece of chromosome under selection, the
mean value of the generation did not come out quite as hi^ as would he
expected on the basis of incooplete dominance. The great portion of the
population was grouped in the lower one third of the curve. This phenomenon
suggests the existence of partial dominance of the "low modifiers. "
3 Selection exercised on regions 1,2 and 3 in the sex
chromosome does not affect the crossover value of the region outside of this,
as far ae we can see from our data.
KPOK
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- 17 -
V
ACK'NDWLEDGEdENT .
The writer is very grateful to Prof. J. A. Detlefsen for the
opportvinity to work on this prohlera. It ia with a deep sense of appreciation
to say that hut for his helpful suggestions and his generosity in vc^ulping
the writer with necessary laboratory conveniences this work would not have
been possible.
-18-
VI
LITEUfiTUHE CITED.
Bridges, C. B. 1915 A linkage variation in Drosophila. Jour. Expt. Zool. ,
vol. 19, pp. 1-21
Detlefsen, J. A. and Robert s £. 1921 Studies on Crossing Over. Jour. £3cp.
Zool. vol. 32, no. 2, pp. 333-354.
Detlefsen, J. A. 1920 Is crossing over a function of distance? Proc. Nat'l.
Acad, of Sci. vol. 6, No. 11, pp. 663-670.
Plou^, H. H, 1917 The effect of temperature on cfossing over in
Drosophila. Jour. Ej^t. Zool., vol. 24, pp. 147-210.
Sturtevant, A. H. 1919 Inherited linkage variations in the sedond
chromosome. Puh. Carnegie Inst. Wash. D. C, , no, 278, pp. 305-341.
il
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-19-
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