A Database for Triticeae and Avena
III. 2. The uses of trisomics in identifying inverted chromosomes
T. Tsuchiya, Department of Agronomy, Colorado State University, Fort
Collins, Colorado 80523, U.S.A.
METHOD 1. Analysis of trisomic F1 hybrids between trisomics and inversion
A. Cross trisomics with inversion homozygotes
B. Analyze meiosis of trisomic F1 hybrids
Trisomic (AAA) x Inversion
Here A represents the normal chromosome, and a the inverted
In a few cells, at anaphase I, especially at early anaphase I, the trivalent
or relic (trace) of trivalent could be recognized. In the critical lines
shown above bridge/fragment figures shown in Figure 1a will be found in
some sporocytes at meiotic anaphase I. For details on the configurations
to be found at AI - TI and/or AII - TII readers refer to McClintock (1938).
Figure 1. Chromosome configurations at meiotic anaphase
I in the F1 hybrids between primary trisomics and paracentric inversion
homozygote. a, critical combination showing trivalents with bridge/fragment.
b, noncritical combination showing trivalent without bridge/fragment.
In noncritical lines the bridge/fragment in the trivalent (Figure lb)
will never be found. Of course, fairly large numbers of sporocytes should
be studied: the number of sporocytes to be analyzed depends upon the frequency
of crossing over. It is not quite sure, but the crossover values in some
regions might be higher in trisomic than in diploid because of the trisome
condition (see for detail, Swanson's Cytology and Cytogenetics, p. 264-267).
It is desirable to study meiosis of parent trisomics in order to make sure
that there is no inversion in the parents.
METHOD II. Analysis of back cross hybrids (BC1)
Trisomic portion could be used for the analysis of meiosis in the same
way as in Method I.
As shown above, the segregation ratio for presence and absence of fragment,
bridge/fragment or bridge at AI-TI/AII-TII would be 2:1 in diploid portion.
The diploid portion would be much easier to handle (analyze), because of
easiness of collecting materials and of analyzing the figures. In the diploid
portion just analyze the presenc or absence of fragment, bridge/fragment,
or bridge at AI-TI/AII - TII and compare the ratio in all seven trisomic
In the noncritical combinations the ratio for presence and absence of
fragment/bridge figures would be 1:1, while in the critical line the ratio
for presence and absence = 2 (Aa) : 1 (aa) as shown before.
As a whole, the analysis of diploid portion in back cross F1 (BC1)
would be the simplest way of analysis and should be most accurate. If some
75 to 100 diploid plants from one cross combination are analyzed it would
be sufficient enough to find the difference between 1 Aa : 1 aa
and 2 Aa : 1 aa (Hanson, 1959).
In case of anather back cross, AAa x AA, the ratio should
be different from AAa x aa mentioned above.
The following descriptions and tables show all expected cases mentioned
(A) In critical combinations:
(B) In noncritical combinations:
The situation is exactly the same as diploids. The trivalent has nothing
to do with inverted chromosome. The trivalent is completely free from bridge/
fragment. In both trisomic and diploid portion of BC1 hybrids
the ratio of inversion-hetero and -homo is 1:1 as shown in
the following table.
METHOD III. Genetical analysis without cytology.
The procedure is exactly the same as Method II. The only difference
is that there is no need to study cytological behavior in meiosis in Method
III. Also only diploid plants are studied in this method.
In the table shown above, the segregating ratios are 2Aa: laa
in AAa x aa and 2AA : 1 Aa in AAa x
AA in critical combination. Aa is heterozygote for an inversion
and should show some extent of semi-sterility, while AA (normal
homo) and aa (inversion homo) should show (almost) complete fertility.
In the noncritical combinations the segregating ratio would be 1:1 for
fertile and semisterile plants.
Method III could be used to obtain at least a supplemental information
for the cytological analysis, or may be used in the preliminary experiment
in order to find the information roughly about the inverted chromosomes.
So that more detailed cytological analysis can be made for only the suspected
Hanson, D.W. 1959. Minimum family size for the planning of genetic
experiments. Agron. J. 51:711-715.
McClintock, B. 1938. The fusion of broken ends of sister half chromatids
following chromatid breakage at meiotic anaphases. Missouri Agric. Expt.
Sta. Res. Bull. 290:48pp.
Swanson, C.P. 1963. Cytology and cytogenetics. Prentice Hall. 596 pp.
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