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Are there "Identical Twin" plants?

Nick Theodorakis nicholas_theodorakis at urmc.rochester.edu
Sat Dec 16 20:51:58 EST 2000

In article <Pine.GSO.4.21.0012161735320.10069-100000 at login.ice.mtu.edu>,
  jmglime at mtu.edu ("Janice M. Glime") wrote:
> How can you ever be certain that sexually reproduced mice are
> identical?  I don't know how many chromosomes in a set mice have, but
> simplicity, let's say they have 15.  (Humans have 23.)  Each original
> parent then had 15 chromosomes in the gamete.  These made two sets
> would sort independently when their offspring produced gametes. That
> permit 2 to the 15th combinations in the gametes of that offspring.
> Unless the strain is homozygous for all traits, which is highly
> genetic identity can never be determined with certainty.  Furthermore,
> cross-overs will occur, further contributing to the diversity of
> combinations.  Back-crossing reduces the variability, but it does not
> eliminate it.  Lots of generations with selective breeding will
result in
> more chances of "breeding true," but unless the mice have only two or
> three kinds of chromosomes, actual identical offspring is a highly
> unlikely event.
>   Since I am not a geneticist, perhaps I am missing something in the
> standard mouse-breeding protocol.




Published in ILAR News Vol 23(1) -- 1980



Institute of Laboratory Animal Resources
Division of Biological Sciences
Assembly of Life Sciences
National Research Council

ILAR News, Vol. XXIII, No. 1, Fall 1979


. . . .



I. Introduction

II. Genetically Defined Stocks
A. Inbred Strains
B. F1 Hybrids
C. Mutant Stocks
D. Coisogenic, Congenic, and Segregating Inbred Strains
E. Recombinant Inbred Strains
F. Inbred Strains Versus Outbred Stocks in Research

III. Stocks Not Genetically Defined

IV. Selection of Experimental Animals

V. Breeding Colonies of Genetically Defined Animals
A. Proliferation of Sublines
B. Documentation of Sublines
C. Coisogenic, Congenic, and Recombinant Inbred Lines

VI. Genetic Quality Control
A. Coat Color
B. Skin Grafting
C. Biochemical Markers
D. Immunological Markers
E. Skeletal Morphology

VII. Nomenclature
A. Inbred Strains of Mice, Rats, Guinea Pigs, and Rabbits
B. Coisogenic, Congenic, and Segregating Inbred Strains
C. F1 Hybrids
D. Outbred Stocks
E. Mutants and Variants



. . . .

I Introduction

For biomedical research involving laboratory animals to be effective in
the sense that results from different laboratories can be compared and
evaluated, there must be recognized criteria for animal care, including
definitions of animal health status and genetic constitution. Genetic
uniformity, which facilitates reproducibility of experiments, is highly
desirable in experimentation with animals and should be utilized
wherever practicable. Comparisons among results derived from work with
different genetically defined types within the same species may add to
the significance of those findings. In certain mammalian species,
especially the mouse, and to a lesser degree the rat, rabbit, guinea
pig, and hamster, the availability of a great variety of genetically
defined stocks provides research workers with precise experimental

Other species, in which genetically completely homogeneous animals are
not available, are also essential for research. For these species,
clear recognition of the genetic status of each stock, provision of as
much genetic information as feasible, and colony management designed to
avoid genetic differentiations within a single colony are all desirable

The basic rules of Mendelian genetics (segregation and independent
assortment), and the acknowledged fact of increased fixation of
homozygous genotypes through matings between related individuals, lie
behind all of the recommendations here summarized. The practical steps
to be taken to ensure optimal genetic quality in a given situation
differ, however, according to the original size and genetic status of
the animal colony involved. Complete genetic fixation is easily
available in some species; it is probably not even approachable in
others. It is necessary, therefore, to deal with maintenance of both
inbred and outbred colonies, and to examine the philosophy of choice,
both of suitable species and of particular genetically defined types
within a single species for any given kind of research. One must also
consider the nature, potential, maintenance, and utilization of
genetically completely defined stocks, and the use and maintenance of
outbred animals. Procedures for maintenance and genetic monitoring of
genetically defined stocks must also be included.


II Genetically Defined Stocks

A brief description follows of the types of genetically defined stocks
available to research workers, including inbred, coisogenic, congenic,
and segregating inbred, F1 hybrid and recombinant inbred strains, and
mutant and outbred stocks. The availability of each of these types in
the more common laboratory species is indicated.


Inbred strains of mice, rats, and guinea pigs have been available for
more than 60 years. Strains in common use today originated more than 40
years ago, although new strains are constantly being developed.
Currently there are available more than 260 inbred mouse strains, 100
inbred rat strains, and 30 inbred hamster strains. Only two completely
inbred rabbit strains and a few completely inbred guinea pig strains
exist today, although others are partially inbred.

Each inbred strain is the result of 20 or more consecutive generations
of single-pair brother x sister mating, in which all animals trace back
to a single breeding pair in the 20th or subsequent generations. As a
result of this pattern of matings, virtually all genetic variation is
eliminated. Individuals of an inbred strain are in many ways similar to
a set of monozygotic twins, and they afford a powerful tool in many
areas of biomedical research. Properties that make them so valuable

1. Homozygosity
All animals of an inbred strain are homozygous at virtually all genetic
loci, such that an animal will breed true if mated to another animal of
the same inbred strain. There will be no "hidden" recessive genes.

2. Isogenicity
The term "isogenic" means that all individuals are genetically
identical. As a consequence, skin grafts and tumors exchanged between
members of the same inbred strain will not be recognized as "not-self,"
and thus will not be rejected. Moreover, if an individual of the strain
can be genetically typed at polymorphic loci that differ between
strains, this typing will apply to the whole strain. For example, the
genotype at the major histocompatibility complex (H-2 in the mouse, RT-
1 (previously H-1 or AgB) in the rat) is already known for most inbred
strains of mice and rats. Because any one individual will have a
complete set of the genes present in the whole inbred strain, daughter
colonies may be set up in different laboratories that will be
genetically identical with the parent colony. (To be strictly accurate,
it must be pointed out that no strain can ever become completely
genetically fixed and is therefore never completely isogenic; for most
practical purposes the above statements are valid.) By contrast, in
outbred stocks of most species (with the possible exception of the
hamster), isografts will usually be rejected, the array of genotypes in
the stock is never completely known, and daughter colonies will contain
but a sample of the genes present in the parent colony.

3. Uniformity
Because they are isogenic, inbred strains will be uniform with respect
to all such highly heritable characteristics as blood and tissue types,
biochemical polymorphisms, and many morphological features. As for such
quantitative characteristics as body weight, litter size, and various
types of behavior, some variability will arise in response to
environmental influences that are not constant for all individuals, but
in general these characters will be more uniform in inbred than in
outbred stocks. Granted there may be exceptions to this general rule if
a given strain happens to be highly sensitive to certain environmental
influences. In other cases inbred strains may appear more variable as a
consequence of the scale effect. Thus a strain with a 20 percent tumor
incidence appears to be more "variable" for tumors than one with a zero
incidence: this is not, of course, variability in the usual sense of
the word.

. . . .

Nick Theodorakis
nicholas_theodorakis at urmc.rochester.edu

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