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Linkages and
crossing overs in pigeons: Exemplified by basic color, dilution and
beak length
Linkages in pigeons was
demonstrated as early as 1919 by Cole and Kelley using the example
of basic colors and dilution. Every general textbook on genetics
covers the topic in detail. Anyone who enjoys using these terms
should have at least once understood their meaning and effects using
an example.
"Exemplary," a favorite word
among educators. Once you understand the "mechanism" using an
example, you should be able to apply it to other phenomena. Anyone
who knows how blue-check and blue-bar pigeons behave when mating
will also be able to apply it to browns, ash red, indigo, reduced,
and other color varieties with these patterns. Anyone who has
understood the meaning of dominant, recessive, and intermediate
using the example of checkered pigeons should also be able to apply
it to indigo when mating with blue. This also applies to linkages.
This is easiest to illustrate and understand using genes on the sex
chromosome. In the female, the chromosome is counterbalanced by a
much smaller W-chromosome. This chromosome primarily carries
information for the development of female sexual characteristics.
Unlike the male, the female therefore possesses only one set of
sex-linked genes, such as those for basic colors and dilution.
Figure 1 shows an authentic
mini-demonstration. The black cock and his diluted dominant red hen
(dominant yellow) regularly breed black young hens and heterozygous
dominant red cocks. The genetic makeup of the chromosomes is shown
in the image. In the following the effects on the females are
central. Unlike cocks, they cannot be heterozygous and are therefore
easier to classify.
Figure 2 shows the back-mating
of the heterozygous male from Figure 1 to a dominant yellow female.
Without crossovers, the young male will produce black females and
dominant yellow females. This is clearly not the case with the two
females, one with the diluted black color (dun).
This can be explained by
crossing over in Fig. 3. In the final phase of preparation of the
chromosomes for fusion, the two chromosomes (here from the
heterozygous ash red//black and heterozygous dilute//intense cock)
lie together, cross over, divide again, and fuse together in a new
combination.
Due to crossing over, the
offspring also include black diluted females (dun) and intensely
colored reds. We concentrate on females with a black base color
since they can be classified in the nest. Dun-colored females among
these are crossing overs, blacks, blue-checked females, etc. are
not.
In the report by Cole and
Kelley, a crossing-over rate of 40% was determined, which suggests a
large distance between the gene loci (p. 199). In the
mini-experiment, limited to the evaluation of black basic color
females, the rate was even 47%, close to the 50% expected if the
genes were independent. The 1919 study identified early, prominent
corners of the sex chromosome for mapping. Both gene loci are far
apart at opposite ends.
With the discovery that the
gene locus for stipper and alleles is close to the gene locus for
the base colors, and the gene locus for reduced and rubella is close
to the gene locus for dilution, the chromosome map could be further
filled. Hollander was uncertain whether the order of dilution (d)
and reduction (r), as well as of St (stipper) and color locus (b for
brown), was correct.
Also not yet identified is the
sex-linked lethal webbed foot, analyzed by Hollander/Miller (1982).
From the data of Christie and Wriedt (1923), Hollander had concluded
that there is also a sex-linked influence on short beaks and that
other factors are likely to be involved (Hollander 1983). Recently,
this conclusion was also confirmed in molecular genetic studies
(Boer et al. 2021). In an earlier experiment of our own, crossing a
pale dominant red male Gimpel (gold gimpel white wing cock) with a
short-beaked female with a black base color, a close linkage of the
gene loci for short beaks and base colors was indicated in the F2
and in backcrosses (Sell 2012). This was closer than the 30%
measured later in the second series of experiments (Sell 2019). The
linkage break can occur at different locations. The dun-colored
female shown in Fig. 3, with the gene combination 'black, long
(wild-type) beak, and dilution,' is the result of a CO behind the
locus for beak length and before the locus for dilution factors (5).
Multiple crossovers are also possible and makes matter more
complicated.
Fig. 5: The chromosome pair of
the heterozygous male before crossover, during crossover, and after
splitting and fusion in a new combination for the gene loci for
basic colors, short beak, and dilution.
Crossing over is not specific to pigeons; it is
described in detail in all genetic textbooks. For example, R.
Goldschmidt's German language "Theory of Inheritance," published in
1927 in the first edition and in 1929 in the second edition. The
print run is impressive, with a total of 10,000 copies. Clearly,
there was a great thirst for knowledge back then. Today, much is
freely available online. Nevertheless, one will have to develop an
understanding for oneself and have the flexibility to apply the
general information presented to one's own questions.
Literature:
Boer EF, Van Hollebeke HF, Maclary ET, Holt C,
Yandell M, Shapiro MD. A ROR2 coding variant is associated with
craniofacial variation in domestic pigeons. Curr Biol. 2021 Nov
22;31(22):5069-5076.e5. doi: 10.1016/j.cub.2021.08.068. Epub 2021
Sep 21. PMID: 34551284; PMCID: PMC8612976.
Christie, W., und Chr. Wriedt, Die Vererbung
von Zeichnungen, Farben und anderen Charakteren bei Tauben,
Zeitschrift für induktive Abstammungs- und Vererbungslehre 32
(1923), S. 233-298
Cole, Leon J., and Frank J. Kelley, Studies on
Inheritance in Pigeons. III. Description and Linkage Relations of
Two Sex-Linked Characters, Genetics 4: 183-201, 1919
Goldschmidt, Richard, Die Lehre von der
Vererbung, Zweite Auflage 6. Bis 10. Tausend mit 50 Abbildungen,
Berlin Verlag von Julius Springer 1929
Hollander, W.F., and W.J. Miller, A New
Sex-Linked Mutation. Web-Lethal from Racing Homers. American Racing
Pigeon News, Oct. 1982
Hollander, W.F., Origins and Excursions in
Pigeon Genetics, Burrton, Kansas 1983
Sell, Axel, Molecular genetics of short beaks,
in: Sell, Axel, Critical Issues in Pigeon Breeding. What we know and
what we believe to know, Part VI Achim 2021, p. 54.
Sell, Axel, Pigeon Genetics. Applied Genetics
in the Domestic Pigeon, Achim 2012
Sell, Axel, Taubenzucht. Möglichkeiten und
Grenzen züchterischer Gestaltung. Strukturen, Figuren, Verhalten,
Zucht und Vererbung in Theorie und Praxis, Achim 2019
AS May 2025
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