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Sex-linked inheritance and sexual dimorphism
Sex-linked inheritance with sexual colour dimorphism
When talking about sex-linked pigeon colouring in general, the first
thing that comes to mind is the Texan. These are not only
'sex-linked' in heredity when mated with other colourings. They also
show a sexual dimorphism in colour: male pigeons are lighter
coloured than female pigeons. When mated with each other, this
sexual dimorphism remains purely hereditary in the strain over the
generations.
Crosses of these sex-dimorph Texans with other colourings
demonstrate that the responsible hereditary factor 'faded' is
located on the sex chromosome: A white or light speckled cock mated
upon a blue hen will produce hemizygous females in the slightly
lightened female colour, and similarly coloured heterozygous cocks.
From the reverse mating, a blue cock with a 'faded' hen, you get
females without faded factor and heterozygous cocks in the female
colour. Similarly act the dominant stipper gene and the recessive
frosty gene, both alleles of faded. Because of the health defects in
most homozygous stipper cocks, and their unattractive whitish
colouring, the cocks at exhibitions are heterozygous stipper and
show the colouring of the hemizygous females. This is also true for
Almonds, where cocks of the colour required in the standard are
always heterozygous for the stipper gene.
Fig. 1: Sexual dimorphisms at Stipper, Faded, and Frosty. Source:
Critical Issues in Pigeon Breeding. What we know and what we believe
to know, Part III (2020)
Sex-linked inheritance without colour sexual dimorphism.
Sex-linked inheritance also exists without clear sexual dimorphism
due to other hereditary factors located on the sex chromosome. This
was discovered earlier than with the stipper and frosty genes.
Namely with the recessive hereditary factor 'dilution' and the
dominant hereditary factor 'dominant red'. Later also with factors
like 'reduced' and 'rubella'.
Homozygous dominant red cocks and hemizygous dominant red female
pigeons do not differ noticeably in colour. Sex-linkage is evident
in the first cross: from a homozygous blue cock and an ash red hen
only blue females and ash red cocks fall, in both sexes, from the
reverse mating of a homozygous ash red and only blue females and red
cocks.
 
Fig. 2: Blue cock x dominant red hen and heterozygous ash red son
and blue daughter
 
Fig. 3: Homozygous dominant red cock x blue hen and dominant red
progeny, a heterozygous dominant red cock and a hemizygous dominant
red hen. Source: Pigeon Genetics 2012, figures 13-16).
Darwin (1868)nknew from French literature (Boitard and Corbié 1824
and Chapuis 1865) that some of the dominant red cocks were
distinguished from other red pigeons by black spots after crosses.
As was later shown, this is not a sexual dimorphism of dominant red.
It is a sign of the heterozygosity of these cocks for black pigment.
Fig. 4: Distinct ‚ink spots‘ in tail and primaries at a heterozygous
ash red cock. Source: Pigeon Genetics, fig. 18.
Sex-limited inheritance (coloured sexual dimorphism without
sex-linkage).
In sex-limited inheritance, the hereditary factors responsible for a
trait are not located on the sex chromosome. They are thus autosomal
genes. Male individuals can have the hereditary factors, they are
also inherited by the offspring, but they occur almost exclusively
in only one sex. When thinking of these characteristics, one first
thinks of the decorative plumage and other attributes of the male
sex in birds. An example from livestock breeding is the laying
performance of females in chickens and that the yield and quality of
milk are passed on via the bull, but that the gene can only have an
effect in the female sex
(http://www.biologie-online.eu/genetik/geschlechtsbegrenzte-vererbung.php).
Direct parallels to this, which have been analysed in more detail,
are not to be found in the pigeon literature. In the case of the
autosomal factor 'Recessive Opal', in combination with the colour
spread factor 'Spread', breeders point out that cocks are usually
light silver-grey and hens considerably darker. In the case of the
platinum factor, which is also considered to be recessive and
autosomal, the cocks differ from the hens already in the bar and
check variety on closer inspection. Cocks have a stronger lightening
of the primaries and tail feathers than females. If spread is added,
the difference becomes even more obvious (Sell 1986, 1994, 2012,
2015).
 
Fig. 5: Platinum check couple (hen at the left) and Spread Platinum
couple with darker hen and light platinum cock
Fig. 6: Spread Platinum (3 cocks and 3 hens) in the 2010th
After the discovery of the factor in the 1960th, the
constantly inherited sexual dimorphism in spread platinum remained
in breeding for decades. After that, much lighter cocks than in the
initial decades and also lighter females suddenly appeared. In these
the colour sexual dimorphism was abolished. Even after crosses of
platinum with blue homing pigeons, such light-silver-greys were
found in some cases in later generations in the last years.
Potential causes, which can only be speculated about, are the
mutation of the original gene into an allele without this
sex-limitation and the occurrence or omission of modifiers by
outcrossing upon other colours.
Following the surprising amplifications of sexual dimorphism in the
Frosty combinations shown below, it is conceivable that a sex-linked
factor with minimal and almost imperceptible sexual dimorphism is
also the basis in platinum and may be also in Spread recessive opal.
It could be superimposed by the effect of another non-sex-linked
factor. If there is such a genetic reaction in frosty-rubella and
frosty-grizzle, there could also be analogous reactions in platinum
colouration. However, the probability that such a connection had not
held so firmly for several decades argues against this. Probably the
platinum-coloured ones will have disappeared before the mystery is
solved.
Enhancement of sexual dimorphism in sex-linked factors by modifiers:
Frosty, Rubella and Grizzle
Frosty-Rubella
In this context, observations on the sex-linked frosty factor
responsible for sexual dimorphism are noteworthy. Homozygous frosty
cocks show a slight colour brightening, similar to that in
heterozygous faded. In females it is not or barely visible (see Fig.
1). Rubella is recessive sex-linked. In contrast to the stipper
alleles rubella bar and rubella check are not sexually dimorphic.
The combination, Frosty + Rubella, results in almost white appearing
silver-grey cocks with sometimes slightly translucent bars or
checks.
 
Fig. 7: Young frosty-rubella cock at the book cover with slightly
visible checks and near to white frosty-rubella cock (not Spread!)
Females become ice-grey and the rubella-coloured pattern in rubella
pigeons are toned down in colour). The sexual dimorphism of frosty
is extremely increased by rubella. In outcrossing upon other colours
it is a fragile combination because rubella and frosty are
relatively far apart on the sex chromosome and in subsequent matings
the linkage can be broken (linkage breakage and crossover).
 
Fig. 8: Frosty-Rubella couple (left) and Frosty-Rubella hen besides
a frosty hen (at the right) Source: Critical Issues in Pigeon
Breeding, Part III and V.
Frosty-grizzle
Amazing also the effect achieved in Frosty by adding the grizzle
gene already in heterozygous state. In hemizygous frosty females the
grizzle factor has the same effect as in blue females without the
Frosty gene. Grizzles are produced in the intermediate colouration
desired in standards. In homozygous 'blue-grounded' frosty cocks'
(Fig. 1), whitish white-storked, as obtained in some breeds in
homozygous grizzles, are produced. We owe the discovery to Frank
Zetzsche during his efforts to rebreed the grizzle colour variety in
Thuringian Self (Thüringer Einfarbige).
Fig. 9: Frosty-Grizzle. Hemizygous frosty hen, heterozygous grizzle
(left), homozygous frosty cock, heterozygous grizzle (photos and
breeder Franz Zetzsche), Source: Critical Issues in Pigeon Breeding
Part II (2021)
Didactic aids
If you do not want to take note of the above statements as mere
assertions, you will have to deal with genetic basics. With the help
of Punnett's squares as a didactic aid, it is easy to understand how
sex-linked and sex-dimorphic inheritance occurs. Also, what the
combination of hereditary factors can do.
It has been condensed into a booklet with exercises and solutions in
English, Dutch and French. On the cover of the supplement to the
Introduction to Pigeon Genetics, the mating of a heterozygous
stipper cock with a black hen is reproduced in Punnett's square.
 
Fig. 10: Cover ‘Introduction to Heredity in Pigeons’ and the
supplement with exercises
Literature
Sell, A., Pigeon Genetics. Applied Genetics in the Domestic Pigeon,
Achim 2012.
Sell, A., Breeding and Inheritance in Pigeon, Hengersberg 1994.
Sell, A., Genetik der Taubenfärbungen, Achim 2015.
Sell, A., Taubenzucht. Möglichkeiten und Grenzen züchterischer
Gestaltung, Achim 2019.
Sell, A., Introduction to Heredity in Pigeons, Achim 2022.
Sell, A., Critical Issues in Pigeon Breeding. What we know and what
we believe to know Parts I-VI, Achim 2020/2021.
Sell, A., Zucht und Vererbung, Hengersberg 1986.
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