Green males that are split for brown and dilute are called passe partout. This article explains that these splits occur in two distinct genetic configurations:in coupling and in repulsion. The utility of these stock birds for the color bred enthusiast is also explained.
An early ambition of the color bred canary fancier was to transfer the dilute gene from the agate, a green dilute, to the brown bird, thus producing an isabel1, a brown dilute. According to Mendel’s law of independent assortment this is impossible. Since both traits are sex linked it should not be possible to shuffle the genes to obtain the desired combination.
+ Wild, Normal type
1) using a brown cock with an agate hen
b+ X +d
this mating gives the following progeny:
50% b+ Green cocks/brown and dilute
50% b+ brown hens
Crossing the cocks from pairing one back to the mother (or any other agate hen) gives these results according to Mendel:
2) b+ X +d
25% b+ Green males/brown and dilute
25% b+ brown hens
25% +d agate males
25% +d agate hens
In theory it seems impossible to get the isabel bird. In practice pairing number two actually gives the following progeny:
(a) b+ Green males/brown and dilute
(b) b+ brown hens
(c) +d agate males
(d) +d agate hens
(e) ++ Green males/dilute
(f) ++ Green hens
(g) bd agate males/brown
(h) bd isabel hens
The birds of genotype (h) will be isabel hens, dilute browns. The birds of genotypes (e), (f), (g), and (h) are products of crossing over.
Why is there this gap between theory and practice?
These traits do not follow the law of independent assortment because they are not independent. They are both located on the same chromosome, in this case the x chromosome.
During the production of eggs and sperm homologous chromosomes, chromosome pairs, are located in close proximity to each other. Sometimes points of contact and breakage are formed. These discrete points are visible under magnification and are called chiasma. At these points single strands, chromatids, of each chromosome may break and switch chromosomes. This whole process is known as crossing over. Genes that are located on the same chromosome are said to be linked or in linkage. Crossing over implies linkage but linkage does not necessarily imply crossing over.
There are two plans by which linked traits may become involved in crossing over:in repulsion and in coupling. Genes are linked in coupling when both mutant factors are located on the same chromosome.
(1) b+ (2) bd
Figure one symbolizes traits linked in repulsion. This is the genotype of the males from mating one. Clearly, our original example is of linkage in repulsion. Also, for the sex linked traits, only the male is able to produce gametes that are products of crossing over, since only the male possesses two x chromosomes.
The second formula represents linkage in coupling. Such a bird may be produced by mating a green male to an isabel hen.
(3) ++ X bd
50% ++ Green males/brown and dilute 50% ++ Green hens
The males are again green/brown and dilute but it must be noted that the genes are positioned differently on the chromosomes.
Crossing a male from mating three to an agate hen gives the following results:
(4) ++ X +d
(a) ++ Green males/dilute
(b) bd agate males/brown
(c) ++ Green hens
(d) bd isabel hens
(e) +d agate males
(f) +d agate hens
(g) b+ Green males/brown and dilute
(h) b+ brown hens
The genotypes listed as (e), (f), (g), and (h) are the products of crossing over. Since the isabel hens are here produced according to independent assortment, without crossing over, a higher percentage will probably be produced than in mating scheme number two.
Green males that are carriers of brown and dilute are known as passe partout. The original plan, that used by Mr. Helder, pioneer color bred canary fancier, for producing such birds, mating number one, yields passe partout in repulsion. Mating number three also gives us passe partout birds, but this time linked in coupling.
These passe partout are very valuable birds, for with them all four of the classical canary colors may be very economically produced. The four classical colors are Green, brown, isabel, and agate. Though these passe partout birds are important as stock birds, they are useless for show. Most of them have light beaks and pale legs. Many also show a brown suffusion in the feathers
The observant reader may have noted that I failed to give percentages when discussing any pairing that involved crossing over. In these cases the percentages can not be computed theoretically. The probability of crossing over and the implied frequencies of the progeny may only be inferred from breeding results. The closer the two linked genes are located the less likely is the chance of breakage and the consequent crossover. Genes located at a relatively great distance apart have a much greater likelihood of crossing over.
This probability is expressed as a percentage and varies, for different traits, from 0% to 50%. The probability of crossing over for any two specific traits, as it is a function of the location of the genes, is nearly constant. To calculate thus figure it is necessary to cross a heterozygote for the traits with a homozygote. The following formula may then be applied:
PERCENTAGE OF = NO. OF CROSS OVER INDIVIDUALS X 100
CROSSING OVER TOTAL NUMBER OF INDIVIDUALS
Without having any exact figures available, as an educated guess, we amy assume that the progeny that are not the result of crossing over are more likely to appear than those that are due to crossing over. Remember, crossing over, though predictable, is the result of a microscopic accident. It can at best, as a limit, only equal the frequency of independent assortment. It is most often less likely.
What all this means is that in matings (2) and (4) progeny (a), (b), (c), and (d) are more likely to be produced than (e), (f), (g), and (h). If your intention is to produce isabel hens, use passe partout derived from a Green X isabel, a passe partout in coupling. If you want to rear agate hens, use a passe partout derived from a brown X agate, a passe partout in repulsion.
As far as I know, the percentage of crossing over is unknown for any of the traits in the canary. The fancier may make a real contribution to science by keeping good records and performing the appropriate calculation. Thus we will be better able to produce the exact phenotype that we desire.