The traditional method of selective breeding practiced by East Asian and South American cultures is defined as
Backcrossing is a crossing of a hybrid with one of its parents or an individual genetically similar to its parent, in order to achieve offspring with a genetic identity which is closer to that of the parent. It is used in horticulture, animal breeding and in production of gene knockout organisms.
Advantages
* I
f the recurrent parent is an elite genotype, at the end of the backcrossing programme an elite genotype is recovered
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As there is no "new" recombination, the elite combination is not lost
Disadvantages
* Works poorly for quantitative traits
* Is more restricted for recessive traits
* In practice, sections of genome from the non-recurrent parents are often still present and can have deleterious traits associated with them
* For very wide crosses, limited recombination may maintain thousands of alien genes within the elite cultivar
Backcrossing may be deliberately employed in animals to transfer a desirable trait in an animal of inferior genetic background to an animal of preferable genetic background. In gene knockout experiments in particular, where the knockout is performed on easily cultured stem cell lines, but is required in an animal with a different genetic background, the knockout animal is backcrossed against the animal of the required genetic background. As the figure shows, each time that the mouse with the desired trait (in this case the lack of a gene (i.e. a knockout), indicated by the presence of a positive selectable marker) is crossed with a mouse of a constant genetic background, the average percentage of the genetic material of the offspring that is derived from that constant background increases. The result, after sufficient reiterations, is an animal with the desired trait in the desire genetic background, with the percentage of genetic material from the original stem cells reduced to a minimum (in the order of 0.01%).
Due to the nature of meiosis, in which chromosomes derived from each parent are randomly shuffled and assigned to each nascent gamete, the percentage of genetic material deriving from either cell-line will vary between offspring of a single crossing but will have an expected value. The genotype of each member of offspring may be assessed to choose not only an individual that carries the desired genetic trait, but also the minimum percentage of genetic material from the original stem cell line.
Tosa Onagadori
For example, Japanese researchers in Hawaii experimented with the most parsimonious method to reach the Onagadori stage of the long tailed cultural treasure. Following Japanese tradition, they utilized a specific form of backcrossing- more specifically, matrilineal backcrossing- that is,
male progenitor to matriarch over successive generations.
First, an appropriate female was selected. She was from a very homogenous Tosa-Kojidori X Tosa Mikawa strain. This is the Japanese equivalent of a Leghorn cross battery egg producer. Neither of light weight egg producing progenitor breeds, the Kojidori or Mikawa are long tailed breeds. The researchers chose the egg producer because the genome of the strain was completely mapped and the strain is closely bred to the point that each egg is produced by each hen is nearly identical in size, weight and shape.
The " Tosa Tomu" production egg layer hen was paired with a wild junglefowl collected in the Mariana and Marshall Islands that is known as the Firefox or Marquesas Island Junglefowl. It was introduced by seafarers hundreds of centuries ago, Japanese oral history has it that the founders of the Ongadori included wild roosters brought back from the Mariana Islands which are just south of Japan in the Philippine Sea.
Mariana/Marquesas Island Junglefowl.
The f1 progeny produced from the Mariana junglecock bred to the Tosa Tomu hen were selected from, with the males with the desired traits being bred back to the mother itself or one of her full sisters. The rest of the birds were removed from the breeding program. The junglefowl sire and a few of his male progeny of the f1 were set aside. The f2 progeny produced through backcrossing with the maternal line were again selected from. The males with the most desired traits were conserved- the rest of the stock removed from breeding. This continued for eight generations. On the 9th generation, females of the f9 generation were now held back and bred back to the original Junglecock sire or on his f1 sons. This generation-is considered a new generation and is called F( capitol) a1.
Generation Fa1 new sire.
The researchers selected for white plumage as it is was very easy to distinguish at hatching. By the 18th generation, a male ( pictured below) was produced that could now be bred to an f1 hen ( pictured as well) Their offspring breed fairly true to type and extremely elongated tails and saddles are becoming increasingly common with each successful completion of an eight generation backcross.
I've learned to use this backcrossing method to improve egg colour in Marandaise and Araucanian hens.
In short straight language, the matriarch is the most important gene stock not the male. People often make the mistake of line breeding a male on to his own daughters which would work better in mammals than birds ( see founder effect and Haldane's rule). In poultry, the most parsimonious route will be to keep breeding successive generations of roosters with the most desireable aesthetic back to their mothers or the full sisters of their mothers. That first generation of females is termed the " the matriarchate". Only use a single male founder and backcross this way. If you outcross even once before three full terms- ( 8 generations X 8 generations X 8 generations of backcrossing with no new founders) you will lose your way and have to start from scratch again. New females can be brought in after the third full eight generation backcross generation ( Fc) but not before then. If any new male material is brought in you will have to start your backcrossing again.
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