Gregor Johann Mendel was a Silesian monk that in 1822 set the main principles of the genetics. His theories are, still today, the basis for developing inheritances laws and theories. His “laws of genetics” are still learned in high schools and colleges.
Mendel’s First Law
The meanings of DOMINANT AND RECESSIVE we seen are essential for the good understanding of Mendel’s theories. Let us make the things easier working with “pure” bloodlines.
“bloodline is the group of direct descendants from a common ancestor.”
That means the sons, grandsons, grand grands, etc from a determined bird. Nowadays it is very difficult to get pure bloodlines due to the enormous number of mating done. So a single bird carries several mutant genes that were added from generations of previous mating.
This does not mean we have hybrids, or mestizos. We just have carries of several pairs of mutant genes; i.e., heterozygotes, or splits, for several mutations. The term split is commonly used in aviculture as a synonym for heterozygote. (D’Angieri, A. –“The Colored Atlas of Lovebirds” – TFH-1997)
“PURE line is one that carries just one pair of mutant genes”
So first we will work just with pure lines: Pastels will transmit just pastel, the dilutes just dilute and so on. The generations of descendants are designated as F1 (first generation), F2 (second generation), F3 (third generation), etc.
|100% GREEN /RECESSIVE PIED (split for recessive pied).|
We have seen above that the genes are given by each one of the parents which originates “Paupau” birds: green split recessive pied.
The green color is a schizochroic interaction of several genes. The gene “Pau” IS NOT responsible for the green color, but just ONE OF THEM. Here it shows just the absence of the mutant factor.
|GREEN/RECESSIVE PIED||×||GREEN/RECESSIVE PIED|
|Green||green / recessive pied||recessive pied|
|75% green phenotype||25% recessive pied phenotype|
|green||0.5Pau × 0.5Pau||0.25=25%|
|green / recessive pied||2 ( 0.5Pau × 0.5pau )||0.25=25%|
|recessive pied||0.5pau × 0.5pau||0.25=25%|
Here we observe a 3 to 1 rate (non-pied : pied). This rate occurs in pure bloodlines due to the chromosome segregation. This is Mendel’s First Law.
“The gametes are transmitted to their descendants in a 50% rate”
|GREEN/RECESSIVE PIED||×||RECESSIVE PIED|
|2 (0.5P × 0.5p) = 50%||green / recessive pied|
|2 (0.5p × 0.5p) = 50%||recessive pied|
Mendel’s Second Law
Mendel’s First Law is very restricted to be used in Agapornis breeding as gene interaction is not involved.
The schizochroic nature of Agapornis coloration is such that all characters are the result of genic interaction. Therefore, the second Mendelian law, which considers two or more allele pairs at the same time, is more pertinent. (D’Angieri, A. –“The Colored Atlas of Lovebirds” – TFH-1997)
Let us better illustrate the phenomenon of the independent segregation of gene pairs: each pair of parental genes will produce gametes that carry just one its allele like in first law!
|american dilute edged o (gaga)||×||aqua (pp)|
|N.B: The genes “Ga” and “P” show the absence of the (ga) e aqua (p) respectively.|
The double split GagaPp is GREEN. Both genes are in heterozygosis so we have a NON-DILUTE EDGED NON-AQUA green bird which does not show any mutant color as both involved genes are recessive.
The Mendel’s Second Law proportionate a 1/16 (6,25%.) frequency in each square above.
|GagaPp||25.00||green/dilute edged, aqua|
|gagaPp||12.50||dilute edged./ aqua,dilute edged aqua|
|gagapp||6.25||dilute edged aqua (former silver cherry)|
The double homozygosis gagapp is an interaction producing a third phenotype inexistent so far: the former silver cherry. There will be four different color is this pair: green, aqua, dilute edged green and dilute edged aqua.
This is a typical example of the way of action of mutant genes of the Agapornis and very good illustrates the genetics and its DOMINANCE, RECESSIVE AND GENE INTERACTION PHENOMENA.
|aqua / dilute edged||×||aqua / dilute edged|
Pay attention to aqua factor in homozygosis from both parents which leads the frequencies to the Mendel’s first law.
|blue personatus||×||pastel green personatus|
|100% personatus green/blue, pastel|
|Aaidid||green pastel /blue|
Let us try three pairs of genes together:
|pastel blue personatus||×||DD green personatus (olive)|
|100% medium green (D)|
Twelve different phenotypes were produced:
|medium green||(D)I_ A_D_A_dd|
|green||I_ A_ dd|
|DD green||I_ AaDD|
|DD pastel||idid A _ DD|
|pastel medium green||idid A _ DD|
|D blue||I_ aa D_|
|pastel D blue||ididiaaD_|
|DD blue (dark blue)||I_aaDD|
|pastel DD blue (dark blue)||ididaaDD|
Crossing Over And Linkage
The tendencies of the genes to keep together in chromosomes is called “linkage” that will show us the Mendel’s second Law is related only to different chromosomes (non-homologous). Indeed, a chromosome consists of a chain of genes. They are lined up, and, when they are duplicated, identical genes should result. However, the practical results show us that gene recombination can nevertheless occur, as a result of another phenomenon: crossing over .Crossing over is an occasional exchange, without preset rules and at no particular point, of parts of the filaments (chromatids) of homologous chromosomes, in a tetrad (the group of four chromatids formed in the course of duplication of the homologous chromosomes).
After an initial intertwine, the filaments repel one another, allowing an exchange of fragments bearing several genes. This mechanism accounts for just one aspect of the impossibility of recombination predicted by Mendel’s second law.
The crossing-over rate between two genes is directly proportional to the distance along the chromosome between them: the farther apart, the greater the crossing rate, up to a maximum of 50%.(D’Angieri,A – The Colored Atlas of Lovebirds – TFH -1997).
In Agapornis, the linkage phenomenon together with crossing-over occurs clearly in roseicollis: Let us consider the turquoise Dark factor (D) and the Aqua factor (p). When there is a complete linkage between them, each gamete shall carry ether DP or dp and there will be no recombination. In this case there will be just two kinds of gametes: DP (50%) and dp (50%). However, we know that Dark Pastels (Mauves) do occur; this is genetically Dp, indicating that crossing have occurred.
Example 8: Dark Green (DDPP) x AQUA (ddpp) yields 100% Medium Green (DdPp). These are next paired among themselves: DdPp x DdPp.
|F1:||DdPp||100% D green (medium green)|
Theoretically, we expect Dark Pastels (DDpp) at a rate of 1/16, or 6.25%. We have observed that their real frequency is 1/93, or 1.07%. A crossing rate of 1.07% means that approximately 1.10% was crossed over. So we have Dp = 0.55% and dP = 0.55%, yielding a sum of 1.10%. Thus 98.90% experienced no crossing over. (D’Angieri – “The colored Atlas of Lovebirds”):
There are two types of splits for aqua, just the type II will produce DD aquas:
|type I||type II|
It is the occurrence of genes that are linked to sex chromosomes which must me designated by Z and W letters although the majority of the authors insists to use X and Y.
This should no be done as the use of X and Y indicates automatically that males do express the heterogametic sex (XY). This does not occur in birds. The Z and W system was first observed in genus Abraxas of insects, later in fishes and birds. There insects and fishes X and Y typed but so far no bird was observed. (D’Angieri,A, The Colored Atlas of Lovebirds-TFH -1997)
“The use of XY with birds is complety wrong, because once it is applied, it is assumed that the heterogametic sex is represented by the males, which isn’t the case with birds” (D’Angieri,A, The Colored Atlas of Lovebirds-TFH -1997)
Females determine the sex in birds; it is done by W chromossome.
That means MALES are ZZ and FEMALES ZW. That explains there are more females than males of sex-linked characters, they occur just in roseicollis and are the opaline, lutino, pallid and cinnamon factors.
You must keep in mind that birds have a chromosome map with diploid chains 40-80, that means linkage phenomenon is a rule not exception in birds (Bucley, 1987)
|100% green/opaline, aqua males|
|100% opaline green/aqua females|
Polyallelia is quite representative in lovebirds. Recently the pastel factor in personatus and its transgenics have been proven to be polyalleles of ino factor.
A typical case of multiple alleles, which means there are more than one pair of genes in the same locus. The ino factor in roseicollis is allele to pallid .It is also said by some authors that turquoise and aqua might be alleles. In our experience they are not as we are going to discuss further.
The roseicollis’ ino factor and the pallid (former Australian cinnamon) are the first example known between two multiple alleles that occurred apart in the world.
U.S.A in 1973 and Australia in1957 respectively.
Pallid is indeed a partial ino factor, schizochroic color of lutinism.-(D’Angieri, A – The Colored atlas of Lovebirds-TFH-1997)
|100% pallid (par-ino)|
|males:||ZIZia||green / pallid (par-ino)|
|ZIZi||green / ino|
Epistasis (Non Visual Birds)
Epistasis is a genetic phenomenon that occurs when a certain character dominates over another non-allelic gene.
One good example involves the Violet and Ino factor, which are found on different chromosomes. When together in the same bird, the ino trait dominates. We cannot detect the Dark factor, although the bird in question is genetically a Violet Lutino .
We can say that the Ino factor is epistatic to the Violet factor, or that the Violet is hypostatic to the Ino. It is important to note that only the Ino factor’s American allele is epistatic to the Violet factor. The Pallid factor interacts with Violet factor to produce a violet ramped dull yellow bird.
Let us have a better idea with its genetic formulas:
It is important to say that Violet inos are non-visual violets.
“Epistasis is the phenomenum that turns one expected visual bird to a non-visual bird. It may be either dominant or recessive”.
There are factors that are known as deleterious genes, or better say they provides such a kind of immune deficiency that increases lethality or diseases. They might be the pale fallow roseicollis and bronze fallow personatus. There is also the Japanese dilute that is quite deleterious to female birds. In fact they are so few in numbers that should be better studied.
So far there is no proven lethal gene described in lovebirds.