Top 10 Genetic Myths in Avian Genetics by Linda S

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Top 10 Genetic Myths in Avian Genetics by Linda S. Rubin All Rights Reserved Introduction pay attention to what is written on the in- Not surprisingly, and despite innumer- ternet, or passed on as good advice by the able benefits, aviculturists continue to find well meaning, to sometimes discover that it daunting to explore the principles that what is stated from “experts” may not be as explain heredity. Even the word, “genet- factual as hoped for. The following attempts ics” may cause some to inwardly cringe and to address some of the top ten myths that shudder. Yet, genetic principles are crucial are still perpetuated today in avian genetics to our understanding, because knowledge so that we can side-step these errors. of genetics can empower aviculturists with the ability to produce healthier strains of vi- Genetic Myth #1: Mutations are able breeding stock and future generations synonymous with hybrids. False! of robust species. When correctly bred, There is still much confusion around the breeding lines pass on such set qualities as differences between breeding color muta- increased egg production, higher fertility, tions within a species, and producing hybrid strong immune systems - and when prop- offspring between two different species. Linda S. Rubin erly applied - the ability to produce primary For clarity, the term “mutation” refers to Amazon parrot to a Lutino mutation Red- and rare color mutations to order. An un- changes in the outward color or pattern of lored Amazon parrot, would produce all derstanding of simple Mendelian rules and a bird. So, what are color mutations? Muta- Red-lored Amazon parrots. In other words, other modes of inheritance need not be tions may be caused by a variety of factors, all resulting offspring - whether nominant complicated, and many models, once un- but generally arise in aviculture as a sudden (wild colored) Red-lored Amazons, or Lu- derstood, are surprisingly simplistic. onset of a variation known as a spontaneous tino mutation Red-lored Amazons, would Although our first inclination is to ap- mutation which produces a new, anomalous all be the same species - 100% Red-lored ply genetic principles to outward color mu- color or pattern that differs from the nomi- Amazon parrots (Amazona autumnalis). tations and patterns in birds, the laws of nant (wild form) of the species. Some spon- Therefore, generally speaking, when ex- genetics equally pertain to all the traits in- taneous, partial mutations - if not fully de- amining a new color or pattern mutation, herited in a bird’s genotype. When applied veloped - may require a planned, selective we are looking at an example of a color mu- correctly, the laws of genetics allow us to breeding program requiring several genera- tation in the nominant wild species. We produce large, saleable baby birds raised by tions to achieve the full maturation of the are not looking at the offspring of dissim- fertile parental generations who success- final mutation. ilar species, but rather where both parents fully incubate, feed, and wean their own Viable mutations are able to reproduce are the same species. An example of a color babies, and also allows us to show the most because the genes - when inherited in the mutation would be a Blue Indian Ringneck exquisite bird to top perfected show stan- progeny - are either passed on as visuals (ho- Parakeet, bred from Indian Ringneck par- dards. In other words, the principles of he- mozygous), or carried as splits (heterozy- ents. An example of a hybrid would be off- redity affect every inherited trait - both visi- gous), depending upon the full genotype of spring from dissimilar species, e.g., one In- ble and invisible - in a bird’s genotype or full the parental generation. dian Ringneck Parakeet parent and one genetic makeup. Producing hybrids, on the other hand, Alexandrine Ringneck parent. Many of us learn avian genetics from requires mating a bird of one species to a Although color mutations are encour- our own hands-on work “in the field” (i.e., bird of a different species. For example, aged and sought out by many avicultur- the aviary), and from information gathered breeding Red-lored Amazon parrots to ists, most avicultural organizations frown and contained in our own studbooks, rather Green-cheeked Amazon parrots would re- upon the production of hybrids for an im- than through formal study. Our budding sult in offspring that are neither 100% ge- portant reason. Producing hybrids and knowledge and gradual absorption of facts netically Red-loreds (Amazona autumna- breeding their successive young will dilute over time contribute to the subtle and seem- lis), or 100% Green-cheeked Amazons ( the limited gene pool of stock available for ingly effortless process of learning through Amazona viridigenalis). Instead, the two the species involved. To continue to do so osmosis. Admirable. Except that even the different species would produce hybrid off- may affect the future availability of spe- most successful aviculturist may not possess spring that would not be recognized as ei- cies. Whereas, producing color mutations the text book knowledge to separate scien- ther species - autumnalis or viridigenalis. is simply changing the color and/or pattern tific fact from popular misinformation. We On the other hand, breeding a Red-lored

58 Volume XLII • Number 2 & 3 • 2013 of the species outwardly and in no way af- characteristic or trait. DNA in a gene can be the wild due to such factors as a more lim- fects their future establishment in captivity caused to mutate by two major occurrences. ited gene pool, the increased chance of sur- when bred responsibly. A mutation can be acquired due to envi- vival, and possibly an increase in genetic ronmental effects such as radiation or other drift. For example, this may be why we are Genetic Myth #2: Color mutations, agents, or when errors occur when a cell beginning to find more color mutations oc- compared to the nominant copies its DNA prior to the cell dividing. cur with the increase of producing some of species, are inherently weak. Mutations can be inherited randomly the larger Psittacine birds compared to de- from one or both parents. If a parent has cades ago. False! Mutations are a permanent change a mutation in its DNA, that mutation is Although established mutations can be in the DNA sequence of a gene affecting then passed on to its offspring. Mutations purchased and bred to reproduce in the avi- the genetic information of a hereditary occur more frequently in captivity than in ary, new mutations occur randomly and cannot be deliberately created, but rather, happen by chance. A common misconception is that color mutations, when compared to the nominant (wild) species, are inherently weak. Al- though there are a few autosomal recessive mutations in aviculture that have had inherent difficulties, it is not a given that new color and pattern mutations will be associ- ated with problem outcomes or lethal genes. One example of positive intervention occurred with the supposed original emer- gence of the (autosomal) Recessive Silver Cockatiel in Europe during the early 1960s, when it was first reported to have produced chicks that were born blind. While first breeding cockatiels in the mid-1970s, the author became aware of Recessive Sil- vers imported from Europe, some few years later. These birds, however, did not have the inherent blindness or weakness of the ear- lier birds and were, in fact, quite healthy. It appears the lethal genes were either selec- tively bred out, or equally likely, as rumor had it, a new strain of Recessive Silvers were developed, one which did not carry any lethal factors. Whenever another gene or allele becomes available, the one easiest to work with often finds favoritism. For example, in exhibition Budgerigars, once the Australian Domi- nant Grey color mutation became available, which appeared identical to the recessive Grey, interest waned in working with the English Recessive Grey that required a min- imum of two generations to produce visuals. Aviculturists naturally favored the Austra- lian Dominant Grey, which would produce visuals in half the nest in the first generation. These Dominant Greys, carrying large

AFA Watchbird 59 size, good head qualities, and other coveted show characteristics often found their way table 1 to the top of the show bench. It was not un- Sex-linked inheritance usual to find the (nominant wild) Normal Greens, Dominant Greys - and the cross – Cock Hen Progeny 1. Sex-linked Non-linked = 50% Non-linked/Sex-linked Grey-greens, to make up the top ten bench cock at many exhibition budgerigar shows. 50% Sex-linked hens There are instances of weakened ino (i.e., 2. Sex-linked Sex-linked = 50% Sex-linked cocks Lutino or Albino) mutations that have been 50% Sex-linked hens labeled as defective in some way, e.g., weak, 3. Non-linked Sex-linked = 50% Non-linked/Sex-linked inferior, “poor doers,” and who inherit a cocks multitude of problems from eyesight diffi- 50% Non-linked hens culties to immune compromised defenses. 4. Non-linked/Sex-linked Non-linked = 25% Non-linked/Sex-linked Although some birds may be affected ini- cocks tially in some species, aviculturists have 25% Non-linked cocks proven that line-breeding and affective out- 25% Sex-linked hens crossing can breed out problems initially ef- 25% Non-linked hens fecting a line of birds. 5. Non-linked/Sex-linked Sex-Linked = 25% Non-linked/Sex-linked In fact, there are quite a number of ro- cocks bust, healthy inos frequently seen at bird 25% Sex-linked cocks shows, including many winning top bench 25% Sex-linked hens ino mutations, and champion pedigreed 25% Non-linked hens inos who keep winning top awards, repro- ducing winning progeny. As an example, while it had been common to see only Nor- Key: Where “Non-linked” is Normal (the nominant wild form), or any other color that is not mals win a best in show in many parrot spe- sex-linked. This table may be used to check the results of individual sex-linked mutations, one at a time. While the charts demonstrate the graphing of several sex-linked mutations cies divisions, it is not unusual today to find together (with other colors or modes of inheritance), each sex-linked trait may be double mutations included among the top bench checked, one at a time, in the above table. birds. Unfortunately, new bird owners may not always select healthy birds from strong Example: Non-linked/Sex-linked cock x Non-linked hen bloodlines, or be educated on how to pro- P1 XL X | X Y Normal/Lutino cock (XL X) | Normal hen (XY), where L = Lutino vide adequate nutrition, exercise space and | | optimum management practices when be- XL X | XL Y 25% Normal/Lutino cocks | 25% Lutino hens ginning to keep and breed birds. Such prac- | | tices can and do affect gene pools and future X X | X Y 25% Normal cocks | 25% Normal hens generations of stock. Any bird, whether of the nominant wild coloration, or a color Note: It is easier to understand why a hen can never be split (heterozygous) to a sex- mutation, will only be as hardy as its genes linked trait when looking at a chart. As a hen’s sex chromosomes (XY) have only one “X”, and the breeding program employed from the only possibility is to be visual for the trait (XL Y), or not (XY). This is because hens do which it originated. If a bird comes from not have a second “X” sex chromosome to cover or hide a recessive sex-linked trait on a healthy well-bred stock, produced from companion “X” sex chromosome. Additionally, it is believed that the “Y” sex chromosome is proven bloodlines (that are not necessarily too small to carry locus sights (designated addresses) for sex-linked color traits to reside. limited to exhibition stock), then the result will be a more hardy bird. Even show breeders know the old genetic adage, “like produces like.” If you start is the nominant wild color or a variant color a viable, healthy strain, before ever attempt- off with problem birds (e.g., highly inbred, mutation. ing to let go of such stock. compromised stock, etc.), then that is what Occasionally, some new mutations, es- As aviculturists, producing healthy you will produce: weak, sickly birds. The pecially autosomal recessives, can be linked bloodlines is our responsibility. And this laws of genetics apply to any bird, whether it to lethal factors. However, the responsible responsibility is not merely limited to pro- breeder will work with the line establishing ducing color mutations, but includes any

60 Volume XLII • Number 2 & 3 • 2013 stock we work with, be it a family line, table 2 strain, or stud of birds. The genetic modes DOMINANT X RECESSIVE MATINGS of inheritance such as those employed with producing color mutations can theoretically P1 Cock x Hen = F1 Progeny be applied to the many valued traits in the 1. Dominant x Dominant = 100% Dominant establishment of all species, e.g., health, fer- 2. Dominant x Dominant/Reces- = 50% Dominant tility, longevity, resistance to disease, and sive 50% Dominant/Recessive other important traits we wish to preserve. 3. Dominant x Recessive = 100% Dominant/Recessive 4. Dominant/Recessive x Dominant/Reces- = 25% Dominant Genetic Myth #3: All sive 50% Dominant/Recessive chromosomes are alike. False! 25% Recessive Chromosomes are found inside the cell 5. Dominant/Recessive x Recessive = 50% Dominant/Recessive and can be thought of as a single coil of 50% Recessive DNA containing the genes or heredity of 6. Recessive x Reces- = 100% Recessive life. While chromosomes always travel in sive pairs, in most instances they are identical,

Example Budgerigars:: with the exception of one pair of chromo- Where GG = Light Green; Gb = Light Green/Sky Blue; and bb = Sky Blue somes called the sex chromosomes. Model: 1,2 3,4 (High School Algebra “FOIL” METHOD = First, Outer, Inner,Last) Because most of the avicultural litera- 1,3 ture assigns sex-linkage in terms of “X” and 1,4 “Y” chromosomes, rather than the scien- 2,3 tific notation written as “Z” and “W,” this 2,4 column will continue with the “X” and “Y” 1. GG GG 2. GG Gb notation most aviculturists are accustomed GG GG to. While the sex chromosomes known GG Gb function is to determine the gender of the GG GG offspring, and as we are reminded that chro- GG Gb mosomes always travel in pairs, we find that 100% Normal Green (GG) 50% Normal Green (GG) the sex chromosomes of cock birds are iden- 50% Normal Green/Blue (Gb) tical (XX), but differ in the hen (XY). When an egg is fertilized, each offspring 3. GG bb 4. Gb Gb will inherit one sex chromosome from each Gb GG parent. The cock will always donate an “X” Gb Gb from his “XX” sex chromosomes; the hen Gb bG will donate either an “X” chromosome, or Gb bb a “Y” chromosome from her “XY” sex chro- 100% Normal Green/Blue (Gb) 25% Normal Green (GG) mosomes (with an equal chance of donating 50% Normal Green/Blue (Gb, bG) either an “X” or a “Y” chromosome). 25% Blue (bb) If the cock’s “X” sex chromosome unites with the hen’s “X” sex chromosome to form 5. Gb bb 6. bb bb “XX,” the offspring produced will be male. Gb bb If the cock’s “X” sex chromosome unites Gb bb with the hen’s “Y” sex chromosome, the off- bb bb spring produced will be female. Therefore, bb bb unlike mammals, in avian species it is the 50% Normal Green/Blue (Gb) 100% Blue (bb) 50% Blue (bb) female who determines the gender of the offspring by whichever sex chromosome she Key to above table *White includes Whites of Suffusion (i.e., White Sky Blue, White Cobalt, and White donates to the union. Mauve) that act as double recessives. Looking at the remainder of the chro- **Unknown refers to not enough data has been reported by breeders, or colors do not yet exist. mosomes, we assign the term autosomes to all chromosomes, with the exception of the

AFA Watchbird 61 one pair of sex chromosomes. Autosomes carry a multitude of genes, including genes table 3 for color mutations, which we refer to as HIERARCHY OF RECESSIVES autosomal. The difference between auto- (Copyrighted by Author) somes and sex chromosomes is important, Note: This table, composed by the author, demonstrates the hierarchy of autosomal recessive mutations and does not include sex-linked, co-dominant, or intermediate because, as we will see, while hens can never dominant mutations. be split (heterozygous) to sex-linked colors BUDGIES (and a general guide in parrots that produce similar psittacin pigments): or traits, hens may be split (heterozygous) to Color Dominant to: Recessive to: autosomal recessive colors and traits. Normal Green All Colors None Dilute Green Blue in all Suffusions Normal Green Genetic Myth #4: Hens may be (e.g. Greywing) split (heterozygous) to sex-linked Pastel Green (Yellow) Blue in all Suffusions Normal Green Dilute Green traits. False! Normal Blue All Blue Suffusions All Green Suffusions At present, it is theorized in sex-linked Dilute Blue Pastel Blue (White)* All Green Suffusions avian mutations that only the “X” chromo- (e.g. Grewing Blue) Normal Blue some is sufficiently large enough to carry the Pastel Blue (White) None All Green and Blue Suffusions genes for sex-linked recessive traits. The “Y” chromosome, being significantly smaller, COCKATIELS Example using Normal Grey, Pastelface, Recessive Silver and Whiteface has no locus (e.g., “address”) sites available for such color alleles to reside. It becomes Color Dominant to: Recessive to: increasingly clear when charting sex-linked Normal Grey All Colors None inheritance why hens cannot carry sex- Pastelface Whiteface Normal Grey linked recessive colors in hidden form, as do Recessive Silver Unknown** Normal Grey Whiteface None Normal Grey their male counterparts (Table 1). Pastelface Because the hen’s “Y” chromosome is too small to carry locus sites for color alleles, Key to above table and at the same time is unable to cover, or *White includes Whites of Suffusion (i.e., White Sky Blue, White Cobalt, and White Mauve that act as double recessives. mask, the color alleles on the “X” sex chro- **Unknown refers to not enough data has been reported by breeders, or colors do not yet mosome, the recessive genes on the “X” sex exist. chromosome are revealed and must show themselves. This is why hens (XY) show sex-linked recessive colors more frequently. They need only carry the mutation on their one “X” chromosome for the mutation to table 4 be visible. SINGLE AND DOUBLE FACTOR DOMINANT INHERITANCE Therefore, whenever working with sex- 1. Singe Factor x No Factor = 50% Single Factor linked colors in hens, the old adage, “what 50% No Factor you see is what you get,” could never be 2. Singe Factor x Singe Factor = 25% No Factor 50% Single Factor more true. If you can’t see it, it’s just not 25% Double Factor there. However, this statement only applies 3. Single Factor x Double Factor = 50% Single Factor to sex-linked recessive traits, because hens 50% Double Factor may certainly be split (heterozygous) to au- 4. Double Factor x No Factor = 100% Single Factor tosomal color mutations that work under 5. Double Factor x Double Factor = 100% Double Factor different rules of inheritance. Key: “No Factor” represents a Normal, or any other mutation which is not governed by Single and Double Factor Dominance inheritance. Genetic Myth #5: A bird may be Example: Cockatiels: Single Factor x Single Factor = 1:2:1 ratio with equal chance of a “dominant recessive” color either gender. P1 Single Factor Dominant Silver cock x Single Factor Dominant Silver hen mutation. False! = F1 25% No Factor (e.g., unaffected) Unfortunately, this myth has become a 50% Single Factor Dominant Silver common misconception and does not need 25% Double Factor Dominant Silver to be. To the best of the author’s knowledge,

62 Volume XLII • Number 2 & 3 • 2013 a gene cannot exist in both a dominant Similarly, the Pastelface (Pastel) Cocka- produce a new combination such as a cross, mode of inheritance and a recessive mode tiel is our first autosomal recessive Cockatiel or double mutation. Without the original of inheritance, concurrently. It is either one mutation to interact in a “dominant” rela- act of crossing over (e.g., coupling, or re- or the other (Table 2). A gene may, how- tionship to the double recessive Whiteface. combinant forms), many double mutations ever, interact with other genes or alleles in a For example, one may say the autosomal re- would not exist today. variety of behaviors, depending upon their cessive, Pastelface, is recessive to Normal For example, when this author presented mode of inheritance. This has more to do Grey, but behaves in a dominant manner to a genetics workshop to color breeders in with allelic forms of genes occupying a lo- Whiteface. To call the autosomal recessive Boston during 1989, reference was made to cus site, which is beyond the scope of the Pastelface a “dominant” mutation would be a biologist who formally observed a near 30 present article. genetically incorrect. As far as we know at percent occurrence of crossing over between In this example, however, it may be help- this time, the recessive Pastelface is recessive the Lutino and Pearl genes in a very small ful to think in terms such as single, simple to all cockatiel mutations, with the excep- population of cockatiels during the late recessives, and the less powerful double re- tion of the recessive Whiteface. 1970’s. Such a crossing over from XL XP to cessive. The confusion appears to stem from What Cockatiel, Ringneck, and other XLP XN (where L stands for Lutino; the lack of understanding that while an parrot breeders must learn to understand P stands for Pearl; and N stands for Nor- autosomal recessive color is recessive in its (as do Budgerigar and Lovebird breed- mal Grey) in a male cockatiel - known as mode of inheritance, it may simply behave ers who routinely work with such colors), coupling or cis linkage - was originally re- in a dominant (or commandeering manner) is that such recessive genes merely demon- sponsible for producing a small percent- whenever interacting with a double reces- strate a hierarchy of relationships among au- age of the first Lutino Pearl hen genotypes sive mutation. tosomal recessive color mutations, in partic- (XLP Y). Unfortunately, the information For example, in Budgerigar genetics, ular, how such recessives are inherited when has been taken completely out of context while the autosomal recessive allele for body working with double recessive mutations. and reprinted by subsequent authors as, “A color Sky Blue is recessive to Normal Green, crossover occurs about 30% of the time in recessive Sky Blue will act or behave in a Genetic Myth #6: “Cross Over” male cockatiel chromosomes,” which is sim- dominant manner to the double recessive (Crossing Over) occurrences can ply untrue and without basis. Always know White (i.e., Blue x White = all Blue split be predetermined or predicted. your source. White progeny). This relationship among False! recessives merely indicates their hierarchy Crossing over refers to a phenomenon in Genetic Myth #7: The Albino behaviorally. It does not justify labeling the genetics where chromosome pairs swap seg- Mutation in Cockatiels is a single allele Sky Blue as “dominant,” because ge- ments of their chromosomes (i.e., chroma- mutation, therefore a hen may be netically Sky Blue is an autosomal recessive. tids) during meiosis, resulting in a recom- split (heterozygous) to Albino. Rather, Sky Blue behaves in a dominant bination of linked genes. Such “crossovers” False! manner in the hierarchy of relationships as termed in the fancy, happen by chance The “Albino” mutation in Cockatiels, with double recessives (i.e., White). How- occurrence, and there is no way to predict unlike the majority of inos in other spe- ever, as an autosomal recessive, it would be when or where it will happen in avian color cies, is not a singular, spontaneous color genetically incorrect to label Sky Blue as a genetics. However, once crossing over does mutation. Rather, the “Albino” Cockatiel dominant gene (Table 3 copyrighted by occur, it will result in some of the genes is actually a cross mutation between the author). linking together to sex-linked recessive Lutino mutation, and

AFA Watchbird 63 the autosomal recessive Whiteface mutation. When birds of the correct genotype table 5 are paired they produce the cross mutation, COMBINING AUTOSOMAL RECESSIVES WITH CO-DOMINANT TRAITS Whiteface Lutino that is an all white bird with red eyes, depigmented beak and feet, Example: P1 Whiteface Single Factor Dominant Silver cock x Whiteface Single Factor Dominant Silver hen that is void of all melanin (grey) and psittacin (yellow and orange) pigments. Rather = F1 wSwS = 25% Whiteface Double Factor Dominant Silvers wSw = 25% Whiteface Single Factor Dominant Silvers than call the cross mutation by its genotypic wwS = 25% Whiteface Single Factor Dominant Silvers name, Whiteface Lutino, the phenotypic ww = 25% Whiteface label “Albino” has caught on as the avicul- Note: The above has equal opportunity of producing either gender. These gametes can tural lingo simply because it looks like the be checked with Table 4: Single and Double Factor Inheritance (i.e., #2 Single Factor x inos of other species. Single Factor = 1:2:1 ratio); and Table 2: Dominant and Recessive Matings (i.e., recessive x recessive = 100% recessive). Because the “Albino” Cockatiel is the result of two separate color mutations, one of which is a sex-linked color, it is incorrect split to Whiteface; or Normal Grey split to the trait, the remaining autosome serves to to state that a hen can be split to “Albino.” Whiteface. mask the autosome that carries the recessive Hens, like cocks, can be split to autosomal trait. In avicultural lingo, a bird with a single recessives such as Whiteface, however un- Genetic Myth #8: Single quantity is called “split,” or heterozygous, like cock birds, hens can never be split to and double quantities are denoted by writing the slant sign before the sex-linked recessives such as Lutino (refer synonymous with single and name of the mutation (e.g., Normal/Pied is a normal appearing bird carrying a gene for to Myth #4). Therefore, whenever speak- double factor inheritance. False! ing of “Albino” Cockatiels, a hen could only Autosomal recessive color mutations are the Pied mutation). be split (heterozygous) to Whiteface. This sometimes referred to as having single or On the other hand, single and double is also true of many other species where double quantities. If a color mutation is car- factor color mutations refer mainly to au- ino (i.e., Lutino or Albino) is sex-linked, ried on both autosomes, the bird is said to tosomal mutations in one of the domi- although inos do exist in other modes of carry a double quantity and will therefore nant modes of inheritance. Single and dou- inheritance. display the (homozygous) phenotype for ble factor birds may appear in two distinct Beware of purchasing a Cockatiel hen the autosomal recessive trait. Put another color forms, expressed visually (phenotypi- represented as “guaranteed split to Lutino way, the color mutation, or any recessive cally) in their outward appearance (Table or ‘Albino’,” or you will be disappointed. trait, must be carried on both chromosomes 4). Although such dominant mutations The rule when working with sex-linked hens in order for a bird to appear visual for the may theoretically be charted alongside sex- is “what you see is what you get.” Hens sim- mutation. linked, or autosomal recessive color muta- ply cannot be split to Lutino or “Albino.” However, if the recessive mutation, or tions (within the same individual or when Therefore, when working on the production trait, is only carried in single quantity on charting a pair of birds), it functions as a co- of the “Albino” mutation in Cockatiels, a one autosome, the bird is then split (het- dominant expression since the allelic gene Cockatiel hen may only be visual for these erozygous) for the trait, carrying the trait present in a heterozygous state is expressed mutations: “Albino,” Lutino, or White- in hidden form. Therefore, if only one au- as a single factor phenotypic color form of face; she may also be visual for Lutino and tosome is affected by a single quantity of the mutation. In other words, single factor birds will visually appear as one color;

64 Volume XLII • Number 2 & 3 • 2013 years in captivity, mutations have further table 6 differentiated themselves by being affected DARK FACTOR INHERITANCE by color modifiers. Some modifiers, such as dark factors, can change the appearance of a 1. No Dark Factor x No Dark Factor = 100% No Dark Factor standard color mutation. 2. No Dark Factor x One Dark Factor = 50% No Dark Factor 50% One Dark Factor For example, dark factors are well known 3. No Dark Factor x Two Dark Factors = 100% One Dark Factor in the Budgerigar Fancy. In the nominant 4. One Dark Factor x One Dark Factor = 25% No Dark Factor wild green Budgie - referred to as the Light 50% One Dark Factor Green - the appearance of dark factors work 25% Two Dark Factors to modify the Light Green into the Dark 5. One Dark Factor x Two Dark Factors = 50% One Dark Factor 50% Two Dark Factors Green (one dark factor), and Olive Green (two dark factor) color shades. In the blue 6. Two Dark Factors x Two Dark Factors = 100% Two Dark Factors series, dark factors appearing in the Sky Key: “No Factor” represents a Normal, or any other mutation which is not governed by Blue Budgerigar result in Cobalt (one dark Single and Double Factor Dominance inheritance. factor), and Mauve (two dark factor) color

Example: Budgerigars (applicable to psittacine species with equivalent psittacin pig- shades. Dark factors also affect the Yellow ments). series, Grey series, and other mutations in Budgerigars . Dark Factor Green Series Blue Series (Table 6) None Light Green Sky Blue Similarly, lovebirds and several Psittac- One Dark Factor Dark Green Cobalt Blue ula species such as Indian Ringnecks have Two Dark Factors Olive Green Mauve followed suit with dark factor modifiers, None Light Yellow Light Grey which have permeated their green and blue One Dark Factor Mustard Yellow Medium Grey series and other various colors. Many Cock- Two Dark Factors Olive Yellow Dark Grey atiel breeders already argue that Normal Note: Families of Yellow and Grey correspond with Light, Medium and Dark shades Grey Cockatiels show dark factors, ranging Example 2: Cobalt Blue Lineolated Parakeet x Normal Green Lineolated Parakeet: from Light, to Dark Grey, somewhat paral- P1: One Dark Factor (Cobalt) x No Dark Factor (Normal Green) leling the example of the Grey Budgerigar = F1: 50% No Dark Factor: Normal Green/Blue mentioned above. We also find dark fac- 50% One Dark Factor: Normal Green/Cobalt tors in other parrots from the smaller spe- Note: Test-breeding is required to determine which F1 offspring carries the dark factor cies such as Parrotlets, Lineolateds, and the Cobalt. Australian Parakeets to more rarely, the larger species of Psittacines. double factor birds will usually appear as a modes of inheritance (Table 5). While single and double factor muta- uniquely different and alternate color form The terms “single and double quan- tions (as discussed under Myth #8), gen- of that mutation. tity,” may appear deceptively similar to the erally operate by a dominant mode of in- Common examples of co-dominant and terms “single and double factor.” However, heritance affecting specific mutations (e.g., intermediate dominant mutations include the aviculturist might find it easier to as- Yellowface Budgerigars, Dominant Silver the Dominant Silver Cockatiel, the Yellow- sociate “quantity” differences with homo- Cockatiels, etc.), dark factor modifiers ap- face Budgerigar, and the Grey-green Indian zygous and heterozygous genotypes (i.e., pear to exist in both dominant and reces- Ring-necked Parakeet, respectively. When visuals and splits) in recessive traits; while sive autosomes affecting a number of color such colors are bred with others, new and “factor” differences indicate fully expressed, forms (e.g., Dark Green, Olive, Cobalt, interesting cross mutations may result, e.g., distinctly different colored phenotypes (i.e., Mauve, Dark Yellow, Olive Yellow, Me- Whiteface Dominant Silver Cockatiels, two distinct color forms) appearing in dom- dium Grey, Dark Grey, etc.). Yellow-face Opaline Clearwing (“Rain- inant mutations. bow”) Budgerigars; Yellow-head Gray- Genetic Myth #10: The avicultural green Indian Ring-necked Parakeets, etc. Genetic Myth #9: Single and community does not affect the Such color combinations can require a great double factor inheritance is future gene pool. False! deal more work to chart compared to sim- synonymous with dark factor To understand the big picture and the ple sex-linked recessive or autosomal reces- inheritance. False! avian gene pool, an unlikely yet insight- sive color mutations, especially when com- In many of the popular species in avi- ful comparison would be to discuss the bined with additional existing mutations or culture that have been bred for numerous use of antibiotics in humans. The medical

AFA Watchbird 65 community is distressed by the casual use, lack of parental skills, we contribute these Linda S. Rubin is an avicultural writer, or overuse, of antibiotics in human beings problems to the gene pool and perpetu- avian educator, and national speaker with because each time any of us use these drugs ate them in others’ aviaries. By continuing more than 35 years experience keeping, it enables today’s “super bugs” to grow im- to cull birds (that can and do make excel- breeding and judging parrots, cockatiels and mune to the antibiotics used. Once it be- lent pets in the right homes), we all help to budgerigars. Linda is the editor of the digi- comes resistant to others, such an antibiotic strengthen each others’ stock. tal Cockatiel Mutations Magazine and au- would become ineffective for you or me. Aviculturists must be ever vigilant in thored several books including Ultimate Par- So what does this have to do with the culling their lines for healthy, productive rot Guide, Multiple Bird Households (both avicultural community contaminating the offspring whenever possible and encour- avian gene pool? Just as each of us make our age customers to do likewise. Whether we TFH Publications) and Cockatiel Genetics own medical choices, similarly, as avicultur- need an unrelated outcross for our own avi- Made Easy available at her website, www. ists working with breeding birds we each ary, or a companion pet for a precious child, CockatielsPlusParrots.com. Linda served contribute our breeding lines - healthy or ill we can educate ourselves to produce robust as NCS Genetics Consultant for ten years, - to the overall gene pool. birds that have an excellent chance for a teaching private genetic workshops, and has When we fail to cull offspring from par- long, healthy life, whether for our own avi- been penning columns with an international ents who carry disease, or unwittingly set ary, or for future aviculturists and the fu- byline since 1990. She currently writes fea- traits for disease in bloodlines such as liver ture of birds. tures online as a Bird Breeder Expert for disease, diabetes, weakened immune sys- BirdChannel.com, and CompanionBird- tems, etc., we perpetuate these genetic traits Reference: World.com. Linda has spoken on genetic top- in others’ future breeding stock. Similarly, Rubin, Linda, S. 1999. Cockatiels in ics in Australia, Canada, and extensively in when we fail to monitor top exhibition lines Color. Current Myths in Avian Genetic the USA, including more than a dozen AFA that become weakened by producing birds Nomenclature. Aves Communications & conventions. with low fertility, smaller clutch sizes, or Promotions, Chestnut Hill, MA.

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66 Volume XLII • Number 2 & 3 • 2013