UNDERSTANDING RECESSIVE INHERITANCE
by Michael Stuart Christian
The array of expressions displayed in the recessive variety of budgerigars can be ranked as some of the most attractive seen in this fascinating little parakeet. Regrettably, many enthusiasts find a multitude of problems with perpetuating the various recessive manifestations and believe the recessive rules too complex to pursue. However, once the ratio of transmission is recognised for one, and it is understood that the same rule applies to all, enthusiasts begin to appreciate that recessive transmission is a concept they are more than able to deal with.
Ratio of Transmission:
Enthusiasts who have examined the available literature will have become acquainted with the 25:50:25 ratio, which constitutes the basic laws of Mendelian recessive inheritance. For readers not familiar with this law, the concept is relatively straightforward and occupies little space to explain.
Recessive x Normal=100% Normals
Recessive x Normal/Recessive= 50% Normal/Recessive & 50% Recessive
Normal/Recessive x Normal/Recessive= 25% Recessive, 25% Normal, 50% Normal/Recessive
Recessive x Recessive= 100% Recessive
Number of Varieties:
In Budgerigars, six varieties that are carried as recessive genes are recognised in the Australian National Standard. These are:
Danish (or harlequin) pieds, clearwings, graywings, black-eyed clears, Fallows and blues (any variety). (Dilute is not recognised in the National Australian Standard). Many of the problems enthusiasts have with developing an understanding of the nature of recessive inheritance result from too limited an explanation in the available literature. The majority of writers provide ample information on the ratio of transmission but neglect to discuss the physical means by which these characteristics are perpetuated. This key knowledge is a major failing to the development of a working knowledge of recessive inheritance. The most valuable tools in establishing a sound understanding of the phenomenon are some theoretical background (such as provided in this text), accurate records and experience handled with mindful observation. Genetics is a precise science, and what may appear to be the case is not always so, especially where gauging with a small number of results.
The genetic composition of your budgerigar is determined at conception. Each parent contributes a sex cell (gamete) that contains a half chromosome complement and, therefore, half the Genetic information necessary to form the "blueprint" to construct a new individual. This new cell is called a zygote. The zygote divides to become an embryo and eventually, the new individual. Chromosomes sort themselves into homologus pairs (chromosomes of identical shape, gene, and loci arrangement and size. Each pair of chromosomes then potentially has two genes for a particular characteristic. Since there are two "instructions" on how the organism is to develop, these two "instructions" need to reconcile to one another. They either work together to build, or they "agree" that one gene’s instructions override the other. That is, one gene becomes silent in terms of its instructions. Such a gene is termed recessive.
Genes and Characteristics:
Each recessive characteristic (color, feature, etc.) is determined by a specific gene (or genes; however, multiple gene affects are outside the scope of this article and will not be dealt with here). Chromosomes come in pairs and have finite numbers in an organism. These chromosomes are partitioned into sites called loci (singular form: locus). The loci are very important, they are particular to characteristics. A gene not on the correct locus will simply not be expressed (in terms of the given characteristics). This information is fundamental to accepting why there can be no hierarchy of recessive varieties.
Since chromosomes come in pairs, they pair specifically. Each chromosome contributed by one parent is paired with another identical chromosome from the other parent. Because they are identical, they share the same gene arrangement and have the same loci for various characteristics. With recessive characteristics, each gene from the expression needs to be matched by another identical gene on the same locus existing on the corresponding chromosome. Recessive varieties realistically do not all occupy the same chromosomes, but for our purpose, it is simpler that they do. To understand recessiveness, the three features discussed above must be recognised and understood. It is in the genes occupying loci on corresponding chromosomes that the issue of understanding how one recessive variety can be split for another rests. Once this is accepted, it is immediately understood why one recessive variety is unable to dominate another.
Genes (How They Work):
Genes determine all aspects of the shape and structure of an organism by directing protein synthesis. Color and markings are the result of this process. Where a gene is recessive, for it to affect protein synthesis, it is necessary for it to be present on the loci for the character on both chromosomes in the pair. Where the corresponding chromosome holds the "wildtype" gene on that specific locus, the effect becomes latent (recessive). A gene can also have more than one effect, but, again, that is moving outside the scope of this article.
The fourth element of genetics is not frequently addressed in avian literature, but it has to be introduced and expanded upon. The "wildtype" is the gene on a given focus in its unaltered state. The various colors, varieties and other characteristics that we are accustomed to dealing with in budgerigars (and in other parrots) are the result of a change in the chemical composition of a given wildtype gene. They are mutations. This is regardless of whether they are dominant, sex linked or recessive. As already outlined, for a recessive characteristic to be manifest, the gene for it must be present on the appropriate site (locus) of each of the two homologous chromosomes. For example, for clearwing to be expressed, a gene for clearwing must be present on the loci for it on both chromosomes. Where a clearwing gene is accompanied by a wildtype (that is, the gene on that locus that has not been subjected to the type of mutation causing the clearwing effect), the individual will be a split, or more precisely, the clearwing phenotype becomes latent.
Hierarchy of Recessiveness:
A major impediment to the understanding of recessive transmission is a subscription to a "hierarchy of recessiveness" on the part of many enthusiasts. Part of this belief may be due to the frequency of certain composite phenotypes. Clearwing-fallow composites are, or at least were in the early 1980s, a relatively common occurrence. Such composites explicitly stated that each clearwing of a pair producing clearwing-fallow composites were split for fallow. Breeders quickly recognised that the clearwing could be split for fallow but, since the opposite did not occur with any frequency, failed to explore why this was so. The frequency of this clearwing-fallow phenomenon is a result of the use that breeders put fallow into clearwing production and may be due to the probability that clearwing and fallow are linked--that is, these two varieties do, in fact, share the same chromosome. Consequently, when breeders bred these two varieties (for reasons too numerable to enter into here) as a breeding pair, in many instances the genes crossed over, and the linkage was established. Once a chromosome carried Genes for both varieties, and if the loci for the two varieties is close, they become extremely difficult to separate, and, as already noted, the composite persisted.
Graywing and clearwing appear to be a separate mutation of the same wildtype gene, have a very similar composition and share the same locus. As a result, these two recessive expressions, when present in the one individual, combine to produce a composite effect. That composite is typically called a "fullbody-colored graywing. The fact that the expression is termed a qualified "Graywing" is a matter of semantics; it is not a statement on dominance of one recessive over another. Equally, that the majority of their number typically appear more like a graywing in no way supports the proposition that one is "dominant" over the other or that one is 'more recessive" than the other. That it appears so, is a visual phenomenon. If a full body-colored graywing, or more correctly, a clearwing-graywing composite, were paired to a normal (that is, an individual not carrying a gene for either of these two expressions), 50 percent of the offspring would be split clearwing only, and 50 percent would be split graywing only.
Very few enthusiasts would dispute that normal blue can be split for recessive varieties--that is, other recessive varieties. The fact of the matter is that blue is the result of a specific gene that functions like all other recessive genes. When we consider composites, blue fallow is just as much a composite as is clearwing-fallow. Why we don't think of blue in this light is simply that we have learned to think of them in separate terms--normal! But this is only the way we think about the expression. In reality, blue is a modification of green, the "normal" color. Normal is the natural markings unmodified by any gene in any of the three Mendelian categories. So, by extension, when you have a "blue fallow" or a blue any other recessive variety, you have a modifying gene on each of two homologous chromosomes: one gene affecting color and, equally, another modifying gene on each of two other (or possibly the same) homologous chromosomes affecting markings.
Where two different recessive varieties are mated, the resulting offspring are nearly always normal, split for both - the graywing/clearwing being the exception. (The term normal is not specific for unmodified phenotype, but purely relative to the two or more recessives in question.) Where expectations will vary is where each of the member pair is carrying some other recessive characteristic not accounted for in the mating. If we continue with the blue and fallow example, you could use a fallow (which is green) to a blue (which is normal). The result would be a normal green split for blue and fallow, not blue-fallow composites as some breeders may assume. If you were to mate two normal greens split for blue and fallow, the resulting offspring ratio would be: 25 percent Green/blue and fallow, 12.5 percent Green Fallow/Blue, 12.5 percent Blue Normal/fallow, 12.5 percent Green Normal/fallow, 12.5 percent Green Normal/Blue, 6.25 percent Green Normal, 6.25 percent Green Fallow, 6.25 percent Normal Blue, and 6.25 percent Fallow-Blue.
Gametes: Sperm or ova.
Linkage: The phenomenon where the loci for different characteristics are close together on the same chromosomes; the closer they are, the greater the degree of linkage.
Cross over: A phenomenon where sections of a chromosome cross one another and exchange portions and, in consequence, genes. The more closely Genes are linked, the greater the chances of them being passed together.
More information on GENETICS can be found here.
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