AVIAN GENETICS

An understanding of basic cytogenetics is required before proceeding into specific avian mutation crosses and the expected outcome. Cytogenetics is the science which attempts to correlate cellular events with genetic phenomena.

Genes are made up of DNA and in simple terms one or more genes produce an expressed trait or phenotype. In addition, a single gene can effect several processes and in turn produce several traits. This multiple effect is termed pleiotropism. In the process of mutation(s), a gene may be changed into two or more alternative forms called alleles. The term allele and gene are used interchangeably. A trait, either whole or in part, is produced by a pair of genes, one received from the male parent and one from the female parent. (The exception is sex-linked genes where the trait is expressed in female birds with the gene only present on the single X sex chromosome.) If both genes are identical then this genetic state is termed homozygous. If the pair of genes are different then this is termed heterozygous or hybrid. The gene that can express itself in the homozygous, as well as, heterozygous condition is referred to as the dominant factor. The gene that can only express itself in the homozygous condition is referred to as the recessive factor.

When dealing with a trait that has a dominant and recessive gene, there are three common conditions. The first is homozygous dominant in which both genes present are the dominant ones. The second is homozygous recessive in which both genes present are the recessive ones. The third is heterozygous in which both the dominant and recessive genes are present. Breeders often refer to the heterozygous condition as "split" when addressing a recessive mutation.

There is also a condition, which exists for some genes called codominance. In this case, each gene is capable of some degree of expression in the heterozygous condition. Other terms synonymous with codominance are partial dominance, semidominance, incomplete dominance and additive genes. Examples of codominant genes are the Dominant Silver mutation in Cockatiels and the Dark Factor, Spangle and Violet in the Budgerigars. It should be understood that all of these various codominant terms are used interchangeably throughout genetic writings. Some authors have preferences and some have suggested specific meanings for the various terms. I feel it is best to keep them all with the same meaning to avoid the development of confusion with previous writings.

The heterozygous condition for a dominant mutation is referred to as Single Factor and the homozygous condition is referred to as Double Factor.

When discussing dominant genes and their interactions there is currently a point of confusion within the avian community with regards to lethal dominant genes. A bird cannot survive a lethal dominant mutation. A bird may survive a lethal codominant mutation if it is heterozygous for the lethal mutation. An example of this is the blue mutation in canaries. If the blue mutation were a full dominant mutation, that is to say both the heterozygous and the homozygous condition display the same physiological effect, then there would be no surviving blue canaries. Since canaries survive the blue mutation and display some effects of this mutation suggests that the blue mutation is a partially dominant mutation that when heterozygous with the wild-type gene cannot carry out its full lethal action. The single wild-type gene present in the heterozygous (Single Factor) blue mutant condition is able to continue on the life vital process that is halted by the homozygous (Double Factor) blue mutant condition.

Keep in mind that not all traits in the animal world are controlled by a single pair of genes. A trait can be controlled by numerous genes on the magnitude of 100 or more. Traits that are controlled by numerous genes are termed quantitative traits. An example of this in Budgerigar genetics would be the Dark Factor gene effect on the Blue series trait. Within the Bird Tracker program, the Options section of the Species Screen allows you to specify quantitative traits.

Traits come from genetic DNA. The genetic DNA in conjunction with a protein matrix forms nucleoprotein and becomes organized into structures called chromosomes. Chromosomes are located within the nucleus of a cell. In summary, genes reside on chromosomes.

Figure 1 shows the basic anatomy of a chromosome.

The chromosomes within a non-replicating cell are strung out as fine filaments. It is when a cell is involved in cellular division that the chromosomes take on a condensed "X" appearance as shown in the Figure 1. The two chromatids on the same chromosome are referred to as sister chromatids. Sister chromatids are the result of replication of genetic material and normally, they are a photocopy of each other. There are genetic events, which could make them dissimilar. Those events will be explained later in this discussion. Chromomeres are regions of dense DNA. When stained, they appear as dark regions under the microscope. The location of the centromere will vary depending upon the chromosome. If the centromere is in the center it is referred to as metacentric, if off center submetacentric or acrocentric, if very near one end telocentric.

The exact number of chromosomes (karyotype) possessed by most avian species is unknown. Avian karyotypes can range upwards to 40 - 80 chromosomes.

There are two categories of chromosomes, autosomes and sex chromosomes. With birds, sex is determined by a heteromorphic (i.e. morphologically dissimilar) pair of chromosomes called sex chromosomes. In humans and some other species, these chromosomes are labeled X and Y. A male human has the XY complement (the heterogametic sex) and a female human has the XX complement (the homogametic sex). In some species, such as birds, the complement of sex chromosomes is the opposite of the above. Geneticists have assigned a different lettering scheme to make this distinction. The letters used are Z and W. A male bird has the complement ZZ (the homogametic sex) while the female has ZW (the heterogametic sex). However, you will find that nearly every avian breeding presentation will use the X and Y designation for sex chromosomes in birds. For the remainder of this presentation, I will use XX to describe the male bird sex chromosomes and XY for the female. This is the only deviation I will take from established genetic doctrine in order to prevent any unnecessary confusion with the reader. All chromosomes exclusive of the sex chromosomes are called autosomes. Autosomes occur in morphologically similar pairs referred to as homologues.

Realize that the Y chromosome in birds is most likely void of any genes. In some species of animals, XX (two X chromosomes) is a male and XO (just one X, no Y chromosome) is a female. The O symbolizes lack of a sex chromosome. Therefore, one could reasonably conclude that the XO condition in birds would produce a male. Such events do occur among various species in nature, including humans at a ratio of 1:5000 (Turner syndrome).