It is helpful to understand a few basics of genetics before proceeding to look at the various mutations in the Rock Pigeon (Columbia Livia) and the expected outcome from various mutation crosses. We will start with some simple definitions, which will hopefully create some familiarity with terms. When we are comfortable with the terms then the subject of genetics doesn't seem quite so difficult. Genes are made up of DNA and control the transmission of hereditary characteristics. The expression of the hereditary characteristics -- which for our purposes will be the plumage color -- is called the phenotype. The genotype is the genetic makeup of the individual bird. As we will see later, the genotype may vary while still producing the same phenotype. If we talk about pigeons are different in many ways from mammals. First, pigeons have no external sex organs so it's often difficult to tell male from female -- but you knew that already. Even more interesting, at least from the bird's point of view, is that male pigeons have two XX chromosomes and females only one. Females also carry a little slug of material in addition to their X chromosome. That little slug of material can be passed on to an egg, but for most purposes we can ignore it because it seems to carry very little information of interest to us. The biological vessel (pigeon or any other plant or animal) is broken down into cells. Each cell contains chromosome pairs composed of DNA (Deoxyribonucleic Acid). The DNA protein coding on each chromosome can be broken down into small identifiable sections referred to as genes. Pigeons have 40 chromosome pairs. Chromosomes reproduce themselves in two different ways. The first way is a process called mitosis. This is a process necessary for a cell to double itself. When a pigeon lays an egg, inside is the first cell containing the chromosome which through mitosis will become a pigeon. That first chromosome will double itself countless times until an embryo is developed. This process will continue through growth and far into the life of the pigeon as just like humans, a pigeon is continually reproducing cells to replace damaged or old cells. The second way chromosomes reproduce themselves is a process called Meiosis. This is the process where chromosome pairs divide and 1 of every pair is transferred to a gamete. A gamete is a sex cell which contains 1/2 of the chromosomes of the parent cell. In reproduction, a child receives one gamete from each parent. When these two gametes come together during procreation a new chromosome is formed unique from each of the parent's chromosomes. All chromosomes, including these X chromosomes, carry information on them which the bird's body uses like the commands of a computer program. This information tells the pigeon's body to produce feet, feathers, eyes, pigment -- in short, everything that goes into making a bird the animal it is. Some of this information is stuff that we as breeders want, and some is stuff which we don't. The stuff we want might include instructions to grow feathers which curl on the wing shield, or to produce pearl eyes instead of orange ones. Some stuff we don't want might include instructions to grow extra toes or crest or webs on the feet.
Sex Chromosomes
Of the 40 chromosomes in pigeons one is known as the sex chromosome. In humans, we call this the "Y" chromosome and it is carried by the male. In pigeons we call this the "W" chromosome and it is carried by the female. The sex chromosome carries little known information but determines sex. If a gamete receives the sex chromosome then it will produce a hen. The other chromosome of the pair that the sex chromosome is a member contains genetic information. When a characteristic is called sex linked it means it is carried on this chromosome (the one in the pair opposite the sex chromosome). In a hen, who has the sex chromosome she will only receive one gene. For cocks, they do not have the chromosome, they will receive two. Here is an example using the color gene since it is sex linked. B represents the color Black/Blue
Cock (B/B)
B /B B/B
Hen (B/"W")
B/"W" B/"W"
All gametes with "W" are hens with only one color gene. All cocks have two color genes instead of the sex chromosome. For recessive genes on the chromosome it simply means a cock must inherit the gene on both chromosomes before the gene is expressed. This is in such cases as dilute. For a hen, she only needs one recessive gene on the chromosome opposite the sex chromosome in the chromosome pair. This leads to the saying "A hen simply is what she is." Not necessarily true in genes that are not located on the sex chromosome.
Genes
Genes are DNA coding on a particular location on any chromosome which have been identified has having a hereditary effect that can be observed. For our purposes, these are traits that can be seen. In the purest sense, this would also apply to such things a cycle cell anemia, blood type, hemophilia etc. in humans. In pigeon breeding this would apply to feather coloration, feather quality, eye color, body type etc. There are probably thousands of traits that can not be observed in the hand but have a direct effect on racing. Lung capacity or efficiency, heart strength, muscular processes etc. The possibilities are endless. Often more than one gene can affect a certain trait. For coloration for example there are genes for basic coloration and yet there are other genes (located on a different location of a chromosome or a different chromosome altogether) which may also have an affect on a trait. An example of this would be basic color in combination with the gene (self or spread). A black/blue pigeon becomes solid black, a Red/ash pigeon becomes a true silver and a Brown/Khaki pigeon becomes a solid brown or sometimes called a dun.
Tidak ada komentar:
Posting Komentar