Chromosomes and Genes
www.debmark.com/rabbits/genetics.htm
Chromosomes are strings of DNA. They act as blueprints by which rabbit builds itself. A chromosome is made up of individual points (or loci) called genes. An individual gene determines either the appearance or function of a body part. This gene may act alone or, usually, in concert with other genes to determine a particular appearance or function.
A chromosome string is linked together with one other chromosome string of the same type having the same gene locations. Thus, chromosomes and the genes on them occur in pairs in most of the cells of the body. There are 22 different chromosome pairs existing in each cell of the rabbit except the sex cells and red blood cells. Each chromosome pair controls different functions. For instance, the X and Y chromosomes determine sexual characteristics.
X - X Female, X - Y Male
The same location on each chromosome of the pair controls the same characteristic. There are thus two genes involved - one on each chromosome at the same location. A particular gene location may allow only one type of gene to be present there; whereas, another location may allow different types of genes to occupy it. Those locations that allow only one type of gene on both chromosomes do so because any other type would cause either deleterious or fatal effects. An example of this would be the genes that control the shape of the teeth. If any other than the expected gene type is present at this location, the teeth would not grow in right, which could cause starvation.
Some locations allow different types of genes to stay there, causing different expressions of the same characteristic without deleterious effects. For instance, a hair color gene location may allow 2 genes of different types to occupy that location. One type of gene may be the gene that produces black hair, the other brown. Each chromosome may house either the black or the brown gene.
Some of the genes match their counterparts on the other chromosome exactly and some do not. When two genes match, such as, two black genes, this is called a homozygous pairing (homo- meaning the same). When the genes are not the same, such as one black and one brown gene, this is called heterozygous pairing (hetero- meaning different). Another name for heterozygous is hybrid. You've heard this term used in such common language as hybrid corn or hybrid tomatoes. These vegetables have one type of gene on one of the chromosomes, and a different gene on its companion chromosome.
When we speak of a gene location, we're really talking about two points - one on chromosome A and one on its companion chromosome B. Since there are two chromosomes involved, there are two genes involved - one from each chromosome. The points on each chromosome match up perfectly with the points on its companion chromosome. Thus, a location is made up of two genes.
Dominant genes are represented as upper case letters for ease of identification though out this site
Dominant and Recessive Genes
When two different genes occupy the same location, one of the genes expresses itself in the characteristic and the other either doesn't or does so to a lesser extent, modifying the effects of the other gene. When one gene expresses itself more than the other, it is called the dominantgene. The other gene is called the recessive gene. When the dominant gene expresses itself completely, to the exclusion of the recessive gene, this is called complete dominance. Most dominant genes express themselves completely. When the recessive gene modifies the expression of the dominant gene in some way, the relationship of the dominant gene to the recesive gene is called incomplete dominance.
When a buck produces sperm or a doe produces eggs (both of these cell types are called gametes), the chromosome pairs in the cells that create these gametes divide, putting one chromosome of each type into the gamete. When the doe's egg is fertilized by the buck's sperm, the chromosome from the sperm unites with the same chromosome type in the egg and the chromosome pairing is once again restored. Whatever genes that came from the buck are now matched up with the genes of the doe. The expression of these genes in the resulting offspring depends on their dominance and how the other genes relate to each other.