The influence that different alleles within a locus have on each other are discussed here. An explanation about gender related properties and linked properties will follow at the end of the article.
Dominant and recessive
In the introduction we used the example of the B-locus where the b-allele is dominated by the B-allele. When we look at the combinations for the B-locus we'll find the following possibilities with their respective colours:
The presence of a B-allele completely removes the effect of the b-allele. The B-allele is called dominant and the b-allele is called recessive.
Some alleles work like layers of paint; the more of these alleles are present the stronger the effect. You don't find many examples in animals, but most likely a few characteristics with a gliding scale (milk production, body height, etc.) will be based on this kind of relationship between alleles.
If the effect of one dominant allele does not completely cover the effect of the recessive allele the 'dominant' allele is called incomplete dominant. A (not perfect) example is the Merle-gene: mm represents the normal colour, Mm produces the Merle effect (a diluted coat colour with random patterns) and MM produces the full effect (often white dogs, which can be blind, deaf and/or sterile).
Besides the relationship dominant-recessive one can also find the situation that alleles within a locus do not influence each other. A well known example is the blood group system (ABO-system) in humans.
Such a peaceful co-existence of the alleles is called co-dominance.
Gender related characteristics
This describes two situations:
The characteristics are only visible with one of the genders. An example that comes to mind is milk production (which is to a large degree hereditary). This property can not be measured in a male, even if the locus is located on an autosomal chromosome.
If the characteristic is located on the X-chromosome the gene works like a property on a 'normal' chromosome with females. Males only have one X-chromosome, so recessive alleles can not be masked by the dominant counterpart. This effect is seen in some hereditary diseases. Such diseases appear more frequently in males because female carriers can mask the affected allele with a dominant (normal) allele. In symbols: female: Vv, male: vY
Several characteristics seem to be linked in a way. The link did not appear in 100% of the cases however. The explanation was that the loci for these characteristics were located on the same chromosome. With the chromosome of either mother of father the child receives a set of linked properties.
The fact that the rule was not true in 100% of the cases was later shown to be due to a phenomenon called 'crossing-over', which will be discussed later.