In diploid organisms (humans, dogs...), chromosomes go in duplicated pairs. Both chromosomes of a given pair contain the same genes in the same locations. The same gene can have different types which are called alleles. Genes are said to be either dominant or recessive, but what this really means is that the alleles of a given gene are either dominant or recessive. If an allele is dominant (eg. brown eyes) it only needs to be expressed once (ie. on one of two chromosomes of a given pair) for that trait to be expressed. If it is recessive (eg. 'blue eyes') it needs to be expressed twice (in both chromosomes of one pair) for the trait to be expressed. The genetic coding of an organism is called its genotype, the physical expression of this genotype is called the phenotype (in this case, 'having brown/blue eyes').
In Labradors, two genes code for the coat colour. In other words there are two chromosomes coding for coat colours, and as there are two copies of each chromosome, four chromosomes from two different pairs affect coat colour. Each of the two genes coding for the coat colour has two alleles.
The first gene is the coat colour gene and determines the dark coat colour: black is dominant ('B') and brown is recessive ('b'). The second gene is the expression gene and determines the ability to express the dark colour: the 'ability to express a dark coat' is dominant ('E') and the 'inability to express a dark coat' ('e') is recessive ('e'). Hence there are only two coat colours: black and brown. Yellow dogs are the result of the inability to express a dark coat colour.
Puppies inherit one coat colour gene (B or b) and one expression gene (E or e) from each parent. So... a coat colour gene on a dog can be BB, Bb or bb. An expression gene can be EE, Ee or ee. The combination of these two genes gives the puppies' phentoype.
