Genotype & Phenotype
Now that we have reviewed gene structure and expression, let's take a few steps back out into the 'big picture' and see how this molecular information relates to visible traits.
Recall that each somatic cell contains two copies of each chromosome, one from the maternal parent and one from the paternal parent. Thus, each individual also has two copies of each gene, one located on each of these homologs. These copies, or alleles, can be identical; that is, both parents contributed a chromosome whose gene had exactly the same nucleotide sequence as the other parent's gene, or they can differ by one or more nucleotides. Individuals with two identical alleles are called homozygotes, while individuals with differing alleles are called heterozygotes. The physical location of a gene on a chromosome is called a locus.
The alleles that an individual carries are referred to as its genotype, while the traits caused by those alleles are referred to as its phenotype. In this animation, the phenotypes associated with all of the possible genotypes at the W (white) and S (spotting) genes are shown. Note each dog has two alleles for each gene. As you can see, the W allele causes a white phenotype, and the S allele causes spotting. Thus, a dog with both W and S is white with spots.
A dash "_" after an allele symbol indicates that the individual can have any of the other possible alleles as their second allele, and the phenotype will not change. To see why this is the case, surf on to the next topic, Simple Inheritance.