Final answer:
Genes are expressed for a particular trait by the alleles received from each parent, with dominant alleles typically expressed over recessive ones. The genotype consists of these alleles and their expression results in the observable phenotype. Gene expression is regulated and can be polygenic, affecting traits such as skin color or height.
Step-by-step explanation:
Genes are expressed for a particular trait through the process of genotype influencing phenotype. A genotype consists of the alleles an individual inherits; it can be homozygous (identical alleles) or heterozygous (different alleles). When it comes to expression, the phenotype—observed traits like purple or white flowers—is the result of this genetic inheritance. For instance, the phenotype for flower color in Mendel's pea plants was influenced by alleles for color, with the dominant allele (B) expressing the purple flower trait whether in a homozygous (BB) or heterozygous (Bb) state, while the recessive allele (b) would only express the white flower trait when in a homozygous recessive state (bb).
Additionally, genes must be 'turned on', or expressed, to produce their respective proteins. This gene expression is regulated through mechanisms such as transcription factors and involves processes like mRNA transcription. Differential gene expression, where certain genes are turned on or off as needed, leads to diverse cell functions and contributes to an organism's phenotype. Notably, traits can be influenced by polygenic inheritance, where several genes contribute to aspects of a common phenotype without direct interaction.