|1. Genes control all phases of life in an organism, including its metabolism, size, color, development, and reproduction. Genes occur in pairs and are located on homologous chromosomes, which are inherited from parents. Through the use of monohybrid and dihybrid crosses with the garden pea, Gregor Mendel discovered the basic principles that govern the inheritance of genes.
2. Mendelian genetics explains the inheritance of genes that are located on separate chromosomes and have two allelic forms, but other patterns of inheritance occur that are not governed by Mendel’s principles. Incomplete dominance and codominance occur when neither allele is completely dominant. Multiple phenotypes can occur when more than two alleles exist for a given trait or when that trait is controlled by more than one pair of genes. Genes that occur on one chromosome are inherited together or linked. Crossing over can result in the recombination of linked genes.
3. James Watson and Francis Crick described the structure of the DNA molecule as a double helix of nucleotides. A gene is a segment of the DNA molecule that codes for the formation of one polypeptide. Through transcription and translation, the information coded in the sequence of bases is expressed as a specific sequence of amino acids. Some mutations arise from changes in the DNA that result in altered proteins with different characteristics. Since the genetic code is a universal code, the genetic information from organisms can be translated by another organism. Recombinant DNA technology is now being used for practical applications.