Characteristics or traits are passed from parents to offspring at the genetic level. Sex cells which include sperm and oocytes each contain 23 chromosomes. Each chromosome within the sperm has a corresponding homologous chromosome within the oocyte. When the sperm and oocyte unite during fertilization, the result is an egg containing 46 chromosomes. In other words, the egg contains 23 homologous chromosomes. Each chromosome contains double stranded DNA (deoxyribonucleic acid). Within each strand of DNA, one finds specific genes which code for characteristics or traits associated from each parent. For instance, consider homologous chromosome 5 out of 23 homologous chromosomes. The fertilized egg will contain one chromosome 5 from the sperm and the other homologous chromosome 5 from the oocyte. This chromosome will contain a collection of genes which code for specific traits from each parent. Through a complex genetic process, some of these genes are turned on whereas some of these genes are turned off. Depending upon which ones are on and which ones are off, one observes specific characteristics or traits from each parent.
As an aside, the characteristics or traits that are being referred to above are those which can be biologically linked to observable ones (e.g., eye color, height, Sickle Cell Anemia, etc.). There are some characteristics or traits which are not directly observable that may be the result of learning or experience rather than genetics (e.g., temperament, likes and dislikes, personality, etc.).