The document summarizes key concepts about genetics and heredity, including: - Genes contain DNA instructions that determine traits like appearance and behavior. Humans have about 25,000 genes across 46 chromosomes. - Development begins at conception when sperm and egg fuse to form a single cell called a zygote containing a full set of genes. This cell then differentiates and multiplies to form all the body's cells and tissues. - Genes interact with each other and the environment to determine a person's phenotype, or observable characteristics. While genes influence traits, environmental factors also play a role.

1. Part I Heredity and Environment Prepared by Madeleine Lacefield Tattoon, M.A. Chapter Three The Genetic Code From One Cell to Many From Genotype to Phenotype Chromosomal and Genetic Abnormalities