The Primary Unit of Heredity DNA is the primary unit of heredity in all living organisms . When organisms reproduce , a portion of their DNA is passed along to their offspring . The transmission of all or part of an organism ’ s DNA helps ensure a certain level of uniformity from one generation to the next , while still allowing for minor changes that contribute to the evolution and diversity of life .
A Biological Instruction Manual Humans are multicellular , complex organisms . Scientists have come a long way in estimating the number of cells in the average human body . Most recent estimates ( using a man between 20 - 30 years of age , weighing 70 kilograms , and measuring 5 ’ 7 ” in height , as a reference ) put the number of cells at 37.2 trillion ! These can be divided into about 200 different types of cells in the body , all of which work in harmony to carry out all the basic functions necessary for humans to survive . DNA acts as an instruction manual , specifying the function of each cell and providing the instructions necessary for each cell to carry out their functions , develop , and replicate .