DNA is a substance that makes up genes. Everything alive has genes. Plants have genes. Animals have genes. You have genes. Genes are the basic units of heredity  . Heredity means all the characteristics you inherit from your parents. You get your genes from your parents. You inherit half of your genes from your mother. You inherit half of your genes from your father. Genes are a kind of code. A tree's genes tell what shape its leaves will be. A cat's genes tell what color1 its fur will be. Your genes tell what color1 your eyes will be. Your genes tell what color1 your hair will be. Everything about you comes from the code in your genes. Genes line up on strands called chromosomes in cells. Everything alive is made up of cells. Chromosomes are in the center, or nucleus, of cells.   An Austrian monk named Gregor Mendel first saw inherited patterns in pea plants. He experimented with pea plants in the 1860s. One of the things, or traits  , Mendel studied was what makes some pea plants tall and some short. He said that the traits must come from units of heredity passed from the parent plants. These units were called genes.   In the mid-1900s, scientists discovered that genes are made of DNA. In the 1970s, scientists learned how to change DNA with genetic engineering. Scientists also learned that problems with certain genes cause diseases. Muscular dystrophy, cystic fibrosis, and hemophilia are some genetic diseases--diseases caused by problems in genes. Today, scientists are looking for ways to cure genetic diseases by  _ genes through a process called gene therapy. In the last sentence, the word "altering" can best be replaced by   _  .
changing preserving deserving digesting