Sometimes, animals' offspring start out looking quite different from their parents: Think about tadpoles and frogs or caterpillars and butterflies. Even you didn't look exactly now like you did when you were born. As sexually reproduced organisms grow, their appearances change. This is because of a genetically programmed growth pattern provided by their parents. Your cells contain the genetic material to build you and determine how you look and to get you from infancy to adulthood.
The genetics of living organisms are coded in organisms' cells. Typically, the information is stored in the cell's nucleus in the form of chromosomes, which are made of proteins and DNA. The bits of DNA, called genes, determine what traits will be inherited when the cell reproduces. Specialized cells called gametes contain only half of the chromosomes that a regular somatic cell has. They are formed through a process called meiosis, which is when similar chromosomes in a cell duplicate, form pairs and interconnect. Then the cell divides into four cells and half of each pair of chromosomes goes to a different cell. When two gametes join - - like an egg and sperm - - a new organism is created, along with its inherited genetic code.
Genetic material also can change. Gene mutation means that the structure of a gene has changed; chromosomal aberration means a chromosome has more genes than usual, fewer than usual or in a different pattern than usual. The causes of gene mutations and chromosomal aberrations are not really understood or controllable. Generally, these abnormalities are harmful and can even lead to death. Sometimes, however, mutations are beneficial to new generations and can even mark the beginning of a new species and the continuation of evolution.
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