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GENETIC ENGINEERING IN NATURE People have bred animals and plants with desired features for many centuries. Farmers would pick the sheep with the thickest wool and breed from them, perhaps using the less-woolly sheep for food. They would gather seed from the strongest wheat plants to sow for the next crop. Choosing to breed from particular individuals in this way is called selective breeding . Historically, our farm animals and crops have been developed through centuries of selective breeding. But we now understand that the characteristics we want in a plant or animal are determined by its genes. We are learning which genes control which features in certain animals and plants. GENETICS BETWEEN GENERATIONS

All plants and animals have characteristics that they inherit from their parents. What they look like and how they work are passed on from one generation to the next. Some features are fixed for the species of plant or animal—all blackbirds have two legs and a beak, for example. But others vary between individuals—some spaniels are brown while others are white. The “recipe” for an individual plant or animal is carried in its genes. This includes information for the features that are always the same and for those that differ between individuals. Genes are passed on from one generation to the next through the process of reproduction. A living thing starts as one cell, which divides again and again to form more cells. Each time a cell divides, its genes are copied into the new cells. Not all your characteristics are coded in your genes; some come about as a result of how you are brought up, where you live, the type of diet you eat, the experiences you have had, and so on. These are called environmental factors. You may have the genes to grow tall, for instance, but if you are underfed, it may not happen. DNA AND GENES Imagine that you have to build a complex machine from thousands of parts. To fit them together, you need a set of instructions. A living organism is far more complicated than In the Middle Ages, hunting dogs were bred for their speed, strength and ability to track animals. Although no one understood how selective breeding worked, they made use of it to produce the animals they wanted.

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