The secret of heredity An age-old mystery What makes us the way we are Why do some of us have dark hair and others have light hair Why do cats only have kittens and never puppies For centuries no one knew the answers to these questions It took many years of inquiry and research to understand how organisms living things inherit characteristics from their parents Two kinds of study Two different kinds of study led to this understanding One involved breeding plants and animals and noticing how living things pass their characteristics such as color size and shape to their offspring This is called heredity Another area of study looked at how organisms are constructed Scientists learned that organisms are all made up of tiny building blocks called cells The very smallest living things such as bacteria are made up of just one cell while large organisms such as humans have trillions of cells THAT’S A FACT Some bacteria are so small that a million can cover the head of a pin Sheep like other living things produce offspring like themselves The young inherit characteristics from their parents In this case young lambs like adult sheep will have four legs will eat grass and will have thick woolly coats 6
This photograph shows bacteria being grown in a laboratory Bacteria are single-celled organisms that reproduce by dividing in two They reproduce very quickly to make new organisms that are like the original ones Eventually scientists discovered that living things pass on their characteristics through something that is found in living cells That something is an extraordinary substance called deoxyribonucleic acid or DNA A great breakthrough Discovering DNA and how it works was one of the greatest breakthroughs in Life needs life Until as late as the 1800s many scientists believed that some living things appeared spontaneously that is without any parents For example food that was left out would spoil when tiny germs we now call them bacteria appeared in it French biologist Louis Pasteur set out to show that even the tiniest life-forms had to come from somewhere In a set of experiments he boiled beef broth to kill any existing germs Then he kept the broth in flasks with special filters to keep out any dust or particles No germs grew in the broth But when he opened the flasks to the air the broth soon spoiled science in the last 100 years Today scientists know not only how DNA works but also how to use DNA to give living things characteristics they could never have naturally They can make clones exact copies of organisms They are beginning to create life synthetically from scratch in a laboratory I prefer to think that life comes from life rather than from dust Louis Pasteur Breakthrough Pasteur showed that bacteria did not appear spontaneously which confirmed earlier work by 17th-century scientist Francesco Redi Louis Pasteur proved that life does not appear spontaneously His experiments with bacteria showed that living things are produced only by other living things like themselves 7
Heredity in practice The first farmers Even in ancient times people discovered they could influence heredity When early humans started to settle in one place they began to grow the food they needed instead of hunting and foraging for it These were the first farmers Over many years they turned wild grasses into cereal crops such as wheat barley rice maize millet and sorghum They did this in a process called selective breeding They collected the seeds from the best plants of one generation of crops to produce the next generation of plants Better tastier vegetables were also produced this way Farmers realized that animals could be changed by selective breeding too They bred animals that had characteristics or traits they wanted such as cattle with more meat and dogs that were easy to train for hunting or herding But even though farmers had discovered that they could influence heredity they did not understand what made this possible These pictures are from ancient Egypt and date from around 4,000 years ago They show us that people had by then already developed grain crops from grasses and bred farm animals from wild ones 8
Old beliefs survive For centuries people knew from experience that living things produce others like themselves that a human mother could not for example give birth to a rabbit Yet not everyone was quite sure whether this was impossible and they did believe that a mother’s experiences affected the way her young looked For example many people thought that if a baby was born with a cleft palate a condition that affects the top lip sometimes called a harelip the mother must have come into contact with a hare while pregnant Similar superstitions still survive all over the world THAT’S A FACT In 1726 an English woman named Mary Toft claimed to have given birth to rabbits Somehow she managed to convince two doctors that it was the result of dreaming about rabbits and craving rabbit meat Most doctors were sure this was wrong Eventually she admitted it was all a hoax done for money About 12,000 years ago people began to train wild wolves like this one to help with hunting Since then they have used selective breeding to produce the different kinds of dogs we see today Breakthrough People learned from experience that living things produce young like themselves They learned to choose the best plants and animals to breed Over the years they developed productive crops from wild plants and farm animals from wild creatures 9
