Heredity : What is passed ....
Heredity : What is passed on to us by the Sperm and the Egg
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Remember that two hundred egg cells placed side by side would take up only an inch ; five hundred sperm cells lined up head to tail would measure the same length. Yet each of these microscopiclly small cells contains many little bodies called chromosomes, which transmit a complicated code of directions.
This code determines the here- dity of the child that will be born if a seprm and an egg unite : the colour of the hair, skin, and eyes the shape of the nose, intelli- gence at birth ; and the thou- sands of other things that make this human being physically different from all other human beings. You know that children often look like their parents. All the resemblance is carried by the chromosomes in a single sperm and a single egg. For a moment, let's consider in more detail how this code of heredity is carried.
Scientists are making amazing discoveries about this process. In each sperm cell and egg cell, small as each is, there are molecules of a chemical called DNA. Each molecule looks some what like a twisted ladder, weighs about on ten thrillionth (1/10,000,000,000,000) of a ounce, and contains many genes. In the genes of the DNA molecule is stored a set of che- mical directions so complicated that to write them down in English would require several hundred volumes the size of a dictionary.
Every cell in your body contains these molecules. They are so small that if all the DNA molecules in all of the cells in all of the over three billion people now on the earth were piled close together like logs of wood, the whole pile would fit into a cubical box with an edge less than 1/8 of an inch, like this : You might say that, in a sense, all the people in the world in miniature could fit into that little box. The science of heredity is called genetics (from the word gene), and what you are now and will become is determined partly by your genes and partly by your surroundings, called environment. There is more about this in later chapters.
