Each chromosome can be considered as being composed of small sections called _genes_, usually pictured as being strung along the length of the chromosome. Each gene is considered to be responsible for the formation of a chain of amino acids in a fixed pattern. The formation is guided by the details of the gene’s own structure (which are the “instructions” earlier referred to). This gene structure, which can be translated into an enzyme’s structure, is now called the _genetic code_. [Illustration: _Stained section of one cell from salivary gland of_ Drosophila, _or fruit flies, reveals dark bands that may be genes controlling specific traits_.] If a particular enzyme (or group of enzymes) is, for any reason, formed imperfectly or not at all, this may show up as some visible abnormality of the body—an inability to see color, for instance, or the possession of two joints in each finger rather than three. It is much easier to observe physical differences than some delicate change in the enzyme pattern of the cells. Genes are therefore usually referred to by the body change they bring about, and one can, for instance, speak of a “gene for color blindness”. A gene may exist in two or more varieties, each producing a slightly different enzyme, a situation that is reflected, in turn, in slight changes in body characteristics. Thus, there are genes governing eye color, one of which is sufficiently important to be considered a “gene for blue eyes” and another a “gene for brown eyes”. One or the other, but not both, will be found in a specific place on a specific chromosome. The two chromosomes of a particular pair govern identical sets of characteristics. Both, for instance, will have a place for genes governing eye color. If we consider only the most important of the varieties involved, those on each chromosome of the pair may be identical; both may be for blue eyes or both may be for brown eyes. In that case, the individual is _homozygous_ for that characteristic and may be referred to as a _homozygote_. The chromosomes of the pair may carry different varieties: A gene for blue eyes on one chromosome and one for brown eyes on the other. The individual is then _heterozygous_ for that characteristic and may be referred to as a _heterozygote_. Naturally, particular individuals may be homozygous for some types of characteristics and heterozygous for others. When an individual is heterozygous for a particular characteristic, it frequently happens that he shows the effect associated with only one of the gene varieties. If he possesses both a gene for brown eyes and one for blue eyes, his eyes are just as brown as though he had carried two genes for brown eyes. The gene for brown eyes is _dominant_ in this case while the gene for blue eyes is _recessive_. Parents and Offspring How does the fertilized ovum obtain its particular set of chromosomes in the first place?