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hold them are paired. Genes hold "information" for the production of proteins; this information determines the way in which the organism will develop.

In rare cases, a spontaneous or sudden change occurs in the DNA, and as a result the cell does not replicate itself exactly. The resultant change is called a mutation. Mutations are permanent and frequently produce radical and harmful effects on human offspring.

Genetic Influences. Once the zygote is formed, the presence or absence of various genes determines whether a particular characteristic will be observed in the offspring. In some cases, a characteristic of an offspring is determined by only one or two genes. In other cases, many genes work together to control some aspect of the offspring's development. Regardless, the total genetic pattern or script for any individual is called a genome and is created by the particular combination of chromosomes received from the biological parents.

EXAMPLE 4.2. A single defective gene causes a disease called phenylketonuria (PKU) in which the body is unable to convert the protein phenylalanine into a usable form. If phenylalanine builds up to toxic levels, it attacks the cells of the central nervous system, causing mental retardation. (Research has indicated that more than 150 genes affect brain development. Therefore, that many or more must function correctly for a normal brain to develop.)

The characteristics determined by genes include skin color, eye color, hair color, height and weight tendencies, internal organ development, and the possibility of birth defects. Much debate continues about whether intellectual and personality characteristics can be inherited within the genome. Perhaps the most commonly accepted conclusion is that certain tendencies may be passed on from one generation to the next but specific behaviors learned by the parent cannot. Trying to separate the effects of hereditary and environmental factors in such studies has proved very difficult. (See the comments on twins below.)

Dominant and Recessive Genes. Each pair of genes controls specific activities of development. The members of the pair are not always alike. If the paired genes are not alike, one of the pair (the dominant gene) will act as a controller while the other (the recessive gene) does not affect the process of development. Recessive characteristics appear in an offspring only when both members of the gene pair are recessive.

EXAMPLE 4.3. Skin color may be either normal (which is dominant) or albino (which is recessive and is characterized by a lack of pigmentation). The offspring will be albino only if both parents transmit that trait in the germ cells that form the zygote. It is possible that both parents could have normal skin coloring and still produce an albino child, as Fig. 4-1 shows. Although both parents show normal skin coloring, on average one of four conceptions could be albino (aa).

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