Genes do two things. They provide a mechanism of inheritance, and they influence how organisms develop. When genes do the former, they effect a connection between generations – parents pass genes along to their children. When genes do the latter, they participate in processes that occur within a generation; they affect how a fertilized egg – a single cell – divides and differentiates, and eventually becomes an adult, who has numerous traits that were not present at conception.
In saying that genes provide a mechanism of inheritance, the point of the indefinite article is to highlight the fact that there are nongenetic pathways whereby parents influence the traits of their children. A child who hears English spoken while growing up will come to speak English, but this is not because English-speaking parents transmit a gene for speaking English to their offspring. Imitation and learning can lead children to resemble their parents. Cultural context can engender additional similarities that are not genetically mediated; when children inherit money from their parents, this is neither genetic transmission nor learning.
When we turn to the second role that genes play, the indefinite article is again appropriate. Genes are one cause of the traits that organisms develop, but there are others. These nongenetic causes are lumped together under the heading of “environment.” Genes contribute to an individual's being tall, but so does the amount of nutrition consumed when young. In discussing how development proceeds, it is important not to equate “genetic” influences with “biological” influences.
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