control animal behavior have had

to deal with many uncertainties.
In

Line
the first place, most behaviors are

(5)
governed by more than one gene,

and until recently geneticists had no

method for identifying the multiple

genes involved.
In addition, even

when a single gene is found to control

(10)
a behavior, researchers in different

fields do not necessarily agree that it is

a “behavioral gene.”
Neuroscientists,

whose interest in genetic research is to

understand the nervous system (which

(15)
generates behavior), define the term

broadly.
But ethologists—specialists

in animal behavior—are interested

in evolution, so they define the term

narrowly.
They insist that mutations in

(20)
a behavioral gene must alter a specific

normal behavior and not merely make

the organism ill, so that the genetically

induced behavioral change will provide

variation that natural selection can act

(25)
upon, possibly leading to the evolution

of a new species.
For example, in the

fruit fly, researchers have identified

the gene Shaker, mutations in which

cause flies to shake violently under

(30)
anesthesia.
Since shaking is not

healthy, ethologists do not consider

Shaker a behavioral gene.
In con-

trast, ethologists do consider the gene

period (per), which controls the fruit

(35)
                    
fly’s circadian (24-hour) rhythm, a

behavioral gene because files with

mutated per genes are healthy; they

simply have different rhythms.

It can be inferred from the passage that which of the following, if true, would be most likely to influence ethologists’ opinions about whether a particular gene in a species is a behavioral gene?

A    The gene is found only in that species.

B    The gene is extremely difficult to identify.

C   The only effect of mutations in the gene is to make the organism ill.

D   Neuroscientists consider the gene to be a behavioral gene.

答案选C。有点不能理解，因为21行说不能仅以make the organism ill来判断是否是行为基因，请求解答，谢谢了。