Ch. 2 — Introduction • 33 
Genetics in the 20th century 
During the first few decades of the 2()tli cen- 
tury. scientists seardied for progressi\ely 
simpler experimental organisms to clai'ity pro- 
gressi\ ely more complex genetic concepts. First 
was Thomas Hunt .Morgan’s Drosop/j;7a— gnat- 
sized fruit flies v\ ith hulhous eyes. These insects 
ha\e a simple array of four easily distinguish- 
able chromosome paii's per cell. They repro- 
duce rapidly and in large numbers under the 
simplest of laboratory conditions, supplying a 
new generation e\ery month or so. Thus, re- 
searchers could carry out an enormous number 
of crosses employing a whole catalog of dif- 
ferent fruit tlv traits in a relativ ely brief time. 
It became ohxious from the extensi\e Dros- 
ophila data that certain traits were more likely 
to be inherited together than others. \'ellow 
bodies and ruby eyes, for instance, almost al- 
ways went together, w ith both in turn, appear- 
ing more frequently than expected with the 
trait known as "forked bristles. " .All three traits, 
however, showed up onlv randomly with 
curved wings. Certain genes thus seemed to be 
linked to one another. The entire Drosophila 
genome, in fact, fell into four distinct linkage 
groups. The physical basis for these groups, not 
surprisingly, consisted of the four fruit fly 
chromosomes. Linked genes behaved as they 
did because they were located on the same 
chromosome. 
Soon, scientists learned that they could not 
only assign particular genes to particular Droso- 
phila chromosomes but could identify tbe rela- 
tive locations of different genes on a given 
chromosome. This gene mapping was possible 
The riddle of the gene 
W ith all this research, nobody yet knew what 
the gene was made of. The first evidence that 
it consisted of deoxyribonucleic acid (DNA) 
emerged from the work of Oswald Avery, Colin 
MacLeod, and Maclyn McCarty at the Rockefel- 
ler Institute in New York in the early 1940’s. 
Avery’s group took as its starting point some in- 
hecause linkage itself was not permanent, 
linked genes sometimes separated. For instance, 
w hile yellow bodies, ruby eyes, and forked bris- 
tles were all linked traits, tbe first two stayed 
together far more frequently than either did 
with the third. 
The degree of linkage between two genes was 
hypothesized to be directly proportional to the 
distance between them on the chromosome, 
mainly because of a unic|ue event that occurs 
during the development of germ cells. Before 
the normal chromo.some number is halved, the 
chromosomes crowd together in the center of 
the cell, coiling tightly around each other, prac- 
tically fusing along their entire length. It is in 
this state that crossing-over (or natural recombi- 
nation)— the actual physical exchange of parts 
between chromosomes— occurs. No chromo- 
some emerges from the exchange in the same 
condition as before; the lengths of chromo- 
somes are reshuffled before being transferred 
to the next generation. 
The idea of linkage meant that Mendel’s for- 
mulations had to be modified. Clearly, genes 
were not completely independent units. Further 
work with Drosophila in the 1920’s showed that 
genes were also not" permanent and could 
change over time. Although natural mutations 
occurred at a very slow rate, exposing fruit flies 
to X-rays accelerated their frequency enor- 
mously. Exposure of a parental fly population 
led to an array of new traits among their off- 
spring-traits which, if they w'ere neither lethal 
nor sterilizing, could be passed from one gen- 
eration to the next. 
triguing observations made a decade earlier by 
a British physician, Fred Griffith. He had 
worked wdth two types of pneumococcus (the 
bacteria responsible for pneumonia) and with 
two different bacteria within each type. One 
bacterium in each type was coated in a polysac- 
charide capsule; the other was bare. Bare bac- 
