30 • Impacts of Applied Genetics — Micro-Organisms, Plants, and Animals 
passed from parents to offspring, to a study of 
the underlying agent by which this transmission 
is accomplished. That shift began in the garden 
of Gregor Mendel, an obscure monk in mid-19th 
century Austria. By analyzing generations of 
controlled crosses between sweet pea plants, 
Mendel was able to identify the rudimentary 
characteristics of what was later termed the 
gene. 
Mendel reasoned that genes were the vehicle 
and repository of the hereditary mechanism, 
and that each inherited trait or function of an 
organism had a specific gene directing its devel- 
opment and appearance. An organism’s observ- 
able characteristics, functions, and measurable 
properties taken together had to be based some- 
how on the total assemblage of its genes. 
Mendel’s analysis showed that the genes of 
his pea plants remained constant from one gen- 
eration to the next, but more importantly, he 
found that genes and observable traits were not 
simply matched one-for-one. There were, in 
fact, two genes involved in each trait, with a 
single gene contributed by each parent. When 
the genes controlling a particular trait are iden- 
tical, the organism is homozygous for that trait; 
if they are not, it is heterozygous. 
In the Mendelian crosses, homozygous plants 
always retained the expected characteristics. 
But heterozygous plants did not simply display a 
mixture of their different genes; one of the two 
tended to predominate. Thus, when homozy- 
gous yellow-seed peas were crossed with homo- 
zygous green-seed plants, all the offspring were 
now heterozygous for seed color, possessing a 
“green” gene from one parent and a "yellow” 
from the other. Yet all of them turned out to be 
indistinguishable from the yellow-seed parent: 
Yellow-seed color in peas was dominant to 
green. 
But even though the offspring resembled 
their dominant parent, they could be shown to 
contain a genetic difference. For when the het- 
erozygotes were now crossed with each other, a 
certain number of recessive green-seed plant 
again appeared among the offspring. This oc- 
curred whenever an offspring was endowed 
with a pair of genes that was homozygous for 
the green-seed trait— and it occurred at a rate 
consistent with the random selection of one of 
two genes from each parent for passage to the 
new generation. (See figure 5.) 
Genes were real— Mendel’s work made that 
clear. But where were they located, and what 
were they? The answer, lay within the nucleus 
of the cell. Unfortunately, most of the contents 
of the nucleus were unobtainable by biologists 
in Mendel’s time, so his published findings were 
ignored. Only during the last decades of the 
19th century did improved microscopes and 
new dyes permit cells to be observed with an 
acuity never before possible. And only by the 
Figure 5.— The Inheritance Pattern of Pea Color 
Y = yellow gene g = green gene 
Homozygous yellow-seed peas have the genetic compost- 
tion; YY. 
|N^WS|fgous green-seed peas have the genetic carspoBt' 
ion: gg. 
Each parent contributes only one seed-color gene to the off- 
spring. When the two YY and gg homozygotes are crossed, 
the genetic composition of all offspring is Yg: 
All Yg offspring are heterozygous, and all have yellow 
seeds, indicating that the Y yellow gene is dominant over 
the g green gene. 
When these Yg heterozygotes are crossed with each other: 
Vi of the total are homozygous YY, having yejlow seeds 
V4 ofjhe total are homozygous gg,' having srmh aa«^ | 
Vi of the total are heterozygous Yg, having yellow seeds 
Thus, % of these offspring will have yellow seeds, but their 
individual genetic composition, YY of Yg, may be different 
SOURCE: Office of Technology Assessment. 
