Agriculture Reference
In-Depth Information
Before considering an example of one of Mendel's
experiments, there are a few general points to be made of
his experiments. Others, before Mendel, had made con-
trolled hybridizations or crosses within various species.
So why did Mendel's crosses, rather than those of ear-
lier workers, provide the basis for the modern science
of genetics? First and foremost Mendel had a brilliant
analytical mind that enabled him to interpret his results
in ways that defined the principles of heredity. Second,
Mendel was a proficient experimentalist. Therefore he
knew how to carry out experiments in such a way as to
maximize the chances of obtaining meaningful results.
He knew how to simplify data in a meaningful way.
As parents in his crosses, he chose individuals that dif-
fered by sharply contrasting characters (now known to
be controlled by single genes). Finally, as noted above,
he used true breeding (homozygous) lines as parents in
the crosses he studied.
Among other elements in Mendel's success were the
simple, logistical sequence of crosses that he made and
the careful numerical counts of his progeny that he kept
with reference to the easily definable characteristics on
whose inheritance he focussed his attention. It should
be noted that many of the features listed as reasons for
Mendel's success are very similar to the criteria neces-
sary to carry out a successful plant breeding programme
(including the '
luck element
').
Let us consider some of Mendel's experiments as an
introduction to qualitative inheritance. These results
are presented in Table 5.1. When Mendel crossed plants
from a round-seeded line with plants from a wrinkled-
seeded line, all of the first generation (F
1
)
5474 round-seed and 1850 wrinkled-seed, a ratio of
2.96 : 1.
Mendel found it notable that the same general result
occurred when he made crosses between plants from
lines differing for other characters. Another example is
when he crossed peas with yellow cotyledons with ones
with green cotyledons the F
1
s all had yellow cotyledons
while he found a ratio of about 3 yellows to 1 green
in the F
2
. Almost identical results were obtained when
long-stemmed plants were crossed with short-stemmed
plants and when plants with axial inflorescence were
crossed to plants with terminal inflorescence.
What did Mendel make of his generalized findings?
One of the keys to his solution was his recognition that
in F
1
the heredity basis for the character that fails to
be expressed is not lost. This expression of the charac-
ter appears again in the F
2
generation. Recognizing the
idea of dominance and recessiveness in heterozygous
genotypes and the particulate nature of the heritable
factors was the overwhelming genius of Gregor Mendel.
This laid the foundation of genetics and hence the
explanation underlying the most important features of
qualitative genetics.
Genotype/phenotype relationships
Within genetic studies there are two inter-related
points. The first is concerned with the actual genetic
make-up of individual plants or segregating populations
and is referred to as the genotype. The second is related
to what is actually expressed or observed in individ-
ual plants or segregating populations and is termed the
phenotype.
In the absence of any environmental variation (which
can often be assumed with qualitative, single, major
gene, inheritance) the most frequent cause of difference
between genotype and phenotype is due to
dominance
had round
seeds. The characteristic of only one of the parental
types was therefore represented in the progeny.
In
the next generation (F
2
)
, achieved by selfing the F
1
,
both round-seed and wrinkled-seed were found in the
progeny. Mendel's count of the two types in the F
2
was
Table 5.1
Results from some of Mendel's crossing experiments with peas.
Phenotype of parents
F
1
progeny
# of F
2
progeny
F
2
ratio,
dominant : recessive
by phenotype
round (r)
×
wrinkled (w), seed
round
5,474 e : 1850 w
2.96 : 1
×
Yellow (y)
green (g), coty.
yellow
6,022 y : 2001 g
3.01 : 1
×
long (lo)
short (sh), stem
long
787 lo : 277 sh
2.84 : 1
×
axil (ax)
terminal (ter), inf
axil
651 ax : 207 ter
3.15 : 1
