Biology Reference
In-Depth Information
Fig. 6.3 Mendel's first (a)
and second (b) experimental
procedures and results
a
A
×
a
(true-breeding)
cross
Aa
×
Aa
(hybrid)
self-fertilization
A
(true-breeding)
+2
Aa
(hybrid)=
3 (dominating)
A
(true-breeding)
1 (recessive)
b
AB
×
ab
(true-breeding)
cross
AaBb
×
AaBb
(di-hybrid)
self-fertilization
1
AB
2
ABb
2
AaB
4
AaBb
1
Ab
2
Aab
1
aB
2
aBb
1
ab
9
3
3
1
G5. The behavior of each pair of differing traits in a hybrid association is indepen-
dent of all other differences in the two parents. (G5 is directly extracted from
lines 144-146 of Mendel's text. See Corcos and Monaghan, p. 113, p. 118)
One can see that these five generalizations are formulated in terms of traits, true-
breeding, hybrids, and other observable objects. No terms from gene theory are
used. By contrast, the formulations of the two Mendelian laws of heredity, as later
presented in many biology textbooks, include many theoretical terms such as
“gene,” “allele,” “locus,” “meiosis,” and others from gene theory and cell theory.
Because these five generalizations were obtained from Mendel's experiments, one
can see that they together describe the two diagrams shown in Fig.
6.3a, b
, which
represent Mendel's experimental process. In Fig.
6.3a
,
A
and
a
refer to the traits
representing different forms of true-breeding individuals;
Aa
refers to the trait
representing the hybrid from crossing
A
with
a
. In Fig.
6.3b
,
A
and
a
, and
B
and
b
refer to a pair of forms of two different traits;
ABb
,
AaB
, and
AaBb
refer to a form of
hybrids.
Ab
and
Aab
refer to the second form of hybrids, and
aB
and
aBb
refer to the
third one. So the numbers in the four forms of true-breeding and hybrids imply a
ratio of 9:3:3:1, but Mendel himself did not express this ratio explicitly.
Figures
6.3a, b
can be understood as two data models, because they do not
include theoretical or hypothetical terms such as “Mendelian factors,” “gene,”
“locus” (in chromosomes), and the like. Rather, they organize the raw data from
Mendel's series of experiments into intelligible phenomena, model the experimen-
tal results, and envisage the need to search for an underlying mechanism to be
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