Agriculture Reference
In-Depth Information
effects. For example, consider a single locus and two
alleles ( A and a ). If two diploid homozygous lines are
crossed where one has the genotype AA and the other
aa , then the F 1 will be heterozygous at this locus (i.e.
Aa ). If this resembles exactly the AA parent then allele
A is said to be dominant over allele a . On selfing the F 1 ,
genotypes will occur in the ratio
When the tall F 1 is backcrossed to the dwarf parent,
a ratio of 1 tall : 1 dwarf is obtained in the resulting
progeny. Similarly, when the six-row F 1 is crossed to a
homozygous two-row a ratio of 1 six-row : 1 two-row
is obtained. Evidently, the difference between tall and
dwarf behaves as a single major gene with the tall allele
being completely dominant to the dwarf allele; and the
difference between six-row and two-row is also a single
gene with six-row being completely dominant to two-
row. In terms of segregating alleles (i.e. genotypes), the
above example would be:
1
1
2
1
4 aa .If
however, the allele A is completely dominant to a , the
phenotype of the F 2 will be
4 AA :
Aa :
3
1
4 a . Therefore com-
plete dominance occurs when the F 1 shows exactly the
same phenotype as one of the two parents in the cross.
Compare these ratios to those observed by Mendel
(Table 5.1). You will notice that Mendel's data do not
fit exactly to a 3 : 1 ratio that would be expected, given
that each trait is controlled by a single completely dom-
inant gene. It should, however, be remembered that
gamete segregation in meiosis and pairing in fertiliza-
tion are random events . Therefore it is highly unlikely,
because of sampling variation that exact ratios are found
in such experiments. Indeed many more recent genetic
researchers have found that Mendel's data appears to
be 'too good a fit' for purely random events. Whether
these geneticists are correct or not does, however, not
detract in any way from the remarkable analytical mind
of Mendel.
4 A :
×
×
Parents
TT
tt
SS
ss
F 1
Tt
Ss
F 2
TT : Tt : tt
SS : Ss : ss
1:2:1
1:2:1
Where TT and Tt (or SS and Sc ) have the same phe-
notype, it results in the 3 : 1 phenotypic segregation
ratio. If each allele exerts equal effect (additive, and no
dominance) then we would have expected three pheno-
types in the ratio of 1 tall, 2 intermediate and 1 short,
according to the 1 TT :2 Tt :1 tt Mendelian ratio.
Now assume that a large number of the F 2 plants
were allowed to set selfed seed, what would be the
ratio of phenotypes and genotypes in the F 3 genera-
tion? To answer this consider that only the heterozy-
gous F 2 plants will segregate (i.e. TT and tt types
are now homozygous for that trait and will breed
true for that character). Of course the heterozygous
F 2 has the same genotype as the F 1 and so it is
no surprise that it segregates in the same ratio. At
F 3 we therefore have
Segregation of qualitative genes in diploid
species
To illustrate segregation of qualitative genes, and indeed
many following concepts, consider a series of simple
examples. First, consider two pairs of single genes in
spring barley ( H. vulgare L.). Say that dwarf barley
plants differ in plant height from tall types at the t-
locus with tall types given the genetic constitution TT ,
and dwarf lines tt . 6-row barley ears differ from 2-row
ears, where the central florets do not set, at the S-locus,
with 6-row SS being dominant over 2-row types, ss . Let
us consider the case where two homozygous lines are
artificially hybridized where one parent is homozygous
tall and 6-row and the other is dwarf and 2-row. First
consider the segregation expected of each trait.
(
1
/
4 TT
+
1
/
2
×
1
/
4 TT
)
:
1
which results
in 3/8 TT : 2/8 Tt : 3/8 tt . Similar results of 3/8
SS : 2/8 Ss : 3/8 ss would be obtained for 6-row ver-
sus 2-row. Expanding this to later generations we have:
/
2
×
1
/
2 Tt :
(
1
/
2
×
1
/
4 tt
+
1
/
4 tt
)
F 4
7
/
16 TT
:2
/
16 Tt
:
7
/
16 tt
F 5
15
/
32 TT
:2
/
32 Tt
:15
/
32 tt
F 6
31
/
64 TT
:2
/
64 Tt
:31
/
64 tt
Parents
Tall
×
Dwarf
6-row
×
2-row
.
F
F 1
Tall
6-row
/
/
1
2 TT
:
0 Tt
:
1
2 tt
 
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