QUANTITATIVE INHERITANCE - KERNEL COLOR IN WHEAT

Nilsson-Ehle (1909) and East (1910, 1916) documented first significant evidence of
quantitative inheritance by their individual works in wheat.
 Their analysis started from one-locus control which continued to two locus control
and concluded at three-locus control.

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QUANTITATIVE INHERITANCE - KERNEL COLOR IN WHEAT
 Nilsson-Ehle (1909) and East (1910, 1916) documented first significant evidence of
quantitative inheritance by their individual works in wheat.
 Their analysis started from one-locus control which continued to two locus control
and concluded at three-locus control.
ONE-LOCUS CONTROL
 One-locus control inheritance involves two
alleles.
 For example, let’s consider a gene with two
alleles - dominant allele ‘A’, recessive allele
‘a’.
 Dominant allele ‘A’ produces red pigment
and recessive allele ‘a’ results in no
pigment production.
 Each dominant allele produce one unit of
color and each recessive allele produces no
color.
 When a particular strain of wheat having
red grain Kernel (AA) is crossed with
another strain having white grain (aa), all
the F1 plants have kernels that are
intermediate in color.
 The F1 heterozygote, Aa is intermediate
(incomplete dominance).
 When these plants are self-fertilized, the
ratio of kernels in the F2 is;
1 red: 2 intermediate: 1 white



This ratio is similar to typical
monohybrid ratio and doesn’t explain
range of kernel color.

TWO LOCUS CONTROL
 Two-locus control inheritance involves four alleles.
 For example, let’s consider two genes A and B; with four alleles - two dominant
allele’s ‘A’ and ‘B’, two recessive alleles ‘a’ and ‘b’.
 Dominant alleles ‘A’ and ‘B’ produces red pigment and recessive alleles ‘a’ and ‘b’
results in no pigment production.
 Each dominant allele produce one unit of color and each recessive allele produces no
color.
 When two stocks of wheat with red kernels (AABB – 4 color units) and white kernels
(aabb – 0 color units) are crossed, the intermediate AaBb – 2 color units (medium red)
F1 is obtained.
 The self-fertilization of F1 progenies results in five kernels colors, with f2 ratio as;
1 Dark red: 4 Medium dark red: 6 Medium red: 4 Light red: 1 White
Or
1:4:6:4:1
 This ratio is similar to typical dihybrid ratio and doesn’t explain range of kernel color.

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Parental phenotype : Dark red kernel color X White kernel color
Parental genotype : AABB X aabb
Gametes : AB X ab
F1 offsrings : AaBb
Medium-red kernel color
F1 selfing (F1 X F1) : AaBb X AaBb
F2 ratio= 1 Dark red: 4 Medium dark red: 6 Medium red: 4 Light red: 1 White
THREE LOCUS CONTROL
 H Nilsson-Ehle in 1909 crossed two wheat strains, one with red-kernel and other with
white-kernel grain that yielded plants in the F1 generation with grains of intermediate
color. When F1 generations were self fertilized, he observed seven kernel color classes
from red to white were distinguishable in a ratio of 1:6:15:20:15:6:1
 This result can be explained by assuming that 3 loci are assorting independently,
each with two alleles; such that one dominant allele produces a unit a red color and
the other recessive allele does not produce any color.
Parental phenotype : Dark red kernel color X White kernel color
Parental genotype : AABBCC X aabbcc
Gametes : ABC X abc
F1 offsrings : AaBbCc
Medium-red kernel color

F1 selfing (F1 X F1) : AaBbCc X AaBbCc




AB Ab aB ab
AB AABB
Dark-Red
4 units
AABb
Medium-Dark Red
3 units
AaBB
Medium-Dark Red
3 units
AaBb
Medium - Red
2 units
Ab AABb
Medium-Dark Red
3 units
AAbb
Medium - Red
2 units
AaBb
Medium - Red
2 units
Aabb
Light - Red
2 units
Ab AaBB
Medium-Dark Red
3 units
AaBb
Medium - Red
2 units
AaBb
Medium - Red
2 units
Aabb
Light - Red
2 units
Ab AaBb
Medium - Red
2 units
Aabb
Light - Red
2 units
aaBb
Light - Red
2 units
aabb
white
0 units

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F2 Generation :



Ratio 1 6 15 20 15 6 1
Phenotype Deep
Red
Dark
Red
Red Medium
Red
Light
Red
Very Light
Red
White

This example shows how we can get a spectrum of slightly different phenotypes
(continuous variation) with just three genes.

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