A general account of Quantitative (Multiple factor or Polygenic) Inheritance; Examples : Kernel colour in Wheat, Ear size (Cob length ) in Maize(Zea mays) ; Differences between Qualitative and Quantitative Inheritance

1. QUANTITATIVE INHERITANCE
Dr Saji Mariam George
Associate Professor (Retired)
Assumption College Autonomous
Changanacherry

2. QUANTITATIVE INHERITANCE
• Quantitative inheritance is also known as
Polygenic or Multiple factor inheritance.
• Quantitative characters (metric traits) are
measurable such as yield of crops (fruits,
seeds etc.) ; height, weight, skin colour,
intelligence etc. in man ; egg production in
poultry ; milk production in cow etc.
• They do not show clear cut differences and
show continuous variation. That is, there are
many intermediate types between the
parental traits. Continuous variation was first
reported by Joseph Kolreuter (1760) with
regard to the height of Nicotiana (Tobacco)
plants.
Joseph Gottlieb Kolreuter
https://collections.nlm.nih.gov

3. • Yule (a British mathematician,
1906) suggested that quantitative
characters are controlled by many
genes (cumulative genes or
multiple factors or polygenes -a
term coined by Mather).
• Polygenes are non-allelic and each
gene has a small and similar effect
and the effect of several such
genes are additive or cumulative.
The net effect on the trait will
depend upon the number of
contributing alleles or effective
alleles present(Multiple factor
hypothesis).
George Udny Yule
https://royalsocietypublishing.org/doi/pdf/
10.1098/rsbm.1952.0020
Kenneth Mather
https://www.jic.ac.uk/about-us/history-of-plant-microbial-science-at-john-innes-centre

4. • Analysis of quantitative traits can be done by statistical
methods.
Examples:
1. Kernel colour in Wheat
2. Ear size (Cob length) in Maize(Corn, Zea mays)

5. 1. Kernel colour in Wheat
• The inheritance of kernel colour in
Wheat was studied by the Swedish
geneticist Herman Nilsson Ehle (1908).
He had carried out several crosses
(hybridizations) between red kernel
coloured and white kernel coloured
Wheat varieties.
• The F1 heterozygotes (hybrids) in all
crosses showed a lighter red kernel
colour (intermediate colour) than the
homozygotes (red kernel coloured
parents). He got different F2 phenotypic
ratios in various hybridizations such as
3 red : 1 white ,
15 red : 1 white and
63 red : 1 white.
Moreover, there was a continuous variation
in the red kernel colour to white in F2
generations.
Herman Nilsson Ehle
https://commons.wikimedia.org/wiki/Fi
le:Herman_Nilsson-Ehle.jpgg

6. • Based on the results of hybridizations between red kernel
coloured and white kernel coloured Wheat varieties, Nilsson -
Ehle proposed that kernel colour in Wheat is controlled by
three genes. Each gene has two alleles, one the pigment
contributing allele or effective allele giving red grain colour
and the other, the non-contributing allele causing white grain
colour .
• Each contributing allele for red kernel colour contributes a
small degree of red colour to the Wheat kernel and the net
phenotypic effect is due to the additive or cumulative effect of
all the pigment contributing alleles. That is, each dose of a
pigment contributing allele increases the intensity of the red
colour.

7. • Let us consider a cross between a red kernel coloured Wheat variety
with a white kernel coloured Wheat variety where two genes (each
gene has 2 alleles) are involved.
• Let AABB (4 contributing or effective alleles) be the genotype of the
red kernel coloured Wheat variety and aabb (No contributing alleles,
has only non-contributing alleles) be the genotype of the white kernel
coloured Wheat variety.
• The genotype of the F1 hybrids will be AaBb (two contributing
alleles). Since the F1 hybrids have only two pigment contributing
alleles, the kernels will have medium red colour (Intermediate
colour) .
• The F1 hybrids with the genotype, AaBb produce 4 types of gametes
such as AB, Ab, aB and ab during gametogenesis.
• The random fusion of the gametes will result in 16 possible
combinations (4 ♀ x 4 ♂) in the F2 generation. Out of these, 15 will
have red kernel colour (shows continuous variation in red colour
which depends on the number of contributing alleles, whose effects
are similar and additive ) and 1 will be with white kernels.
That is, 15 red : 1 white.

8. Red kernel coloured Wheat x White kernel coloured Wheat
where two genes (each gene has 2 alleles) are involved.
Punnett square

9. • The F2 progeny show continuous variation in red kernel colour
depending upon the number of contributing alleles, whose effects are
additive (cumulative).
Among the progeny,
• 1/16 has 4 contributing or effective alleles for red colour(Genotype
AABB). Hence the kernel colour is same as that of the red
grandparent. i.e., deep red.
• 4/16 have 3 contributing alleles each (Genotypes AABb, AaBB) and
the kernels have dark red colour.
• 6/16 have 2 contributing alleles each (Genotypes AaBb, AAbb, aaBB)
and the kernels have medium red colour.
• 4/16 have 1 contributing allele each (Genotypes Aabb, aaBb) and the
kernel colour is light red.
• 1/16 has no contributing alleles for colour. It has only non-
contributing alleles (Genotype aabb) and hence is white.
• Thus, F2 progeny occur in five phenotypic classes such as 1 deep red :
4 dark red : 6 medium red : 4 light red : 1 white
i.e., in a ratio 1 : 4 : 6 : 4 : 1

10. • Let us consider a cross between a red kernel coloured Wheat variety
with a white kernel coloured Wheat variety where three genes (each
gene has 2 alleles) are involved.
• Let AABBCC (6 contributing or effective alleles) be the genotype of the
red kernel coloured Wheat variety and aabbcc (No contributing alleles,
has only non-contributing alleles) be the genotype of the white kernel
coloured Wheat variety.
• The genotype of the F1 hybrids will be AaBbCc (three contributing
alleles). Since the F1 hybrids have only three pigment contributing
alleles, the kernels will have medium red colour (Intermediate colour) .
• The F1 hybrids with the genotype, AaBbCc produce 8 types of gametes
such as ABC, ABc, AbC, Abc, aBC, aBc, abC and abc during
gametogenesis.
• The random fusion of the gametes will result in 64 possible combinations
(8 ♀ x 8 ♂) in the F2 generation. Out of these, 63 will have red kernel
colour (Shows continuous variation in red colour. The intensity of the
colour depends on the number of contributing alleles whose effects are
similar and additive ) and 1 will be with white kernels.
That is, 63 red : 1 white.

11. Red kernel coloured Wheat x White kernel coloured Wheat
where three genes (each gene has 2 alleles) are involved.

12. F2 : Punnett square 63 red : 1 white

13. • In the F2 progeny, there is continuous variation in the red colour of Wheat
kernel to white which depends upon the number of contributing or
effective alleles whose effects are additive (cumulative).
Among the F2 progeny,
• 1/64 has 6 contributing or effective alleles for colour and will be like red
grandparent (Genotype AABBCC) with deep red coloured kernels.
• 6/64 have 5 contributing alleles each for colour (Genotypes AABBCc,
AABbCC, AaBBCC, ) and have dark red coloured kernels.
• 15/64 have 4 contributing alleles each for colour ( Genotypes AABbCc,
AaBBCc, AaBbCC, AABBcc, AAbbCC, aaBBCC) and have red coloured
kernels.
• 20/64 have 3 contributing alleles each for colour (Genotypes AaBbCc,
AABbcc, AaBBcc, AAbbCc, AabbCC, aaBBCc, aaBbCC) and have medium
red coloured kernels.
• 15/64 have 2 contributing alleles each for colour (Genotypes AaBbcc,
AabbCc, AAbbcc, aaBbCc, aaBBcc, aabbCC) and have light red coloured
kernels.
• 6/64 have 1 contributing allele each for colour (Genotypes Aabbcc, aaBbcc,
aabbCc) and have very light red coloured kernels and
• 1/64 has no contributing alleles for colour and has only non-contributing
alleles (Genotype aabbcc) and has white kernels.

14. • Thus, there are seven phenotypic classes which show
continuous variation such as
1 deep red : 6 dark red : 15 red : 20 medium red : 15 light
red : 6 very light red : 1 white .
i.e., in a ratio 1 : 6 : 15 : 20 : 15 : 6 : 1
• Kernel colour in Wheat being a quantitative trait , is also
modified by environmental factors such as temperature
(which changes enzyme controlled reaction rates), sunlight
and nutrition etc. (which change the level of enzymes and
substrates for reaction ). Such environmental effects cause
individual grains of Wheat to vary within each genotype.

15. Quantitative inheritance
Kernel colour variations in Wheat…
https://agrodaily.com https://www.amu.ac.in

16. Kernel colour variations in Wheat…
https://www.grainews.ca

17. Kernel colour variations in Wheat…
https://greatlakesstapleseeds.com

18. Kernel colour variations in Wheat
https://www.amu.ac.in

19. 2. Ear size (Cob length) in Maize(Corn, Zea mays)
• The ear or cob of a Corn is the
spike that contains kernels,
protected by leaves called
husks.
A cob with attached Corn kernels
https://en.wikipedia.org
https://www.cookstr.com

20. • The inheritance of ear size or
cob length in Maize was studied
by American geneticists
Emerson and East (1913). They
crossed a long eared Black
Mexican Sweet corn (ear length
ranges from 13 cm to 21 cm,
average length 16.8 cm) with a
short eared Tom Thumb
Popcorn (ear length ranges
from 5cm to 8 cm, average
length 6.6 cm).
Rollins Adams Emerson
Edward Murray East
http://www.nasonline.org

21. Long eared Black Mexican Sweet corn
(ear length ranges from 13cm to 21 cm,
average length 16.8 cm)
Short eared Tom Thumb Popcorn
(ear length ranges from 5cm to 8 cm,
average length 6.6 cm)
© 2011 - Green Barn Gardens, LLC.
https://www.victoryseeds.com

22. • In the F1 progeny , ears (cobs) were of intermediate length (ear
length ranges from 9cm to 15 cm , average length 12.1 cm).
• In the F2, the progeny showed a continuous variation with a
wide range of cob length.
• Some corn plants produced short ears similar to that of the
short eared grandparent and some other plants have long ears
similar to that of the long eared grandparent.
• But, majority of the F2 progeny have intermediate ear length
similar to that of F1.

23. • According to Emerson and East, the ear size in Maize is
determined by two genes (Each gene has two alleles) which
show quantitative (polygenic) inheritance.
• In the absence of contributing or effective alleles, the
average cob length is about 6.6 cm as in the short eared Tom
Thumb Popcorn parent.
• The ear length depends upon the number of contributing
alleles . Each contributing allele has a small but similar
effect. It was found that each contributing allele added
2.55cm to the basic length of the cob, 6.6 cm
(That is, 16.8 – 6.6/4 = 2.55 cm /contributing allele, where
16.8 – Average ear length of Black Mexican Sweet corn
6.6 – Average ear length of Tom Thumb Popcorn
4 – Total number of alleles(Two genes- each gene has two
alleles - affecting the ear length in Maize)
• The net phenotypic effect in ear size is due to the additive or
cumulative effect of all the contributing alleles.

26. Among the F2 progeny,
• 1/16 has 4 contributing or effective alleles for ear size ( Genotype AABB)
and the average ear size is 16.8 cm similar to that of Black Mexican Sweet
corn grandparent.
(Basic length 6.6cm + 2.55 [each contributing allele add 2.55cm to the basic
length ] x 4 [ Number of contributing alleles AABB]. i.e. 6.6 + 2.55 x 4 = 16.8).
• 4/16 have 3 contributing alleles each for ear size (Genotypes AABb, AaBB).
Average ear size is 14.2 cm.
( 6.6 + 2.55 x 3 = 14.2 )
• 6/16 have 2 contributing or effective alleles each for ear size (Genotypes
AaBb, AAbb, aaBB). Average ear size is 11. 7 cm.
(6.6 + 2.55 x 2 = 11.7)
• 4/16 have 1 contributing or effective allele each for ear size (Genotypes
Aabb, aaBb). Average ear size is 9.1 cm.
( 6.6 + 2.55 x 1 = 9.1)
• 1/16 has no contributing alleles for ear size and has only non-contributing
alleles (aabb). Average ear size is 6.6 cm similar to that of Tom Thumb
Popcorn grand parent.
• Here, there are five phenotypic classes which exhibit continuous variation in
ear length in a ratio, 1 : 4 : 6 : 4 : 1
• Ear(cob) size in Maize being a quantitative trait is also influenced by
environmental factors such as temperature, sunlight, nutrition etc.

27. Qualitative traits/ Inheritance Quantitative traits/Inheritance
Non- measurable. Measurable - can be
statistically analysed.
Monogenic - controlled by a
single gene. Its effect is
distinguishable.
Polygenic - controlled by many
genes. Each gene has a small,
but similar effect and the net
effect is due to the additive or
cumulative effect of all the
genes. So, single gene effect is
indistinguishable.

28. Qualitative traits/ Inheritance Quantitative traits/Inheritance
Characters show discontinuous
variation since there is a
distinct line of separation
between one group and
another and form distinct
phenotypic classes.
e.g. Flower colour, seed shape
etc.
Characters show continuous
variation – show many
intermediate types. Do not
form distinct phenotypic
classes.
e.g. Grain yield, fibre yield;
height, weight, skin colour,
intelligence etc. in humans.
Not susceptible to
environmental modifications.
Quantitative traits are
modified by environmental
factors.

29. Qualitative traits/ Inheritance Quantitative traits/Inheritance
There is dominance -recessive
relationship.
There is no dominance. There
exists only pairs of
contributing and non-
contributing alleles.
Epistasis may be present. No epistasis.
There may be linkage. No linkage.

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