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QUANTITATIVE INHERITANCE SMG

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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

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

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QUANTITATIVE INHERITANCE SMG

  1. 1. QUANTITATIVE INHERITANCE Dr Saji Mariam George Associate Professor (Retired) Assumption College Autonomous Changanacherry
  2. 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. 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. 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. 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. 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. 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. 8. Red kernel coloured Wheat x White kernel coloured Wheat where two genes (each gene has 2 alleles) are involved. Punnett square
  9. 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. 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. 11. Red kernel coloured Wheat x White kernel coloured Wheat where three genes (each gene has 2 alleles) are involved.
  12. 12. F2 : Punnett square 63 red : 1 white
  13. 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. 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. 15. Quantitative inheritance Kernel colour variations in Wheat… https://agrodaily.com https://www.amu.ac.in
  16. 16. Kernel colour variations in Wheat… https://www.grainews.ca
  17. 17. Kernel colour variations in Wheat… https://greatlakesstapleseeds.com
  18. 18. Kernel colour variations in Wheat https://www.amu.ac.in
  19. 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. 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. 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. 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. 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.
  24. 24. 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.
  25. 25. 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.
  26. 26. 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.
  27. 27. 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.
  28. 28. THANK YOU

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