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COURSE Principles of inheritance and variations Lesson 15 Polygenes and Polygenic inheritance (Quantitative inheritance) Dr. Praveen Kumar Agrawal M.Sc., Ph.D., CSIR NET (JRF). SRF, GATE 21Years of Pre-medical teaching experience
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POLYGENIC INHERITANCE (Quantitative inheritance) It is another kind of inheritance where the quantity of the genes determines the nature of inheritance. . The quality of genes has a little or no role to play in such inheritance. Hence this type of inheritance is also called quantitative inheritance Mendel's rules are not applicable in such type of inheritance. In majority of cases, a character has two or more sharp alternatives. The inheritance of such characters .For example, skin colour in man has no sharp alternatives. Similarly height in man has no sharp . There are many grades of phenotypes. Similarly gradation may be seen in seed size of beans, colour in In all these cases a continuous gradation in the quality of the character is observed. Thus the kind of is governed by Mendel's rules. But some characters are there, which do not have sharp alternatives. alternatives wheat or cob length in corn inheritance which involves the continuous gradation in the quality of the character is called polygenic or quantitative inheritance. The genes, which are involved in this inheritance, are called polygenes (or multiple factors).
. When there are no clear and sharp alternatives, the analysis of the character can not be made by simple counting method. It is instead done by statistical methods, which are based on the quantity of the genes involved. That is why this inheritance is also called quantitative inheritance. . The genes involved (i.e., polygenes) in this type of inheritance are completely non-allelic. They have different locations within the genome of the individual. . These genes have cumulative or additive effect i.e. more the number of dominant genes, more will be the grade (degree of expression) of the character.
Examples of polygenic inheritance Kernel colour in wheat Kernel colour in wheat has no sharp alternatives however it is of five main types dark, medium, intermediate, light and white. The inheritance of these colours is governed by two non allelic genes A and B. The phenotypes and genotypes for the five colours can be shown as below . a. Dark-All four dominant alleles (genes) AABE b. Medium - Any three dominant alleles (genes) - AABb, AaBB, aABB, AAbB. c. Intermediate - Any two dominant alleles (genes) - AAbb, aaBB, AaBb. d. Light Any one dominant allele (gene) - Aabb, aAbb, AABb, AAbB. e. White No dominant allele, all recessive alleles (genes) aabb. . It can be noted that how the increasing number of dominant alleles affects the phenotype grade. When a dark variety (AABB) is crossed with a white variety (aabb), in F1 all intermediate plants are produced. On selfing, dark, medium, intermediate, light and white plants are produced respectively in 1:4:6:4 1 ratio in F2 generation.
AABB aabbWhite AB AB ab ab F1 AaBb AaBb ABbAaBb All individuals are intemediate
Selfing of F1 AaBb AaBb Male Female AB aB ab AABB AABb AaBB AaBb AB Medium Medium Inter- mediate Dark AABb AAbb Aabb AaBb Inter- mediate Ab Inter- mediate Light Mediumm AaBB AaBb aaBB aaBb Inter- mediate Inter- mediate Mediumm Light AaBb Aabb aaBb aabb Inter- mediate Light Light White
Results Phenotypic ratio1:4:6:4:1 (1 Dark, 4 medium, 6 intermediate, 4 light, 1 white)
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