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COURSE Principles of inheritance and variations Lesson 8 Inheritance of two genes and Mendel's law of Independent assortment Dr. Praveen Kumar Agrawal M.Sc., Ph.D., CSIR NET (JRF). SRF, GATE 21Years of Pre-medical teaching experience
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INHERITANCE OF TWO GENES (Mendel's Dihybrid Cross) In his dihybrid crosses, Mendel crossed two plants, taking into consideration 2 pairs of contrasting characters For example, in one such cross, he considered two characters 1. Shape of seed and 2. Colour of seed. These characters had following contrasting (alternating) forms. Character Allelomorphic (Alternative) Forms Dominant Recessive Shape of seed Wrinkled (r) Round (R) Yellow (Y) Colour of seed Green (y)
Mendel crossed a pure (homozygous) round and yellow seeded plant (YYRR) with a pure (homozygous) wrinkled and green seeded plant (yyrr) Pure Yellow Round YYRR yyrr Pure Green Wrinkled Y R YR All individuals had yellow-round seeds F1 YyRr YyRr YyRr YyRr
All individuals had yellow-round seeds F1 YyRr YyRr Selfing of F1 YyRr Male Female Yyr YR Yr yR yr YYRR YYRr YyRR YyRr YR Yellow- Yellow- Yellow- Yellow- YYRr YYrr YyRr Yyrr Yellow- Yellow- Yellow YyRR YyRr yyRR yyRr yR Yellow Round Green Round Green- Round YyRr yyRr yyrr Yellow- Yellow- Green- Green Results Phenotypic ratio 9 : 3 : 3: 1 (Yellow-round 9. Yellow-wrinkled 3, Green-round 3. Green wrinkled 1) Genotypic ratio RRYY RRYy rrYY: RrYY RrYy rrYy: Rryy: Rryy rryy 4 :2
In F2 generations 4 types of plants were produced in 9:3:3: 1 ratio. Yellow-round 9 Yellow-wrinkled 3 Green-round 3 Green-wrinkled 1 When the results were analysed separately for each character (considering single character at a time), then usual monohybrid ratio (3 :1) was obtained. For shape of seedTotal round 12 Total wrinkled 4 Total yellow 12 Then ratio between yellow and green 12:4 or 3:1 Then ratio between round and wrinkled 12:4 or 3 1 For Colour of seed Total green 4 Conclusion This separate analysis for each character clearly suggests that the inheritance of two characters is completely independent of each other. It means that the genes for these two characters do not influence each other. This conclusion prompted Mendel to formulate his principle of independent assortment. . .
Mendel's Law of Independent Assortment) According to it when two pairs of traits are combined in a hybrid, then segregation of one pair of trait is independent of the other pair of traits. It states that the genes of one character do not influence the genes of another character in any way. During inheritance they remain completely independent of each other and assort independently. Mendel was not aware of linkage, crossing over and gene interaction. Later findings revealed out the fact that genes, which are present on same homologous chromosomes, show some degree of interaction. Therefore Mendel's third principle is modified as-the genes of one character do not influence the genes of another character, if they are present on different homologous chromosomes.
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