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multiple factor hypothesis presentation pbg.
- 1. Submitted To:- Dr. PamirelliRanjith sir Assistant Professor (Dept. of Genetics & Plant Breeding) Submitted By:- Klarissa Hembram(241310039) Debasis Behera(241310040) MULTIPLE FACTOR HYPOTHESIS
- 2. NILSSON-EHLE : THEMIND BEHIND THE THEORY • First demonstrated polygenic inheritance using wheat kernel colour in 1908. • Showed that multiple genes control a single trait and established concept of additive gene effects. • Helped develop the Multiple Factor Hypothesis and united Mendelian and NILS HERMAN NILSSON-EHLE (12 FEBRUARY 1873 – 29 DECEMBER 1949)
- 3. • People canhave different heights, skin colours, and eye colours. • These differences come from many genes working together to create these characteristics. We call this polygenic inheritance. • Many genes contribute additively to a single trait.Each gene has a small, additive effect on phenotype. • Unlike characteristics controlled by one gene, polygenic characteristics involve multiple genes and a range of possible characteristcs. INTRODUCTION
- 4. WHAT ARE POLYGENIC CHARACTERISTICS? •Polygenic characteristics are influenced by multiple genes. • Each of these genes contributes a small amount to the final appearance. • This explains the wide variety in characteristics like height, skin color, and eye color.
- 5. EXAMPLES OF POLYGENIC CHARACTERISTICS •Many everyday characteristics are polygenic. • For example, human height is influenced by multiple genes, not just one. • The same goes for skin colour, where multiple genes determine how much melanin the skin produces.
- 6. QUALITATIVE CHARACTERISTICS • Controlledby one or few major genes. • Show discontinuous variation. • Traits fall into distinct categories. • Example: flower color, blood group, seed shape.
- 7. QUANTITATIVE CHARACTERISTICS • Controlledby many genes (polygenes). • Each gene contributes a small additive effect. • Show continuous variation. • Example: height, skin color, grain yield.
- 8. QUALITATIVE VS QUANTITATIVE TRAITS QualitativeTraits • It deals with the inheritance of traits of kind, viz., form, structure, colour, etc. • Discrete phenotypic classes occur which display discontinuous variations • Each qualitative trait is governed by two or many alleles of a single gene. • The phenotypic expression of a gene is not influenced by environment. Quantitative Traits • It deals with the inheritance of traits of degree, viz., heights of length, weight, number, etc. • A spectrum of phenotypic classes occur which contain continuous variations. • Each quantitative trait is governed by many non-allelic genes or polygenes. • Environmental conditions effect the phenotypic expression of polygenes variously.
- 9. MULTIPLE FACTOR HYPOTHESIS •The Multiple Factor Hypothesis explains how quantitative traits (traits that show continuous variation) are controlled by multiple genes, each with a small and additive effect. • A quantitative trait is controlled by two or more genes, called polygenes. • Each dominant allele contributes an equal, small, cumulative effect toward expression. • No single gene shows a large effect. • More the number of dominant alleles stronger the expression of the trait. → Formula: Number of phenotypic classes = 2n + 1 (where n = number of polygenic pairs) Examples are:- • Human height • Skin colour • Grain yield • Wheat kernel colour
- 10. WHEAT KERNEL COLOUR EXPERIMENT(NILSSON–EHLE) • Kernel colour is determined by three independently assorting gene pairs:A/a, B/b • Each dominant allele contributes a small, equal amount of red colour i.e.,additive effect • No dominance among the polygenes • Each dominant allele acts independently • Environment does not affect the basic genetic ratio (but may alter shade intensity). • Most important experiment for polygenic inheritance Key Assumption of the Experiment Crossing the Parents (P1 × P2) AABB × aabb F1 → generation AaBb(F1) Each F1 has 2 dominant alleles Therefore F1 kernels appear medium red (intermediate shade)
- 11. WHEAT KERNEL COLOUR EXPERIMENT(NILSSON–EHLE) F1 Selfing (AaBbCc × AaBbCc F2) → • The F2 generation shows wide variation in kernel colour shades due to recombination. • The range depends on number of dominant alleles, not on specific gene positions. • Gives 5 phenotypic classes (because 2n+1 = 2×2+1 = 5) 2 gene pairs total possible dominant alleles → = 4 Thus, F2 individuals can have between: • 0 dominant alleles white → • 4 dominant alleles dark red →
- 12. WHEAT KERNEL COLOUR EXPERIMENT(NILSSON–EHLE) Gametes of AaBb: Punnett Square Structure (4 × 4 Grid):-
- 13. WHEAT KERNEL COLOUR EXPERIMENT(NILSSON–EHLE) F2 Phenotypic Classes and Ratio:- Number of Dominant Alleles Shade of Colour Expected → → Ratio:- Phenotyipic ratio:1 : 4 : 6 : 4 : 1 Genotypic ratio:-1 : 2 : 2 : 4 : 1 : 2 : 1 : 2 : 1
- 14. WHEAT KERNEL COLOUR EXPERIMENT(NILSSON–EHLE) Why This Happens (Genetics Behind It):- 🔸 2 gene pairs segregate independently Each gene pair forms gametes in Mendelian fashion. 🔸 The combination of these gametes forms offspring having: • 0 dominant alleles (all recessives) • 1 dominant allele • 2 dominant alleles • … up to • 4 dominant alleles 🔸 Dominant alleles add colour intensity Not dominance additive effect. → More dominant alleles more red pigment made deeper → → red colour.
- 15. WHEAT KERNEL COLOUR EXPERIMENT(NILSSON–EHLE) Graphical Representation – Bell-Shaped Curve:- When the F2 data is plotted: • Most individuals fall in the middle class (medium red) • Very few show extreme colours (deep red or white) With this Nilsson-Ehle proved that, “Many genes with small additive effects can produce a wide range of phenotypes, explaining continuous variation.”
- 16. CONCLUSION •Multiple genes influencequantitative traits. •Each gene adds a small effect. •Results in continuous variation. •Key concept in plant breeding and quantitative genetics.
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