Mendelian genetics - Father of Modern genetics

Gregor John Mendel is considered as the Father of Modern Genetics.

1. Mendelian genetics Dr. G. Shanmugavel , Assistant Professor of Biology, Department of HSS, Puducherry Technological University Email : [email protected]

2. Gregor Johann Mendel (1 822– 1884) Austrian Monk, born in what is now Czech Republic in 1822 Son of peasant farmer, studied Theology and was ordained priest Order St. Augustine. Went to the university of Vienna, where he studied botany and learned the Scientific Method Worked with pure lines of peas for eight years Prior to Mendel, heredity was regarded as a "blending“ process and the offspring were essentially a "dilution“ of the different parental characteristics.

3. In 1866 he published Experiments in Plant Hybridization , ( Versuche über Pflanzen-Hybriden ) in which he established his three Principles of Inheritance He tried to repeat his work in another plant, but didn’t work because the plant reproduced asexually! If… Work was largely ignored for 34 years, until 1900, when 3 independent botanists rediscovered Mendel’s work. Loved to read especially about natural sciences and was aware of Darwin’s findings. Studied the inheritance of traits in pea plants. Considered the FATHER OF GENETICS!!!!!

4. Mendel’s peas Mendel looked at seven traits or characteristics of pea plants.

5. Mendel was the first biologist to use Mathematics – to explain his results quantitatively. Mendel predicted The concept of genes That genes occur in pairs That one gene of each pair is present in the gametes

6. Reason for selection of pea plant ( Pisum sativum ) by Mendel’s Easy to cultivate and grow in the garden. Flowers are bisexual characteristics. They are self-pollinating, and thus cross pollination can easily be performed. Different physical characteristics were easy to observe and study. Generation time is short.

7. Gametes Gametes : reproductive cells produced by sexually reproducing organisms. Two types: Male gametes = sperm In plants: pollen Female gametes = eggs In plants: ovules Ovules present in carpel

8. Fertilization Fertilization : fusion of egg and sperm Self-fertilized : fusion of sperm and egg from same plant Cross fertilized : fusion of egg and sperm from two different plants Produced hybrids F 1 : first generation F 2 : second generation Cross-pollination Female part Male parts Transfer pollen Pollen grains

9. Cross pollination

10. Mendel’s experiment procedure

11. Monohybrid cross Parents differ by a single trait. Crossing two pea plants that differ in stem size, one tall one short T = allele for Tall t = allele for dwarf TT = homozygous tall plant t t = homozygous dwarf plant TT  tt TT tt

12. Monohybrid cross for stem length: T T  t t (tall) (dwarf) P = parentals true breeding, homozygous plants: F 1 generation is heterozygous: T t (all tall plants)

13. Punnett square A useful tool to do genetic crosses For a monohybrid cross, you need a square divided by four…. Looks like a window pane… We use the Punnett square to predict the genotypes and phenotypes of the offspring.

14. Using a Punnett Square STEPS : 1. Determine the genotypes of the parent organisms 2. Write down your "cross" (mating) 3. Draw a Punnett Square (p-square) Parent genotypes: TT (tall) and tt (dwarf) Cross TT  tt Punnett-square

15. Punnett square 4. “Split" the letters of the genotype for each parent & put them "outside" the p-square 5. Determine the possible genotypes of the offspring by filling in the p-square 6. Summarize results (genotypes & phenotypes of offspring) T t T t T t T t Genotypes: 100% Tt Phenotypes: 100% Tall TT  tt Monohybrid cross: F 1 generation Possible Gametes for parents T t T T t t

16. Monohybrid cross: F 2 generation If F 1 generation is allowed to self pollinate, Mendel observed 2 phenotypes: TT T t T t tt Genotypes: 1 TT = Tall 2 Tt = Tall 1 tt = Dwarf Genotypic ratio = 1:2:1 Phenotype: 3 Tall 1 Dwarf Phenotypic ratio = 3:1 T t t T Tt  Tt (tall) (dwarf) Possible Gametes for parents T t

17. If you have another cross… A heterozygous with a homozygous T t t t  T t t T t t t Genotypes: 50% T t 50 % t t Phenotypes: 50% Tall plants 50% Dwarf plants You can still use the shortcut!

18. Another example: Monohybrid crosses P = purple (dominant) p = white (recessive) Flower color If cross a homozygous purple (PP) with a homozygous white ( pp ) PP x pp P p F 1 generation: All flowers are purple color

19. Self-fertilized of F1 generation P p x P p PP P p P p pp P p P p Phenotype: 3 Purple 1 White Phenotypic ratio = 3:1 Genotypes: 1 PP = Purple 2 P p = Purplel 1 pp = White Genotypic ratio = 1:2:1 F 2 generation: Possible Gametes for parents p P If F 1 generation is allowed to self pollinate, Mendel observed 2 phenotypes:

20. Mendel’s Principles 1. Principle of Dominance : One allele masked another, one allele was dominant over the other in the F 1 generation. 2. Principle of Segregation : When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene.

21. Dihybrid crosses Mating that involve parents that differ in two genes (two independent traits) P = purple (dominant) p = white (recessive) Flower color For example: Stem length T = tall t = short

22. Dihybrid cross: flower color and stem length TT PP  tt pp (tall, purple) (dwarf, white) Possible Gametes for parents and T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p T t P p tp TP TP TP TP TP tp tp tp tp F 1 Generation : All tall, purple flowers (T t P p )

23. Dihybrid cross: flower color and stem length (shortcut) TT PP  tt pp (tall, purple) (short, white) T t P p F 1 Generation : All tall, purple flowers (T t P p ) Possible Gametes for parents and tp TP TP tp

24. Dihybrid cross F 2 generation If F 1 generation is allowed to self pollinate, Mendel observed 4 phenotypes: T t P p  T t P p (tall, purple) (tall, purple) Possible gametes: TTPP TTP p T t PP T t P p TTP p TT pp T t P p T tpp T t PP T t P p tt PP tt P p T t P p T tpp tt P p ttpp Four phenotypes observed Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1) TP T p t P tp TP TP T p t P tp T p t P tp

25. Dihybrid cross F 2 TTPP TTP p T t PP T t P p TTP p TT pp T t P p T tpp T t PP T t P p tt PP tt P p T t P p T tpp tt P p ttpp TP TP T p t P tp T p t P tp 9 Tall Purple 3 Tall White 3 Dwarf Purple 1 Dwarf White Phenotype Ratio = 9:3:3:1

26. Nine genotypes 1 TTPP 2 TTP p 2 T t PP 4 T t P p 1 TT pp 2 T tpp 1 tt PP 2 tt P p 1 ttpp Dihybrid cross F 2 generation Tall, purple ( 9 ) Tall, white ( 3 ) Dwarf, purple ( 3 ) Dwarf, white ( 1 ) Four Phenotypes Phenotypic ratio – 9:3:3:1 Genotypic ratio – 1:2:2:4:1:2:1:2:1

27. Another example: Dihybrid cross seed color and seed shape YYRR  yyrr Y y R r F 1 Generation : All seeds are yellow round (Y y R r ) Possible Gametes for parents YR yr and YR yr (yellow, round) (green, wrinkled)

28. Dihybrid cross F 2 generation If F 1 generation is allowed to self pollinate, Mendel observed 4 phenotypes: Y y R r  Y y R r (yellow, round) (green, wrinkled) Possible gametes: YYRR YYR r YyRR Y y R r YYR r YY rr Y y R r Y yrr Y y RR Y y R r yy RR yy R r Y y R r Y yrr yy R r yyrr Four phenotypes observed - Yellow round (9); Yellow wrinkled (3); green round (3); Green wrinkled (1) YR y R YR YR Y r Y r Y r yr y R yr y R yr

29. Nine genotypes 1 YYRR 2 YYR r 2 Y y RR 4 Y y R r 1 YY rr 2 Y yrr 1 yy RR 2 yy R r 1 yyrr Dihybrid cross F 2 generation Yellow round ( 9 ) Yellow wrinkled ( 3 ) Green round ( 3 ) Green wrinkled ( 1 ) Four Phenotypes Phenotypic ratio – 9:3:3:1 Genotypic ratio – 1:2:2:4:1:2:1:2:1

30. Based on dihybrid cross results, Mendel’s postulated the Principle of Independent Assortment The factors (now genes) for different pairs of contrasting traits segregate or assort independently of each other at the time of gametogenesis in F1 hybrid without affecting or diluting each other. Mendel’s explained that the two traits (stem length and flower color) are not expressed together but they remain independent of each other. The length of stem is not always associated with the flower color; however, the expression of traits based on genotype as an independently.

31. Beyond Mendelian Genetics: Incomplete Dominance and Codominance Mendel was lucky! Traits he chose in the pea plant showed up very clearly… One allele was dominant over another, so phenotypes were easy to recognize. But sometimes phenotypes are not very obvious…

32. Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical In incomplete dominance , the phenotype of F 1 hybrids is somewhere between the phenotypes of the two parental varieties In codominance , two dominant alleles affect the phenotype in separate, distinguishable ways

33. Complete dominance T T  t t (tall) (dwarf) P = parentals true breeding, homozygous plants: F 1 generation is heterozygous: T t (all tall plants)

34. Incomplete Dominance Snapdragon flowers come in many colors. If you cross a red snapdragon (RR) with a white snapdragon (rr) You get PINK flowers (Rr)! R R R r r r  Genes show incomplete dominance when the heterozygous phenotype is intermediate.

35. Incomplete dominance When F1 generation (all pink flowers) is self pollinated, the F 2 generation is 1:2:1 red, pink, white RR R r R r rr R r R r

36. Codominance Codominance , phenomenon in which two alleles (different versions of the same gene) are expressed to an equal degree within an organism. An  example  in humans would be the ABO blood group, where alleles A and alleles B are both expressed. So if an individual inherits allele A from their mother and allele B from their father, they have blood type AB.

37. Test cross When you have an individual with an unknown genotype, you do a test cross . Test cross : Cross with a homozygous recessive individual. For example, a plant with purple flowers can either be PP or P p. Therefore, you cross the plant with a pp (white flowers, homozygous recessive) P ?  pp

38. Test cross If you get all 100% purple flowers, then the unknown parent was PP… P p P p P p P p P P p p P p p p P p p p P p p p If you get 50% white, 50% purple flowers, then the unknown parent was P p…

39. Back cross Back crossing is the process of crossing or mating a hybrid offspring with one of its parents or an individual genetically similar to its parent. Back cross carried out to achieve offspring with a genetic identity closer to that of the parent. Back cross recovers the elite genotype. It is used in horticulture, animal breeding, and production of gene knockout organisms

40. Test cross Back cross Test cross is the breeding of dominant phenotype with its recessive phenotype Back cross is the breeding of F1 hybrid with one of the parents The F1 hybrid is crossed with recessive genotype in test cross F1 hybrid is crossed with either homozygous dominant or heterozygous genotypes in back cross Test cross identifies the zygosity of the dominant phenotype Back cross recovers the elite genotype All test crosses are back crosses Back cross of F1 hybrid with the recessive phenotype can be considered as a test cross

41. Summary of Genetics Chromosomes carry hereditary information (genes) Chromosomes (and genes) occur in pairs New combinations of genes occur in sexual reproduction Mendel’s Principles: Dominance : one allele masks another Segregation : genes become separated in gamete formation Independent Assortment : Members of one gene pair segregate independently from other gene pairs during gamete formation

42. References: Concepts of Genetics (9th ed.), W. S. Klug, M. R. Cummings, C. Spencer, C. A. Spencer, and M. A. Palladino , Pearson, 2008. Molecular Genetics (Second edition), Stent, G. S.; and Calender , R. W.H. Freeman and company, Distributed by Satish Kumar Jain for CBS Publisher. Human genetics : concepts and applications . Lewis, Ricki . (2007). Boston :McGraw-Hill Higher Education. Genetics   (9th ed.), Verma PS and Agarwal VK (2010). Publisher: S  Chand & Co Ltd. Textbook of PUBLIC HEALTH AND HYGIENE , G Shanmugavel and Binu George (2021). Publisher: Darshan Publishers. Textbook of BIOLOGY FOR ENGINEERS , G Shanmugavel (2024). Publisher: Lambert academic publishing.