1. Mendels’ legacy (1822-1884)

2. Mendel Grew up in A Rural Area of Moravia with a Rich Agricultural Tradition

3. Mendel’s Family was very poor Father was a peasant farmer who was very interested in scientific crop improvement Father was a peasant farmer who was very interested in scientific crop improvement Education Education Early education was with local priest and teacher Early education was with local priest and teacher Showed considerable academic capability and so was sent to larger town Showed considerable academic capability and so was sent to larger town Studied mathematics & physics at the University (1840- 43) Studied mathematics & physics at the University (1840- 43) Financial problems plagued him, affecting his health, so Mendel decided to enter monastery as means of support Financial problems plagued him, affecting his health, so Mendel decided to enter monastery as means of support

4. Mendel’s University Career Served as teacher in church-run school (7th - 9th grade): Served as teacher in church-run school (7th - 9th grade): Greek Greek Math & general science Math & general science Returned to University of Vienna to prepare for certification as a “permanent teacher” Returned to University of Vienna to prepare for certification as a “permanent teacher” Took courses in: Took courses in: - Physics, Mathematics - and Botany Could not pass exams and remained a “temporary teacher” Could not pass exams and remained a “temporary teacher” About 1854

5. Back at Monastery Mendel Undertook Series of Plant Breeding Experiments Might seem like an unexpected activity Might seem like an unexpected activity Abbot of the Monastery, Father Napp, had long been interested in agricultural issues Abbot of the Monastery, Father Napp, had long been interested in agricultural issues Recognized that a major problem was no one knew about the nature of hybridization and how to get hybrids that bred true Recognized that a major problem was no one knew about the nature of hybridization and how to get hybrids that bred true

6. Mendel’s Experimental Garden

7. A Mendelian Genetic Primer Genes come in pairs Genes come in pairs The members of the pair may be identical (homozygous) or non-identical (heterozygous) The members of the pair may be identical (homozygous) or non-identical (heterozygous) Each form of a particular gene is an allele Each form of a particular gene is an allele

8. A Mendelian Genetic Primer Only two alleles of a given gene are possible in an individual although many alleles of a gene are possible within a population Only two alleles of a given gene are possible in an individual although many alleles of a gene are possible within a population One allele is dominant over another (or so Mendel believed) One allele is dominant over another (or so Mendel believed)

9. Genes, Alleles and Chromosomes

10. Mendel’s Model Organism: Pisum sativum The garden pea plant was a favorable organism for these studies because it was self-fertilizing/self- pollinating as the reproductive structures were completely enclosed by the petals It was easy to cultivate & matures quickly Artificial cross-breeding was possible It had distinct characteristics

11. Mendel Crossed Pea Varieties with 7 Clearly Distinguishable Traits

12. Mendel Approached The Problem of Hybridization Differently from his Predecessors When he made crosses, he followed only 1 or 2 (out of his 7) traits (characters) at a time When he made crosses, he followed only 1 or 2 (out of his 7) traits (characters) at a time He employed a very consistent method: He employed a very consistent method: Opened flower & placed pollen from one type onto the stigma Opened flower & placed pollen from one type onto the stigma Covered each flower with little bag Covered each flower with little bag When pods were ripe harvested them and planted seeds When pods were ripe harvested them and planted seeds He counted the number of each type of offspring. Why is counting important? He counted the number of each type of offspring. Why is counting important? What other aspect of his experimental set-up would Mendel have had to consider? What other aspect of his experimental set-up would Mendel have had to consider?

13. Mendel modelled the method for scientific investigations Preliminary investigations Preliminary investigations Careful planning, varying one variable at a time Careful planning, varying one variable at a time Meticulous care to avoid introduction of other variables Meticulous care to avoid introduction of other variables Accurate records Accurate records Sufficient data for statistical significance Sufficient data for statistical significance

14. The Reality of “Round and Wrinkled” Two alternative traits of the seed shape character Two alternative traits of the seed shape character Note that each seed is a new individual of a different generation – seeds are not of the same generation as the plant that bears them.

15. A Punnett square Mendel’s Monohybrid Cross A cross between individuals differing in single character is a monohybrid cross A cross between individuals differing in single character is a monohybrid cross P to F 1 P to F 1

16. Mendel’s First Experiment Crossed Pure Tall x Pure Short (Dwarf) Crossed Pure Tall x Pure Short (Dwarf) Predictions: The offspring would be: Predictions: The offspring would be: All tall All tall All short All short All intermediate All intermediate Some would be tall and some short Some would be tall and some short

17. Mendel’s First Experiment Crossed Pure Tall x Pure Short (Dwarf) Crossed Pure Tall x Pure Short (Dwarf) P generation P generation Results: F 1 generation Results: F 1 generation All tall All tall

18. Staying the Course Mendel continued crosses to the F 2 (the grandchildren) Mendel continued crosses to the F 2 (the grandchildren) The green trait was not lost or altered even though it disappeared in F 1 The green trait was not lost or altered even though it disappeared in F 1 So: One trait is dominant to the other in its expression So: One trait is dominant to the other in its expression The reappearance of the recessive trait in ¼ of the F 2 suggests that genes come in pairs that separate in the formation of sex cells The reappearance of the recessive trait in ¼ of the F 2 suggests that genes come in pairs that separate in the formation of sex cells

19. Results of Mendel’s Experiments 1st Exp: 1st Exp: All offspring (F 1 ) tall All offspring (F 1 ) tall 2nd Exp: Bred F 1 2nd Exp: Bred F 1 Results: Ratio of 787 tall to 277 short (3:1) Results: Ratio of 787 tall to 277 short (3:1) Similar to chance events from flipping 2 pairs of coins: Similar to chance events from flipping 2 pairs of coins: Heads = Tall Tails = Short 1 2

20. Mendel’s First Law: The Principle of Segregation Deduced from the analysis of his monohyrbid crosses Deduced from the analysis of his monohyrbid crosses Genes come in pairs that separate in the formation of gametes (and these sex cells unite randomly at fertilisation) Genes come in pairs that separate in the formation of gametes (and these sex cells unite randomly at fertilisation)

21. Segregation The Principle of Segregation Demystified The principle of segregation is explained by the behaviour of homologous chromosomes at meiosis

22. A Punnett Square A handy way of analysing crosses A handy way of analysing crosses An application of the Principle of Segregation An application of the Principle of Segregation Named after Cambridge geneticist R.C. Punnett Named after Cambridge geneticist R.C. Punnett

23. Characters investigated by Mendel Consistency is Good No matter what the character, Mendel observed a 3:1 ratio of characters in the F 2

24. Therefore, the Principle of Segregation indeed is a general principle of genetics. Monohybrid crosses yielded consistent results

25. An albino woman What works for peas also works for humans In the cross Aa x Aa, where A is a dominant allele for wild type (standard) pigmentation and a is a recessive allele for no pigmentation (albinism), ¾ of offspring will be wild type and ¼ will be albino

26. Different Genotypes can Produce the Same Phenotype

27. Mendel’s Next (3rd) Experiment Crossed one of the F 1 tall plants with its dwarf parent: Crossed one of the F 1 tall plants with its dwarf parent: F 1 Tall x Dwarf F 1 Tall x Dwarf Possible Outcomes: Possible Outcomes: All would be tall All would be tall Mixture of Tall & Dwarf Mixture of Tall & Dwarf All would be intermediate All would be intermediate Experimental results —> Experimental results —> This became known as the “back-cross” or “test- cross” This became known as the “back-cross” or “test- cross” 50% 50%

28. Dihybrid crosses Mendel performed these to find out if different characters like colour and shape inherited together or independently. Mendel performed these to find out if different characters like colour and shape inherited together or independently. Mendel’s conclusion: Mendel’s conclusion: Different characters are inherited independently Different characters are inherited independently

29. The alternative and incorrect hypothesis: dependent inheritance. Why did Mendel conclude that the inheritance of one trait is independent of another? Because it’s the only way to explain the pattern of inheritance Because it’s the only way to explain the pattern of inheritance

30. The Punnett Square for a Dihybrid Cross Note that we are simultaneously applying the Principles of Note that we are simultaneously applying the Principles of Segregations and Segregations and Independent Assortment Independent Assortment

31. The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment The alignment of one pair of homologs is independent of any other. Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes. How many possible types of gametes would be formed with our 23 pairs of chromosomes? 2 n =2 23

32. Consider a cross between parents heterozygous for both deafness and albinism. This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape. What works for peas also works for humans

33. Mendel’s Paper Mendel presented his results at two meetings of the Brünn Natural History Society in 1865 Mendel presented his results at two meetings of the Brünn Natural History Society in 1865 The Secretary of the Society recorded that there were no comments The Secretary of the Society recorded that there were no comments The paper was published in 1866 in the Society’s journal & widely circulated The paper was published in 1866 in the Society’s journal & widely circulated Few references to it in the next 34 years Few references to it in the next 34 years

34. Why Was Mendel’s Paper Ignored? Mendel’s name was not known among academic botanists Mendel’s name was not known among academic botanists There are no illustrations There are no illustrations The paper is filled with algebraic expressions and numbers -- not what biologists were used to (see p. 31) The paper is filled with algebraic expressions and numbers -- not what biologists were used to (see p. 31)

35. Support and Opposition after 1900 Animal and plant breeders were initially the most enthusiastic Animal and plant breeders were initially the most enthusiastic Academic biologists were quite skeptical: Academic biologists were quite skeptical: - What were “factors”? - Whole scheme seemed to “theoretical” - Reminded many investigators of older 19th- century theories that relied on hypothetical “particles” for which there was no evidence