5. Mendelian Genetics

1. 5. Mendelian Genetics Chapter 11

2. Meiosis Review • Mendel’s principles require offspring to inherit one copy of each gene from each parent. • Formation of gametes must divide the chromosomes from each parent in half

3. Meiosis – Review • Gametes are reproductive cells (such as egg and sperm) that contain half the DNA of a normal cell for a given species. • Only reproductive cells undergo meiosis.

4. Meiosis – Review • Meiosis: result is that gametes contain one chromosome of each type from the parent.

5. Introduction: • Heredity- the passing of traits from parent to offspring • Heredity is the key to the difference between species and variety within a population • Genetics- study of heredity

6. Early Ideas about Heredity • Blending Inheritance- the idea that offspring were always a combination of traits contributed by the parents (19th Century)

7. Gregor Mendel • Father of genetics • Czech Republic, born 1822 • Monastery Center of Sci Learning (1851) • In charge of garden at the monastery

8. Studied sexual reproduction of pea plants • Male – Pollen • Female – Ovules

9. Pollen producing stamen (male) Ovule producing pistil (female)

10. Pea plants typically self-pollinate • Pollen / Ovules from same plant • Offspring of self pollination are genetically identical to parents

11. Mendel found that self pollination could be prevented • Mendel then cross pollinated plants with different traits and analyzed the results. • Pollen / Ovules from different plants

12. Mendel limited his study to a few isolated traits that were easy to identify • Seed shape (round / wrinkled) • Seed color (green / yellow) • Plant height (tall / short)

14. Genes and Dominance • Mendel crossed plants with different char for the same trait. Ex: tall x short • Hybrids – offspring resulting from cross breeding

15. In the first generation of hybrids there was no blending of characteristics • Hybrids all exhibited one trait. (all tall, all yellow, not mixed) • Char of one parent seemed to have disappeared.

17. Mendel concluded that individual factors do not blend • Gene- a factor that controls a single characteristic • Gene for seed color • Gene for eye color

19. Alleles- Different forms of a gene, producing various char. • Gene for seed color • Yellow allele / green allele • Gene for plant height • Tall allele / short allele

20. Homozygous- having two identical alleles for a given trait. (TT , tt, YY, yy) • Heterozygous- having two different alleles for a given trait (Tt, Yy)

21. Mendel also concluded that some alleles are dominant over others (recessive). • Tall dominant over short • If a tall plant is CP with a short plant, the offspring will have genes for both, but only tall will occur

22. Tall (TT) ----------------Short (tt) Tall (Tt)-------Tall (Tt)

23. Genotype- the genetic make up for a particular char. • TT, Tt, tt • Phenotype- the physical char. that is expressed as a result of genotype • Tall, Tall, Short

24. Segregation • The offspring from prior experiments were allowed to SP • Naming generations: • Original Parents (P) • First generation (F1) • Second generation (F2)

25. Tall------------------------------Short Tall-------Tall Tall Tall Tall Short PF1 F2

26. F1 was allowed to self polinateproducing F2 • Some of the F2 plants showed the traits that had disappeared in the F1 generation (recessive) • Mendel assumed that the disappearance in F1 was because the dominant allele masked the recessive allele

27. In order for recessive traits to reappear, they must have segregated from the dominant allele. • In the F1 generation each plant received one allele from each parent, but only the dominant one was observed…

28. Mendel believed that each generation receives two alleles for each gene, one from each parent • Gene pairs are segregated in order to produce the gametes (sex cells: sperm, egg, ovum, pollen etc). • Punnet Square is used to predict outcomes of segregation

29. Punnet Square- F1 cross

30. Punnet Square- F2 cross

32. The End

33. Independent Assortment • Mendel conducted experiments to assess whether traits segregated independently or were linked to each other. • Is the gene that determines seed color linked to the gene that determines seed shape?

34. He performed a two-factor cross to test (examined two traits at once). • F1 : RRYY x rryy • Homozygous round/yellow • Homozygous wrinkled/green

38. As expected, the resulting F1 were heterozygous, exhibiting only the dominant traits (all round, all yellow). • F1 was then allowed to SP • F2 : RrYy x RrYy • Heterozygous round/yellow

42. Phenotypes? • Mendel’s experiment prod: 9 Yellow Round 3 Yellow Wrinkled 3 Green Round 1 Green Wrinkled

43. Two factor crosses consistently produce these ratios (9:3:3:1) • Many of the characteristics that Mendel studied do sort independently • Further studies showed that genes on the same chromosome are often linked.

44. Applying Mendel’s Principles • Genetics and probability: • Consistent results from Mendel’s work allowed for models to make predictions • Probable outcomes could be calculated

45. Number of times a specific event occurs --------------------- The # of opportunities Probability=

46. Flip a coin (1/2 or 50%) • Roll a die (1/6 or 17%) • Punnet squares can be used to predict the probability of a particular outcome for given cross.

47. Yy xYy

48. Punnet square shows ratios of possible outcomes. • 1 in 4 or 25% chance of YY • 2 in 4 or 50% chance of Yy • 1 in 4 or 25% chance of yy