Mendelian Genetics and Heredity

1. Mendelian Genetics and Heredity Genetics is the branch of biology that deals with heredity and variation of organisms. Heredity is the passing of traits from parents to offspring.

2. Traits, Genes, and Alleles • DNA is wound into chromosomes • DNA encodes thousands of genes • Genes have slightly different versions from person to person • Each alternate form of a gene is called an allele. • Each organism has two alleles, one on each chromosome of a homologous pairs. • Each parent donates only one allele for each gene to an offspring

3. Homologous Chromosomes • Pair of chromosomes that have the same genes at the same loci, but possibly different alleles. • One set from each parent. • New combinations of genes occur in sexual reproduction • Fertilization from two parents

4. Mitosis

5. Meiosis

6. How do we know about genes and inheritance?

7. Gregor Johann Mendel (1822 – 1884) • The father of modern genetics • Austrian Monk spent 10 years in a monastery experimenting with selective breeding of peas. • Why work with peas?

8. Mendel looked at seven traits of garden pea (Pisumsativum).

9. Intro to Mendel • https://www.youtube.com/watch?v=cWt1RFnWNzk • Ted Ed https://www.youtube.com/watch?v=Mehz7tCxjSE • Sci Show: https://www.youtube.com/watch?v=GTiOETaZg4w

10. Mendel Developed Several Hypotheses: - Elements are passed from parent to offspring- Different elements control different traits- Elements unchanged during life of organism passed to offspring- Elements are passed on in the gametes of organisms

11. Pure Line - a population that breeds true for a particular trait • Hybrid is as a result of crossing between two different characteristics (carries two different alleles of same gene) • Monohybrid -a hybrid that is heterozygous to a specific gene • Dihybrid - a hybrid that is heterozygous for alleles of two different genes • An F1 hybrid (or filial 1 hybrid) is the first filial generation of offspring of distinctly different parental types • Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait.

12. More Definitions • P = Parental generation • F1= First filial generation; offspring from a genetic cross. • F2= Second filial generation of a genetic cross • Test Cross – used todetermine the genotype of an individual with a dominant phenotype

13. One of Mendel’s early experiments: • Mendel crossed a purple flowering plant with a white flowered plant (parents differ by a single trait = monohybridcross )

14. ALL offspring of the First Filialgeneration (F1): had purple flowers He let them self-pollinate to form subsequent generations F2 , F3 …

15. The F2 generation contained:75% purple flowers and 25% white flowers. 

16. F3 hybrid cross produced same ratio as F1

17. The Law of Dominance: • Mendel found one allele would mask (was dominant over) the other  in the F1 generation.  • This led to Mendel's Law of Dominance which states that: • One of the factors for a pair of inherited traits (alleles) will be dominant and the other recessive, unless both factors are recessive. • Therefore, a cross between a homozygous dominant and a homozygous recessive will always express the dominant phenotype, while still having a heterozygous genotype.

18. Mendel’s Discoveries and Conclusions: • The concept of genes and that they occur in pairs (alleles) • Concept of Dominance • One allele may be dominant over the other • Segregation of alleles • 2 copies of a gene segregate from each other so that each gamete carries only one allele • Gametes of one trait combine irrespective of the other gene pairs (of traits)

19. Mendel’s Laws FIRST LAW: THE PRINCIPLE OF SEGREGATION • The two members of a gene pair (alleles) segregate (separate) from each other in the formation of gametes. Half the gametes carry one allele, and the other half carry the other allele. SECOND LAW; THE PRINCIPLE OF INDEPENDENT ASSORTMENT • Genes for different traits assort independently of one another in the formation of gametes. THIRD LAW:LAW OF DOMINANCE • one of the factors for a pair of inherited traits will be dominant and the other recessive, unless both factors are recessive. 

21. Genes and Alleles • As we have TWO sets of chromosomes, one from mom, one from dad), we have TWO sets of genes. • Mom and dad may give us each a different form of the same gene (example: gene for brown eyes , gene for blue eyes). • The different forms of each gene is called an allele. • Often one allele may be dominant over the other, recessive allele. • The combination of alleles is called genotype. • Dominant alleles are indicated by capital letter, recessive by lower case letter.

22. Genotype and Phenotype • Homozygous – having two of the same alleles for a particular gene Example BB or bb • Heterozygous – having two different alleles for the same gene: Example Bb • Phenotype is the visible trait that is expressed due to our genotype BB, Bb – Brown eyes bb – blue eyes

23. The Law of Dominance: • A one-eyed purple people eater is crossed with a two-eyed purple people eater. • All of their offspring have two eyes. • Which trait is dominant?

24. The Law of Dominance: • EE = two eyes • Ee = two eyes • ee = one eye What’s the genotype of the offspring if each parent was purebred? EE x ee • What if you crossed the offspring with each other? • How many new offspring would you expect to have two eyes?

26. Mendel didn’t know about chromosomes and genes or meiosisbut figured out that there was some factor for each trait Eg. Tall plant: T vs. t • Mendel did the same experiment for all seven characteristics. • In each F1 generation: one trait disappeared and then showed up again in the F2 plants. • In each F2 generation: 75% had one trait and 25% had the other trait. • Based on these observations, Mendel formulated the Law of Segregation….

27. The Law of Segregation • There are two factors controlling a given characteristic (alleles), and these factors separate and go to different gametes when a parent reproduces. • We now know that the segregation of genes occurs during meiosis in eukaryotes, which is a process that produces reproductive cells called gametes.

28. The Gene-Chromosome Theory • It wasn’t until 1900 that Mendel’s discoveries were fully understood. • New information on cells allowed scientists to realize Mendel’s factor were genes which are found on chromosomes inside the cell.

29. Can organisms with different genotypes have the same phenotypes? 

30. Yes: Example using Monohybrid Cross • 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 T T t t

31. Monohybrid cross for stem length: P = homozygous plants: T T t t (tall) (dwarf) T t (all tall plants) F1 generation is heterozygous:

32. Amoeba Sisters: Alleles and Genes • https://www.youtube.com/watch?v=pv3Kj0UjiLE

35. Punnett Squares

36. Using a Punnett Square We use the Punnett square to predict the genotypes and phenotypes of the offspringCross a TT (Tall) plant with a t t (short) plant T T t t

37. Secret of the Punnett Square • Key to the Punnett Square: • Determine the gametes of each parent… • How? By “splitting” the genotypes of each parent: If this is your cross T T  t t The gametes are: T T t t

38. Once you have the gametes… T T t t  t t T T

39. Punnett square 1. "split" the letters of the genotype for each parent & put them "outside" the p-square 2. determine the possible genotypes of the offspring by filling in the p-square 3. summarize results (genotypes & phenotypes of offspring) T T T T  t t t t Genotypes: 100% T t Phenotypes: 100% Tall plants

40. Cross the F1 generation

41. PUNNET SQUARE • Tt x Tt • Homozygous – Individual has both alleles the same. TT – homozygous tall plant.tt – homozygous short • Heterozygous – Has one of each allele. Tt – heterozygous tall

42. Monohybrid cross: F2 generation • If you let the F1 generation self-fertilize, the next monohybrid cross would be: T t T t (tall) (tall) Genotypes: 1 TT= Tall 2 Tt = Tall 1 tt = dwarf Genotypic ratio= 1:2:1 T t T t Phenotype: 3 Tall 1 dwarf Phenotypic ratio= 3:1

43. Genotype – The combination of alleles that an organism possesses. • TT, Tt and tt are different genotypes • Phenotype – The outward appearance of an organism • The phenotype for TT would be tallThe phenotype for Tt would be tall. The phenotype for tt would be short.

44. Test cross When you have an individual with an unknown genotype, you do a test crosswith a homozygous recessive individual. For example, a plant with purple flowers can either be PP or Pp… therefore, you cross the plant with a pp (white flowers, homozygous recessive) P ? pp

45. Draw a Punnett Square for this test cross.

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

47. Dihybrid Cross (involves two traits) • Cross between two organisms heterozygous for the same traits. • Used to investigate two different genes

48. Dihybrid Cross RR = Round rr = Wrinkled YY = Yellow Yy = Green

49. Dihybrid Cross of F2: When F1 generation is allowed to self pollinate, mendel observed 4 phenotypes Phenotype Ratio = 9:3:3:1 But 9 different genotypes

50. Law of Independent Assortment Genes for different traits are sorted separately from one another so that the inheritance of one trait is not dependent on the inheritance of another (dihybrid cross). Different traits are inherited independently (separately) of one another.