1. Meiosis & Mendel

2. Homologous Chromosomes
23 from “mom”
23 from “dad”
46 total
The 23 chromosomes have different sizes, shapes and number of genes

3. Your cells have autosomes and sex chromosomes
Autosomes: chromosome that contains genes for characteristics not directly related to the sex of an organism
In humans, pairs 1-22
Sex Chromosomes: chromosomes that directly controls the development of sexual characteristics
In humans, pair 23

4. You have body cells & gametes
Somatic cells (body cells): cells that make up the body tissues and organs (except gametes)
2 copies of 23 different chromosomes
46 total chromosomes
Gametes (sex cells): egg or sperm cells
1 copy of 23 different chromosomes
Combination of mom and dad’s genes

5. Body cells are diploid; gametes are haploid
Haploid: A cell or organisms that only has one set of unpaired chromosomes
Diploid: A cell that has a set of paired chromosomes; one from each parent

6. Formation of Haploid Cells
In order to pass on genes, organisms pass on haploid cells to offspring
Meiosis: From the Greek meiōn, meaning “less”
The process of cell division that halves the number of chromosomes
Occurs when forming specialized reproductive cells, such as gametes or spores

7. Cells go through two rounds of division in meiosis

8. Homologous chromosomes and sister chromatids
Homologous chromosomes: one from each parent; separated in meiosis I
Sister chromatids: duplicated chromosomes; separated in meiosis II

9. Meiosis i

10. Right before meiosis begins…
The DNA in the original cell is replicated
Therefore, meiosis starts with homologous chromosomes

11. Prophase I The chromosomes condense The nuclear envelope breaks down
Homologous chromosomes pair along their length (tetrad)
Crossing-over occurs
Portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding chromatid portions of the other homologous chromosome

12. Crossing Over DNA exchange
Adds even more recombination to independent assortment of chromosomes
The number of genetic combinations amongst gametes is practically unlimited

13. Metaphase I
The spindle moves the pairs of homologous chromosomes to the equator of the cell
The homologous chromosomes remain together

14. Anaphase I The homologous chromosomes separate
Spindle fibers pull the chromosomes to opposite poles
Sister chromatids do not separate at their centromeres
Each chromosome is composed of two chromatids, the genetic material, however, has recombined

15. Telophase I Individual chromosomes gather at each of the poles
The cytoplasm divides (cytokinesis)
Two new cells are formed
Both cells or poles contain one chromosome from each pair of homologous chromosomes

16. Chromosomes DO NOT replicate between meiosis I and meiosis II
Round 2
Chromosomes DO NOT replicate between meiosis I and meiosis II

17. Meiosis ii

18. Prophase II Nuclear membrane breaks down
A new spindle forms around the chromosomes

19. Metaphase II The chromosomes lines up along the equator
The chromosomes are attached at their centromeres to spindle fibers

20. Anaphase II The centromeres divide
The chromosomes (now called chromatids) move to opposite poles of the cell

21. Telophase II A nuclear envelope forms around each set of chromosomes
The spindle breaks down
The cell undergoes cytokinesis
Four haploid cells result

22. Comparing mitosis & meiosis

23. Haploid cells develop into mature gametes
Gametogenesis: the production of gametes

24. Meiosis in Males: Spermatogenesis
The process by which sperm are produced in male animals
Occurs in the testes (male reproductive organs)

25. Meiosis in Females: Oogenesis
The process by which eggs are produced in female animals
Occurs in the ovaries (female reproductive organs)

26. The Importance of Genetic Variation
Meiosis and the joining of gametes are essential to evolution
No genetic process generates variation more quickly
Breeding (i.e. domestic animals) slows down genetic variation

27. Mendel and heredity Gregor Mendel
“Father of Genetics”
“Mendelian Genetics”
Austrian monk
Bred different varieties of the garden pea
The first to develop rules that accurately predict patterns of heredity

28. Heredity The passing of genetic traits from parents to offspring
Breeding animals and crops was done before DNA and chromosomes were discovered

29. Genetics
The science of heredity and of the mechanisms by which traits are passed from parent to offspring

30. Mendel laid the groundwork for genetics
Before Mendel people thought offspring were a “blend” of their parents
Example: a tall plant crossed with a short plant would get a medium plant…wrong!

31. Mendel’s data revealed patterns of inheritance

32. The Seven Characters Mendel Studied and Their Contrasting Traits

33. Useful Features in Peas
Male & female reproductive parts of garden peas are on the same flower
Self-fertilization: mating can be controlled
Cross-pollination: pollen can be transferred to another plant
The garden pea is small, grows easily, matures quickly and produces many offspring

34. Monohybrid Cross A cross that involves one pair of contrasting traits
Example: cross a plant with purple flowers and a plant with white flowers
Mendel did his experiments in 3 steps

35. Step 1
True-breeding: Allow each variety of garden pea to self-pollinate for several generations
Ensure offspring will only show one form of the character
Example: a true breeding purple-flowering plant should produce only purple flowers
P generation: The parental generation

36. Step 2 Cross-pollinate two P generation plants with contrasting traits
Example: cross purple flowers with white flowers
F1 generation: offspring of the P generation

37. Step 3 Allow the F1 generation to self pollinate
F2 generation: offspring of the F1

38. results

39. Mendel’s Results Each of the F1 showed only one form of the trait
One of the traits had disappeared!

40. Mendel’s Results
When the F1 self-pollinated, the missing trait reappeared in some of the flowers!
Mendel saw that there was a ratio of 3 purple flowers to 1 white flower (3:1)
Mendel looked at the 7 traits and found the same 3:1 ratio in the F2 generation

41. conclusions
1. Traits are inherited as discrete units, which explains why individual traits persist without being blended or diluted over successive generations
Law of Segregation
2. Organisms inherit 2 copies of each gene, one from each parent
3. Organisms donate only 1 copy of each gene in their gametes

42. Traits, genes, & alleles

43. The same gene can have many versions
Allele: two versions of a gene
Individuals receive one allele from each parent
Each allele can be passed on when the individual reproduces

44. Homozygous
Two alleles of a particular gene are present in an individual
Homozygous recessive (pp for pea flowers)
Homozygous dominant (PP for pea flowers)

45. Heterozygous
Two different alleles of a particular gene are present in an individual
Pp for pea flowers (still shows purple)

46. genome All of an organism’s genetic material
Unless you have an identical twin, you have a unique genome that determines all of your traits!

47. Genotype and Phenotype
The set of alleles that an individual has for a character (PP, Pp, pp)
Phenotype
The physical appearance of a character
Determined by the genotype
PP = purple, Pp = purple, pp = white

48. Dominant alleles Dominant
The expressed form of the character (Mendel’s flowers: purple)
Represented by a capital letter

49. Recessive alleles Recessive
The trait that was not expressed when the dominant form of the character was present (Mendel’s flowers: white)
Represented by a lower case letter

50. Traits & probability

51. Punnett squares illustrate genetic crosses
Diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross

52. A monohybrid cross involves one trait
Homozygous-Homozygous
4:0 dominant to recessive
Heterozygous-Heterozygous
3:1 dominant to recessive
Heterozygous-Homozygous
1:1 dominant to recessive

53. Homozygous-homozygous

54. heterozygous-heterozygous

55. Heterozygous-homozygous

56. Test cross determining unknown genotypes
Used to determine if a dominant phenotype is heterozygous or homozygous
Cross the individual with a homozygous recessive

57. A dihybrid cross involves two traits

58. Heredity patterns can be calculated with probability
Probability: the likelihood that a specific event will occur
Probability = number of one kind of possible outcome
total number of all possible outcomes
Coin toss: either heads or tails, probability of heads is ½ and probability of tails is 1/2

59. Probability of the Outcome of a Cross
Two parents are involved in a genetic cross so both must be considered when calculating the probability
To find the probability that a combination of two independent events will occur, multiply the separate probabilities of the two events

60. Coin tossing…2 Coins Tossed
Probability of 2 heads: ½ x ½ = ¼
Probability of 2 tails: ½ x ½ = ¼
Probability of 1 heads and 1 tails: ½ x ½ = ¼
Probability of 1 tails and 1 heads: ½ x ½ = ¼
Probability of all possible outcomes must add up to 1

61. Meiosis and genetic variation

62. Law of Independent Assortment
The inheritance of one character does not influence the inheritance of any other character
The alleles of different genes separate independently of one another during gamete formation

63. Independent Assortment
The random distribution of homologous chromosomes
Each of the 23 pairs of chromosomes segregates (separates) independently
About 223 (about 8 million) gametes with different gene combinations can be produced from one original cell

64. Random Fertilization
The zygote that forms a new individual is created by the random joining of two gametes
Because fertilization of an egg by a sperm is random, the number of possible outcomes is squared (223 x 223 = 64 million)

65. Sexual reproduction creates unique gene combinations
Crossing over
Genetic linkage