1. Meiosis & mendelian genetics– chapter
Freshman Biology; Semester Two

2. Chromosomes
Def – the genetic information passed down from parent to offspring
Each/every human body cell has 46 chromosomes
44 = non-sex chromosomes (22 pairs)
2 = sex chromosomes  X or Y (1 pair)
All body cells (except sex cells) go through mitosis
Mitosis produces cells that are:
Clones/genetically identical to parent

5. BEFORE chromosome replication AFTER chromosome replication

6. Cell Cycle Review
Asexual reproduction that occurs in body (somatic) cells; not sex cells
Interphase G1 S G2
Mitosis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
End product is 2 cells that are genetically identical to the parent (original) cell

7. Characteristics of Meiosis
Meiosis occurs in gametes (sex cells) ONLY
TWO divisions with 4 phases each (8 phases total) creating 4 unique cells
Cells start out diploid and end haploid

8. Daughter cells vs. parent cells
Initial Comparison
Mitosis
Meiosis
# of cells produced 2 4
Daughter cells vs. parent cells
Identical
Not identical (Why? crossing over)
# of chromosomes
Same
(46  46 in humans)
Cut in ½ (46  23 in humans)
Purpose
To produce new cells (growth, repair old/damaged cells)
To produce gametes -egg and sperm
(for sexually reproducing organisms)

9. Diploid vs. Haploid Diploid (2n) cells have two sets of chromosomes
One inherited from mom; one from dad
All somatic (body) cells are diploid (all cells except sex cells)
Humans’ diploid number is 46, but other species have other numbers.
The chromosomes that are alike from each set are called homologous chromosomes.
Haploid (1n) cells have one set of chromosomes
Gametes (sex cells) are haploid
Humans’ haploid number is 23, but other species have different numbers.
When fertilization occurs, the organism will again be diploid.
23 chromosomes from male parent + 23 chromosomes from female parent = 46 total (diploid)

10. Meiosis I: Prepping for Meiosis
Interphase I
Cells replicate DNA ONCE, forming duplicate chromosomes
There will only be ONE interphase during the whole process of meiosis.
Meiosis I has four stages:
Prophase I
Metaphase I
Anaphase I
Telophase I
Cytokinesis
Page 273, Figure 5

11. Meiosis I: Stage One Prophase I
Each chromosome (2 sister chromatids) pairs with its corresponding homologous chromosome to form a tetrad
Crossing-over occurs
Result: the exchange of alleles between homologous chromosomes and produces new combos of alleles

13. Meiosis I: Stage Two Metaphase I
Spindle fibers attach to the chromosomes
Still attached at the centromere
Forms tetrad (2 homologous c’somes lined up at equator)

14. Meiosis I: Stage Three Anaphase I
Spindle fibers pull apart homologous chromosomes toward opposite ends of the cell
Sister Chromatids are still connected at the centromere!

15. Meiosis I: Stage Four & Cytokinesis
Telophase I
Nuclear membranes form
Sister Chromatids may not be identical due to crossing over
Cytokinesis
The cytoplasm separates (just like in mitosis)
Cell splits into two haploid (n) cells

16. Meiosis II
Meiosis II is very similar to mitosis; however there is NO chromosome replication that takes place before it begins (no interphase II)
Both haploid (n) cells created in meiosis I divide
Ends with four new haploid (n) cells
Sperm or egg cells
Four stages:
Prophase II
Metaphase II
Anaphase II
Telophase II
Cytokinesis
MEIOSIS II

17. Meiosis II: Stage One Prophase II
The two haploid (n) daughter cells that were produced at the end of meiosis I have half the number of chromosomes as the original cell
NO REPLICATION OF CHROMOSOMES happens during meiosis II

18. Meiosis II: Stage Two Metaphase II
Chromosomes line up in the center of each cell
Spindle fibers are attached at centromeres of sister chromatids
Like metaphase in mitosis. Why?

19. Meiosis II: Stage Three
Anaphase II
Spindle fibers shorten
Sister chromatids separate and move apart toward opposite ends of each cell

20. Meiosis II: Stage Four & Cytokinesis
Telophase II
Nuclear envelopes reform in both cells
Cytokinesis
The cytoplasm in both cells splits to form 4 haploid (n) daughter cells with HALF the number of chromosomes as the original cell
So if parent cell has 46 chromosomes, each cell at the end of meiosis II would have 23 chromosomes.
Result: Sexual Reproduction allows for genetic variation
Crossing over makes 4 possibilites!

21. Spermatogenesis & Oogenesis
Formation of sperm
Starts at puberty
Forms 4 sperm during each meiosis
Men will make 5 to 200 million sperm per day!!
Oogenesis
Formation of the egg
Meiosis starts inside the womb and continues in some during every cycle after puberty
1 egg and 3 polar bodies are created after every meiosis
The egg must contain a lot of cytoplasm to support the developing embryo after fertilization
Mitosis/Meiosis Video

22. Genetics
Genetics is the study of traits and how they are passed from one generation to the next.
BrainPop
Greatest Discoveries

23. Gregor Mendel Austrian monk
Performed genetic experiments in the 1850’s and 1860’s
Considered the “Father of Genetics”
His work was performed with no knowledge of DNA, cells, or meiosis!

24. Mendel’s Experiments Worked with pea plants in the monastery gardens
Followed the inheritance patterns of seven different traits (Ex.: Green seed Vs. Yellow seed) in the plants

27. Creating the F1 Generation
For each trait:
Mendel used a true-breeding plant for each form of the trait for the parent (P) generation
Ex- True-breeding purple flower x true-breeding white flower
Cross-pollinated the plants to produce offspring
Created F1 generation which only displayed one form of the trait (hybrids or heterozygous)
Ex- all F1 plants were purple flowered

29. Conclusions
Pea plants were passing a chemical message from one generation to the next that was controlling the trait (Ex- flower color)
This is a gene (Ex- gene for flower color)
Genes are sections of DNA on chromosomes that code for a trait
Different forms of a trait are called alleles
There is a purple and a white allele for flower color

30. More Conclusions Principle of Dominance
One allele is dominant over the other
Dominant will always be displayed when present
Recessive is only seen when it is the only allele present

31. Creating the F2 Generation
For each trait
Mendel self-pollinated plants from the F1 generation
Ex- F1 purple flower is crossed with itself
Created the F2 generation which displayed both traits in a 3:1 ratio
For every 4 flowers, 3 were purple flowered and one was white flowered

33. Conclusions Each pea plant has two copies of every gene
Each individual has three possible types of combinations
Two dominant alleles- homozygous dominant (___)
Two recessive alleles- homozygous recessive (___)
One of each- heterozygous (___)

34. More Conclusions Principle of Segregation
The copy to be put in the gamete is chosen at random
This happens during Anaphase I when the tetrads (____________ _______________) separate

35. Tetrad Separation (Segregation)

36. Predicting Inheritance Outcomes
Probability-
Punnett squares- tool used in genetics to figure out the probability of a genetic cross
Monohybrid cross- Punnett square showing the outcome of the
Dihybrid cross- Punnett square showing the outcome of the

37. Information About Traits
Physical form of the trait seen is the _____________ (show either dominant or recessive)
Genotype is the ________ that an individual has for a trait (2 alleles/trait)
Represented by letters (capital for dominant, lower-case for recessive)
Letter is chosen based on dominant allele
Possibilities (using flower color as example)
Homozygous dominant ___
Heterozygous ___
Homozygous recessive ___
Heredity

38. Setting Up a Punnett Square
One parent’s possible _______ go on the top
Other parent’s possible gametes go on the side
Squares are filled in with the column and row header

39. Mendel’s Dihybrid Experiment
Ex- True-breeding round and yellow peas (RRYY) x True- breeding wrinkled and green peas (rryy)
F1 generation phenotype: all round and yellow ( )
F1 generation was self-pollinated to create F2
F2 generation showed all 4 possible phenotype combinations in a 9:3:3:1 ratio

41. Conclusions Law of Independent Assortment
Each gene segregates on its own
For example, a pea plant that inherited the dominant yellow pea color did not automatically inherit the round (dominant) pea shape.

43. Setting Up a Dihybrid Punnett Square
All possible allele combinations from one parent are placed along the top (4 total)
For example- an F1 round and yellow pea plant (RrYy) could produce RY, Ry, rY, and ry gametes
All possible allele combinations from the other parent are placed along the side (4 total)
Square are filled with the column and row headers (16 squares)
Letters from one trait go first, then the other
Capital letter for that trait are put in front

44. Dihybrid Punnett Square

45. Uses for Punnett Squares
Give all possible outcomes for a cross between two different parents
Predicts ______________ (not actual) ratios among the offspring

46. Revisiting Independent Assortment
Not all genes independently assort
Only happens with _____________________ ___________________________________
Genes on the ________________________________________ (where one goes the others go too)
For example, if
One homologous chromosome has alleles A, B, and c for three genes
The other homologous chromosome has alleles A, b, and C
Then the offspring cannot get A, B, and C or a, b, and c or any other combinations

47. Crossing Over Revisited
Crossing-over can change the combinations of linked genes
The further apart that two genes are on a chromosome, the more likely that they are to cross- over
Gene maps are maps of chromosomes that show the locations of genes and the distances between them