Meiosis & mendelian genetics– chapter Freshman Biology; Semester Two

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

BEFORE chromosome replication AFTER chromosome replication

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

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

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)

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)

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

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

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)

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!

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

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

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

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?

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

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!

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

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

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!

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

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

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

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

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

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 (___)

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

Tetrad Separation (Segregation)

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

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

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

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

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.

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

Dihybrid Punnett Square

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

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

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