Mendel and Heredity What does segregation imply? This happens with your chromosomes We have 2 copies for each chromosome but can only give 1 copy to the gametes So the 2 copies you have separate or segregate when they move to the gametes

Mendel and Heredity Mendel’s conclusions: 1. Traits are inherited as units and are not blended or diluted over generations 2. Organisms inherit 2 copies of each gene, one from each parent 3. Organisms donate only 1 copy of each gene in their gametes.

Traits, Genes, and Alleles These genes have a locus, specific place, or address on the DNA. Alleles are the different forms of a gene that occurs at that locus You have 2 alleles for each gene (you got one from each parent) They are located on homologous chromosomes

Process of Meiosis OBJECTIVE: Understand how diploid cells undergo 2 cell divisions to produce haploid cells Meiosis produces 4 haploid cells from a diploid cell This is broken into Meiosis I and Meiosis II Each round has 4 phases (similar to mitosis) Its important to pay attention to how chromosome numbers are reduced Remember what homologous chromosomes and sister chromatids are…

Process of Meiosis Homologous: separate chromosomes, 1 from mom and 1 from dad (code for same trait) Meiosis I 1)Interphase: G 1, S, G 2 2) Prophase 1: nuclear membrane breaks down, centrioles move to opposite sides of cell, spindle fibers form. Chromosomes condense. Homologous chromosomes pair up Sister: duplicated chromosomes attached at the centromere (copies / identical)

Process of Meiosis 3) Metaphase 1: Chromosome pairs are randomly lined up at the center of the cell 4) Anaphase 1: Paired homologous chromosomes separate and move to opposite sides of the cell ( sister chromatids stay together ) Meaning which chromosomes get passed is random (I’ll explain) Total of 8,388,608 possible combinations of chromosomes (not including mutations) for parent’s gametes

Process of Meiosis 5a) Telophase 1: nuclear envelope reforms and spindles break down 5b) Cytokinesis 1: produces 2 diploid cells MEIOSIS II 1) Prophase 2: nuclear envelope breaks down, centrioles move to opposite sides of cell, and spindle fibers form No interphase! All of these steps are taking place in 2 cells

Process of Meiosis 3) Anaphase 2: sister chromatids are pulled apart and move to opposite sides of cell 4a) Telophase 2: nuclear envelopes form around each set of chromosomes (on opposite sides) and spindle break down. 4b) Cytokinesis 2: produces 4 haploid cells 2) Metaphase 2: spindle fibers align the 23 chromosomes at the center of the cell (sister chromatids still attached)

Process of Meiosis These haploid cells will become gametes following Gametogenesis – process that completes gamete formation Key Differences between Meiosis and Mitosis # of divisions # of cells produced

Process of Meiosis Males Females - sperm -smaller than egg -must swim to egg How? Whiplike flagellum Produces 4 viable haploid gametes Produces only 1 viable haploid gametes -egg -uneven divisions of cytoplasm produces 3 polar bodies. Why? Must nourish the fertilized egg and be easier to find

Meiosis and Genetic Variation Sexual Reproduction gives us unique combinations of genes Remember the possible number of chromosomal combinations? -due to (1) independent assortment (2) Random gamete fertilization 8,388,608 OBJECTIVE: Understand causes of genetic diversity (ex. crossing-over) So, you take that number for each parent and multiply them together

Meiosis and Genetic Variation This is what makes us all genetically unique!! This is where homologous chromosomes exchange segments during Prophase I Figure 6.20 (pg 190) Sometimes occurring many times on the same chromosomes Greater variation occurs during a process called crossing over That gives us over 70 trillion possible combinations of chromosomes

Meiosis and Genetic Variation How come it appears that some traits are inherited with other traits? The reasoning for some of these trait is due to their genetic linkage – tendency for genes to be inherited together Locus plays a role in this linkage. The closer genes are on the same chromosome the more likely they are to be “linked” Recombination – refers to the mixing of parental alleles