Meiosis, Plant Sex, and Mendelian Genetics

    I. Meiosis, the basis of sexual reproduction
        A. Sexual Reproduction
                1. two parents give rise to offspring; off spring have unique combinations of genes inherited from both parents
        B. Variation = inherited differences among individuals of the same species.
        C. Diploid = having two homologous sets of chromosomes = 2n
        D. Haploid = having one set of chromosomes = 1n
        E. Gametes = haploid reproductive sex cells.  Sex cells are 1n.  Sperm (male) and egg (female) are sex cells. (humans: 1n = 23)
        F. Zygote = fertilized egg (results from union of sperm and egg = fertilization)
        G. Homologous chromosomes = a pair of chromosomes. Chromosomes occur in pairs: one from mother and one from father.
                1. Sister chromatids = a chromosome and its exact copy which arose from DNA duplication.
                2. alleles = alternative forms of the same gene that occupy the same position on homologous chromosomes.  Example: Flower color
                    gene; alleles can be purple & white flowers

    II. What is Meiosis?
        A. Meiosis = cell division that produces haploid cells from diploid cells.  2n -->1n
                1. Meiosis occurs only in organisms that reproduce sexually.
                2. Gametes are the only cells in the body that are not produced by mitosis.  Gametes (sex cells) are produced by meiosis.
                3. Human gametes have 22 autosomes and 1 sex chromosome (either X or Y). So, 1n = 23
                4. Flowers are the reproductive structures in plants (pollen and egg cells)
                5. Meiosis compensates for the doubling of chromosome number that occurs at fertilization.
                6. All organisms that reproduce sexually have an alternation of generations = 1n generation alternates with 2n generation.

    III. The steps of meiosis
        A. Non-dividing phase or Interphase: G1, S (DNA replicated), G2.
        B. Dividing phase: involves two successive nuclear divisions:
                1. Meiosis I: homologous pair of chromosomes separate
                    · Prophase I, Metaphase I, Anaphase I, Telophase I.
                2. Meiosis II: Sister chromatids separate
                    · Prophase II, Metaphase II, Anaphase II, Telophase II.
                    · Finally, Cytokinesis occurs forming 4 haploid cells.  All 4 are 1n.
 
 

	Mitosis	Meiosis
Purpose	Produces somatic cells (cells for body, growth, & repair of body tissue)	Produces reproductive cells (gametes)
Process	Cell duplication  (2n--->2n)	Reduction division  (2n--->1n)
# of divisions	1 cell division	2 cell divisions
Product	2 daughter cells (2n)  (each genetically identical to the parent cell)	4 daughter cells (1n) (gametes). Each genetically nonidentical to the parent cell & to each other.

    IV. Mendelian Genetics
        A. Chromosomes carry hereditary information (genes)
        B. Modern genetics began in the 1860’s when Gregor Mendel, an Augustinian monk, discovered the fundamental principles of heredity.
            Mendel derived a set of correct assumptions about heredity that form the Theory of inheritance for all higher organisms.  He bred or
            crossed garden peas and developed two laws:
                1. Mendel’s law of segregation = allele pairs separate during gamete formation (meiosis), and the paired condition is restored by the
                    fusion of gametes at fertilization.
                2. Mendel’s law on independent assortment = each allele pair separates independently of other gene pairs during gamete formation.
        C. Definitions
                1. Homozygous = having identical alleles for the same gene for a particular trait.  Example: Let P represent purple flower color and p
                    represent white flower color.  So, homozygous for the gene flower color purple is PP or for white it is pp.
                2. Heterozygous = having two different alleles for the same gene for a particular trait.  Example: heterozygous for the gene flower color
                    purple: Pp.
                3. Dominant = an allele whose phenotype is expressed in heterozygotes. Example: Let P be dominant.  So, homozygous dominant: PP
                    and Heterozygous dominant: Pp
                4. Recessive = an allele whose phenotypic effect is masked in heterozygotes by the presence of a dominant allele. Example: Let p be
                    recessive.  So, homozygous recessive: = pp
                5. Genotype = genetic make-up. Example: Flower color genotypes: PP, pp, Pp
                6. Phenotype = appearance (genes expressed). Example: purple, white.
        D. Genetic crosses:
                1. Monohybrid Cross = a cross that tracks the inheritance of a single trait: ie. flower color.
                        a) Mendel usually followed heritable traits for at least three generations: True breeding parents = P generation (parental).  Their
                            hybrid offspring are the F1 generation (for first Filial, Latin for ‘offspring’).  Allowing each of the F1 hybrids to self-pollinate
                            or cross produces an F2 generation (second Filial).
                        c) 3:1 ratio occurs in the F2 generation.

                                    Illustrates Mendel's law of segregation:
                                    Example: flower color
                                    Purple flowers = PP and white flowers = pp
                                    P allele codes for purple color
                                    p allele codes white color
                                    If both parents homozygous then:
 

                                    P allele is dominant. So purple is dominant.
                                    p allele is recessive. So white is recessive.

                                    If F1 generation self pollinates or are crossed (crossing heterozygotes), then the F2 will be:

The box is called a Punnett Square: a useful tool for showing all possible combinations of alleles in offspring.

                2. Dihybrid cross = a mating of parents that are heterozygous for two traits.
                                    Illustrates Mendel's law of independent assortment
                                    Example: pea color and pea shape
                                    Round peas = R and Wrinkled peas = r
                                    Yellow peas = Y and green peas = y
                                    Parents: YYRR  and  yyrr
                                    F1 all will be YyRr
                                    F2  displays a 9:3:3:1 phenotypic ratio
                                                    9 will be yellow and round
                                                    3 will be yellow and wrinkled
                                                    3 will be round and green
                                                    1 will be wrinkled and green