Cell Cycle, Mitosis, and Meiosis

• Cell Cycle phases: Interphase (G1, S, G2) and Mitosis (M).
• Chromosomal DNA duplicates during the S-phase, creating sister chromatids.
• Mitosis phases: Prophase, Metaphase, Anaphase, Telophase, and Cytokinesis.
• In metaphase, chromosomes align at the cell's equator.
• During Anaphase and Telophase, sister chromatids move to opposite poles.
• Mitosis results in two genetically identical, diploid (2n) cells.
• Some haploid cells (but not gametes) can undergo mitosis.
• Karyotype refers to a cell's chromosomal complement (e.g., 23 pairs in humans).
• Diploid cells contain one set of chromosomes from each parent (maternal and paternal).

Sexual Life Cycles and Meiosis

• There are three main types of sexual life cycles with notable differences and similarities.
• Animals depend on sexual reproduction and lack a free-living haploid stage.
• Plants and some algae can have both free-living diploid and haploid stages.
• Most fungi and certain protists exhibit free-living diploid and haploid stages.

Overview of Meiosis

• Chromosomal DNA duplicates during the S phase, creating sister chromatids.
• Meiosis I reduces the chromosome number by half (2n to n).
• Meiosis II halves the DNA content, reverting to the amount before the S phase.
• Meiosis reduces the chromosome number by half so that chromosome numbers remain diploid after fertilization.
• Meiosis creates new genetic combinations through independent assortment and crossing over.
• Independent assortment occurs in Meiosis I, resulting from the random alignment of maternal and paternal homologous chromosomes during metaphase, leading to recombination at the "between chromosome" level.
• Crossing over also occurs in Meiosis I, involves the reciprocal exchange of DNA between non-sister chromatids (maternal and paternal), and chiasmata mark the physical chromosomal locations where crossing over happens. This is recombination at the "within chromosome" level.
• Random fertilization significantly increases the possible genetic combinations in diploid zygotes.
• Meiosis, along with mutation, generates genetic diversity in reproducing populations, providing natural selection with more variation to act upon compared to mitosis-only populations.

Mendelian Genetics

• Blending vs. Particulate Hypotheses of Heredity explores historical theories of inheritance.
• Mendel's Experiments used the crossing method with peas.
• True-breeding lines are homozygous for the gene(s) involved.
• Monohybrid crosses involve a single gene.
• P, F1, and F2 generations are tracked in the crosses.
• F1 and F2 results from Purple X White flower crosses support the Particulate hypothesis.
• Inheritance includes dominant and recessive alleles.
• Seven different traits follow Dominant/Recessive inheritance, resulting in a 3:1 phenotypic ratio in F2 progeny.

Mendel's Model

• Alternate versions of genes (alleles) cause variation in phenotypes.
• Each individual inherits one allele for each gene from each parent.
• Only one dominant allele is needed to display that phenotype.
• For each gene, the two alleles segregate into different gametes.
• Genetic nomenclature includes genotype, phenotype, and homozygous vs. heterozygous.
• Testcrosses determine the genotype (PP or Pp) of an individual with a known phenotype (wild-type) by crossing them with a homozygous recessive "tester."
• Dihybrid crosses involve two genes.
• Independent assortment means two genes are inherited independently.
• A cross between two heterozygotes (for both genes) yields a 9:3:3:1 phenotypic ratio.
• Genes are unlinked if they are on different chromosomes or far apart on the same chromosome.

Non-Mendelian Genetics

• Incomplete Dominance: F1 displays an intermediate phenotype (e.g., Snapdragon color).
• Co-dominance: Example, the MN Blood group system.
• Multiple alleles can exist at a single gene locus (e.g., ABO blood groups).
• Pleiotropy: When a single gene influences more than one unrelated trait (e.g., Marfan syndrome).
• Epistasis: When a phenotypic trait is affected by the interaction of two or more genes where the albino gene (c) is epistatic to coat color gene (B) in cattle and coat color in Labrador retrievers (Gene E epistatic to B) producing three coat colors (Black, Chocolate, Yellow).
• Polygenic Inheritance: A single trait is controlled by two or more genes, resulting in a continuum (quantitative) of traits (e.g., height and skin pigmentation in humans).
• Environmental influences affect phenotype (e.g., flower color in hydrangeas).
• Pedigree Analysis is used to determine the mode of inheritance for a trait (e.g., Widow's peak (dominant); PTC tasting (recessive)).

Chromosomes and Genetics

• Sex-linked traits include white-eyed recessive X-linked in Drosophila.
• Reciprocal crosses yield different results if it involves sex-linked genes.
• Examples of sex-linked traits in humans: Red/green color blindness, Duchenne Muscular Dystrophy.
• Pedigree analysis of sex-linked traits looks for the frequency of affected males.
• X Chromosome inactivation occurs randomly during embryogenesis (paternal vs. maternal) in mammalian females, best seen in a Calico cat (X-linked trait; calico only in females).

Sex Determination Systems

• XY system (mammals): the male is heterogametic.
• XO system (many insects): the female is XX, and the male is XO.
• ZW system (birds, some fish, insects): the female is heterogametic.
• Haplo-Diplo system (bees, ants): the female is diploid, and the male is haploid.
• Linked genes reside close enough on the same chromosome.