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Pre-Mendel: sperm and egg contain essences that mix upon conception(pangenesis) to form a new pattern Peas as model organism: cheap, easy to grow, self-fertilizing, can be cross pollinated Mendel’s Success: used true-breeding strains, used contrasting characteristics, quantified his results Conclusions: male and female parents contribute equally(reciprocal crosses gave same results) (no blending) Dominant vs. Recessive Traits - dominant displayed in F1, recessive trait only in F Particulate inheritance - traits are determined by discrete units inherited thru generations - 2 particles in genotype, but only one is passed on Steps for Determining results of a hybrid cross.
Chromosome - physical structure that carries genes - double-stranded DNA, information arranged linearly Eukaryotes: contains DNA, RNA and proteins. Dispersed/chromatin for most of the cell cycle - bundles in division Eukaryotic chromosomes have a distinct shape and size, 1 centromere and 2 telomeres Distinguished by size and location of centromere relative to telomeres (acro, meta, submeta, telo - centric) Chromosome # per cell differs among species, but most eukaryotes have 2 copies per somatic cell (2N/diploid #) 2 copies of each chromosome in a diploid = homologous (same length, same centromere, same gene placement) Must be copied and separated equally during each stage of division Mitosis : 1 cell -> 2 cells that are genetically identical. Creates somatic cells Meiosis 1 cell -> 4 variable daughter cells. ½ number of chromosomes in somatic cells. Creates gametes The Cell Cycle - stages that 1 cell goes through as it grows and divides G 0 - non-dividing phase - cell is stable and at constant size G 1 - 1st gap or growth phase - proteins needed for cell division are synthesized S - Synthesis - all chromosomes are duplicated G 2 - 2nd gap or growth phase - damaged DNA must be repaired Mitosis - cell division Phases of Mitosis : Prophase: chromosomes condense, mitotic spindles form, nuclear envelope breaks down Metaphase: microtubules from spindle pole attach to each chromosome at the centromere
Meiosis : gamete formation. 4 haploids from 1 diploid cell. 2 divisions (meiosis 1 and 2, reduction and equational) Cell goes through s phase before meiosis Phases of Meiosis Early Prophase 1: chromosomes condense, homologs pair/synapse, crossing over may happen at chiasmata, 5 Stages of Propahse 1: describe appearance of chromosomes as they condense, pair and recombine
Studying heredity in humans is challenging:
Lecture 6 - Non - Mendelian Ratios Causes involving multiple loci/environment - environmental modification/gene interaction Causes involving sex-determination - sex linkage, sex limited traits, sex influenced traits Environmental modification - genotype alone does not determine phenotype, environmental conditions can affect it