Sexual Life Cycles

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• Sexual Life Cycles
• Organisms with sexual life cycles typically
  contain two copies of each chromosome
• Two basic cell types
• Somatic cells
• Diploid (2n) Two complete sets of chromosomes
• Chromosome set includes n-1 autosomes 1 sex
  chromosome
• Pairs of autosomes homologous pairs (same loci)
• Females homologous sex chromosomes
• Gametes
• Haploid (n)
• Sperm cells, ova
• Produced by meiosis
• Unite in fertilization to produce diploid zygote
• Sexually reproducing organisms alternate between
  diploid haploid stages

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Fig. 13.5
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Fig. 13.6
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• Meiosis
• Similar to mitosis, except DNA replicated once
  before cells divide twice
• Divisions Meiosis I, Meiosis II

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Fig. 13.7
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Long, Complex Phase gt90 of meiosis (days) -
Synapsis - Synaptonemal complex
Fig. 13.8
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Fig. 13.8
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Fig. 13.9
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Fig. 13.9
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Fig. 13.11
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Fig. 13.9
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Fig. 13.12
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• Principles of Inheritance
• Historical Background
• First person to study inheritance quantitatively
  was Gregor Mendel (1822-1884)
• Selective breeding common
• Known that
• Hybrid plants with same two parents are similar
• Mating of hybrid offspring produces diverse
  progeny (primary hybrids dont breed true)

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• Principles of Inheritance
• Mendels Experiments
• Used garden pea (Pisum sativum)
• Easy to grow
• Many varieties readily available
• Flowers could be pollinated in a controlled manner

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Fig. 14.2
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• Principles of Inheritance
• Mendels Experiments
• Before experiments, Mendel developed
  true-breeding lines with consistent phenotypes
• Used characteristics that could be studied easily
• Strengths of approach
• Clearly-defined traits
• Simple experiments

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• Principles of Inheritance
• Mendels Experiments
• Prevailing thought Traits controlled by fluids
  that blended together in offspring
• Mendel crossed parent plants (P generation) with
  different traits
• Produced offspring resembling only one parent in
  first generation (F1 generation)
• Crossed two F1 hybrids
• Produced offspring (F2 generation) resembling
  both grandparents

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Fig. 14.3
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• Principles of Inheritance
• Mendels Experiments
• Prevailing thought Traits controlled by fluids
  that blended together in offspring
• Mendel crossed parent plants (P generation) with
  different traits
• Produced offspring resembling only one parent in
  first generation (F1 generation)
• Crossed two F1 hybrids
• Produced offspring (F2 generation) resembling
  both grandparents
• Results suggested existence of heredity factors
  (genes) with different forms (alleles)
• Dominant trait masked recessive trait in F1
  generation

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Fig. 14.4
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• Principles of Inheritance
• Mendels Model
• Developed hypothesis to explain inheritance
  patterns
• Alternative versions of genes account for
  variations in inherited characters
• For each character, an organism inherits two
  alleles, one from each parent
• If two alleles at a locus differ, dominant allele
  determines phenotype recessive allele doesnt
  affect phenotype
• Law of segregation Alleles separate during
  gamete formation one allele in each gamete
  (mechanism?)
• Reported findings in 1866 rediscovered in 1900