genetic theory

• Gregor Mendel worked with

  pea plants
• Gregor Mendel discovered

  • dominant and recessive traits
  • concept of gene (heritable factor)
  • formulation of law of inheritance
• a character is a heritable feature of an individual
• a trait is a variant form of a character (the phenotype)
• law of segregation 


  2 forms of a gene (alleles) present in each parent segregate independently
• law of segregation formulated by

  monohybrid crosses
• monohybrid cross is 


  cross between 2 true breeding individuals differing in only one character
• P generation is the true breeding parents
• F1 generation are the first generation of hybrids
• Mendel's observation


  f1 progeny resembled one of the parents
• Mendel's observation

  • f1 progeny resembled one of the parents
  • f2 generation have both P generation traits
  • ratio in f2 was 3:1
• heritable factor for the recessive trait had not been lost in the f1 generation, but masked by presence of factor for dominant trait
• Mendel's model:

  • variation in inherited characteristics are die to the existence of alternative versions of genes (alleles)
  • organism inherits 2 alleles for each character
  • dominant allele always presents itself in the phenotype
  • alleles remain discrete
  • 2 alleles segregate (separate) during gamete formation
• how to do a testcross?

  Crossing an individual with a homozygous recessive individual

  will determine the genotype of an individual with the dominant phenotype
• Law of independent assortment


  each pair of alleles assorts independently of each other pair of alleles during gamete formation
• how did the law of independent assortment occur?


  following the inheritance of 2 characters at the same time

  involves a dihybrid cross
• results of f2 generation in dihybrid cross


  2 new phenotypes (recombinants)

  9:3:3:1 ratio
• meiosis occurs in germ line
• mitosis occurs in somatic cells
• role of meiosis


  produce haploid gametes and introduce genetic variability
• homologous pair are individual chromosomes that were inherited from different parents
• homologous pair=homologues
• homologous pair have different versions of genes at some of their loci
• chromatid is 1 of 2 identical strands of a newly replicated chromosome
• chromatid is different to pair of homologous chromosomes
• sister chromatids are the 2 identical chromatids held together by _ following replication

  centromere
• chromosome terminology

  labels:

  A) homologous pair

  B) sister chromatids

  C) centromere

  
• Sutton observed:

  • chromosomes occur in pairs in somatic cells
  • chromosome pairs segregate equally into gametes
  • different chromosome pairs assort independently
• chromosome theory of inheritance

  • genes are located at specific positions (AKA loci) on chromosomes
  • chromosomes undergo segregation and independent assortment
• genes are units of heredity and determine traits
• law of segregation explained by 


  each allele on different member of a homologous pair of chromosomes and moves to opposite poles in anaphase I
• independent assortment of chromosomes means that there are 4 possible combinations of alleles at 2 loci
• law of independent assortment happens because of 


  the arrangement of chromosomes at metaphase I is random
• heterozygote is a cell/organism carrying 2 different alleles at a genetic locus
• sources of genetic variability in sexual life cycles:

  • mutation
  • independent assortment
  • crossing over
  • random fertilisation
• recombinant phenotype -> phenotype differs from either parent
• recombinant phenotypes occur from

  independent assortment (for genes on different chromosomes) and crossing over (genes on the same chromosome)
• when does crossing over occur?

  during pachytene phase of prophase I
• how does crossing over happen?

  synapsis (pairing of homologous chromosomes)

  synaptonemal complex (zipper) holding homologous chromosomes together in tetrad
• chiasma is the point which chromosomes make contact during crossing over