Mendel and Meiosis

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Mendel and Meiosis
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Mendels Laws of Heredity

• Heredity passing on traits from parents to
  offspring
• Gametes sex cells they have a haploid number
  of chromosomes they have only ½ as many
  chromosomes as normal cells or one copy of each
  gene

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• Fertilization when a male gamete unites with a
  female gamete
• Zygote fertilized cell
• Gene - chemical factors that determine traits
• Alleles different forms of a gene

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Mendels experiments

• Pea plants are self-pollinating male and female
  parts in the same flower
• Only studied one trait at a time and subjected
  his data to statistical analysis
• First cross was between two purebreds or true
  breeding plants (tall x short)
• All offspring were tall

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Mendels experiments (cont)

• The result was a hybrid with one allele for tall
  and one for short
• Then, Mendel allowed the hybrids to
  self-pollinate (hybrid x hydbrid)
• Second generation offspring were ¾ tall and ¼
  short

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Rule of Unit Factors

• each organism has two factors/alleles for each
  trait the two alleles are located on different
  copies of a chromosome, one from each parent

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Rule of Dominance

• in a hybrid where only one trait can be observed,
  that trait is said to be dominant over the trait
  that is not visible the other trait is said to
  be recessive

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Law of Segregation

• In sexually reproducing organisms, offspring have
  one gene from each parent, these are segregated
  in gamete (sex cell) formation

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Law of Independent Assortment

• genes for different traits are inherited
  independently of each other

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Phenotypes and Genotypes

• Phenotype physical appearance of a trait in an
  organism
• Genotype gene/allele combination of an organism
• Homozygous organism that has two of the same
  alleles for each trait
• Heterozygous organism that has two different
  alleles for each trait

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Crosses

• Monohybrid cross Mendels first crosses
  studying one trait and its inheritance at a time
• Dihybrid cross studies the relationship between
  the inheritance of 2 traits

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Punnett Squares

• Monohybrid cross (one trait) is a two by two grid
• One parents genotype is listed at the top one
  allele per column
• The other parents genotype is listed along the
  side one allele per row
• Squares are filled in with one allele from one
  parent (top) and one from the other (side)

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• Dihybrid cross (two traits) is a four by four
  grid
• Very similar to the monohybrid cross, but you
  must account for all possible gene combinations
  from parents

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probability

• chance that you will get one result over another
• Punnett squares are a way to determine
  probability
• the more data you have, the closer to the
  predicted probability your results should be

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Meiosis
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Genes and chromosomes

• Diploid 2n - cell/organism that contains two of
  each type of chromosome (one copy from each
  parent)
• Haploid n - cell (sex cell) that contains only
  one copy of each type of chromosome

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• Homologous chromosomes two chromosomes of the
  same type or a pair of similar chromosomes in a
  diploid cell
• Meiosis process of cell division that reduces
  the number of chromosomes in a cell from diploid
  to haploid used for reproduction
• sperm male gametes
• egg female gametes

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Meiosis I similar to mitosis

• EXCEPT
• In Prophase I, homologous chromosomes come
  together as tetrads
• during metaphase I, tetrads line up along the
  center of the spindle
• the tetrad is pulled apart during anaphase I, so
  the homologs are now separate from each other in
  two different cells

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• when tetrad are formed, crossing over may happen
  this basically means that homologous chromosomes
  could trade ends

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Meiosis II

• no chromosome replication before meiosis II
• chromosomes line up along center of spindle in
  metaphase II
• sister chromatids are pulled apart in anaphase II
• result of both divisions together is 4 haploid
  cells

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Genetic Recombination

• Crossing over just refers to trading ends
  between homologous chromosomes during the tetrad
  grouping in prophase I and metaphase I
• Also caused by the law of independent assortment
  different chromosomes will be segregated
  independently of one another

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Nondisjunction

• when chromosomes fail to separate properly during
  meiosis
• Monosomy lacking a chromosome
• Trisomy extra chromosome
• Tetrasomy 2 extra chromosomes

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Polyploidy

• Organisms have more than 1 set of chromosomes
• Rare in animals usually results in death or, in
  humans, mental retardation
• Plants have a higher occurance rate can create a
  better or healthier plant, commercially or
  biologically

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Gene Linkage and Maps

• Genes on the same chromosome are said to be
  linked
• If they are far apart on the chromosome, crossing
  over is more frequent
• genes maps are charts of chromosomes with gene
  locations on them (ex. Human Genome Project)