Genetics

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Genetics
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I. Terms to know

• Genetics-the study of heredity
• Traits- characteristics

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• Purebred-
• offspring produce traits which are the same
  generation after generation
• Sometimes called true breeding
• Can be the result of self-pollination in plants

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• Hybrid- produced by crossing parents with
  different traits

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• Genes- factors that control traits

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• Alleles- different forms of a gene

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• Dominant- alleles which show up (are expressed)
  when paired with another allele
• Recessive- alleles which are not expressed when
  paired with the dominant allele

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• Gametes- sex cells (egg or sperm)

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• F1 first generation
• F2 second generation
• P parental generation

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• Phenotype physical appearance
• Genotype genetic makeup (usually indicated by
  letters)

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• Homozygous both alleles are the same
• TT aa YY FF bb

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• Heterozygous one dominant allele and one
  recessive allele
• Gg Tt Yy Rr Ss Aa

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II. Mendels Experiments

• Law of Dominance- when a dominant and recessive
  allele are paired, the dominant trait shows up in
  the offspring

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• Example
• AA x aa
• Aa

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• Law of Segregation alleles separate during
  gamete formation
• A 31 ratio of dominant to recessive will show
  up when two heterozygotes are matedthis would
  not happen unless alleles separated!

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Law of Independent Assortment

• Genes for different traits can segregate
  independently during the formation of gametes.
• The inheritance of one trait does not affect the
  inheritance of another trait IF the traits are on
  separate chromosomes.

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The Dihybrid cross
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• The ratios in a dihybrid cross (9331) prove
  that independent assortment occurs.

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Meiosis

• The formation of gametes (sex cells)
• Chromosome number is reduced by one half

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Meiosis
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Stuff to know about meiosis

• Homologous chromosomes separate
• Cells go from diploid to haploid
• 2 divisions (meiosis I and meiosis II)
• Tetrads form
• Crossing over occurs in prophase I
• Crossing over increases genetic diversity

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Crossing Over
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Crossing Over

• Crossing over increases genetic variability (new
  combinations of genes)

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Non-disjunction
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Non-disjunction

• Results in gametes with too few or too many
  chromosomes
• Ex. Downs Syndrome or XXY
• If an entire set of chromosomes is present it is
  called polyploidy.

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Non-Mendelian Genetics

• Incomplete dominance neither allele is
  dominant there is a blending of traits in the
  offspring.
• Ex. Pink flowers from red and white
• Ex. Wavy hair from straight and curly

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Incomplete Dominance
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• Codominance both traits show up in the
  offspring.

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• Multiple alleles More than two alleles control
  a trait
• Ex. Blood types
• Ex. Some rabbit coloring

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Multiple alleles
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• Polygenic traits many genes control a trait.
• Ex. Height in humans
• Ex. Skin color in humans
• These show a wide range of phenotypes

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Sex-linked traits

• Traits that are controlled by genes located on
  the sex chromosomes (X and Y)
• Ex. hemophilia and colorblindness

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Colorblindness
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Pedigrees
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Karyotypes
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Karyotypes

• Show homologous chromosomes
• Humans have 46 (23 pairs)
• 1-22 are called autosomes
• 23 are the sex chromosomes XX or XY

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X-Inactivation

• Females have XX
• Only one X in each cell is active
• The other X is inactive and condenses to form a
  Barr body

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Linked Genes

• Are on the same chromosome
• The closer two genes are on a chromosome, the
  more likely they will be inherited together.

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Thomas Hunt Morgan

• Studied fruit flies (easy to keep and many
  generations are produced in a short time)

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Linkage Maps
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Linkage Maps

• If crossing over happens frequently, the alleles
  must be farther apart on the chromosome.
• Alleles close together are more likely to be
  inherited together

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Linkage Maps

• The frequency of crossing over makes it possible
  to determine where alleles are on a chromosome.
• This is called a linkage map or gene map

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The Human Genome Project

• The sequencing of all DNA on human chromosomes
• Completed in June of 2000