Mendelian Genetics

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Mendelian Genetics
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Gregor Mendel
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Mendel made several wise decisions

• a. choice of peas good easy to grow and crossb.
  peas were cultivated and bred for a long time
  lots of variations (mutations) to work with c.
  peas are self pollinating and easy to pollinate
  by hand
• d. used true breeding plants, i.e. ones that
  showed a constant trait when self-pollinatede.
  Mendel followed the inheritance of each trait
  individuallyf. Mendel used statistics
  (probability) to interpret his results

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• A cross involving pea plants with green vs.
  yellow pods
• Green X Yellow ---gt all Green Parental
  F1 generationGreen X Green (self) -----gt 3
  Green for
• every
  1 Yellow F1 generation F2
  generation
• Mendel got a 31 ratio by counting the number of
  plants that were Green (428) vs. Yellow (152).
  428/152 2.811 or essentially 31.

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• Factor Gene
• Dominant
• Recessive

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• Mendel used the terms dominant and recessive for
  the behavior of traits that stayed constant
  through crossing or dissappeared and reappeared
  in the F2 generation.
• Mendel's results contradicted the popular theory
  of the day that traits blended
• Mendel's Law of Segregation of gene "Each
  organism contains two factors for each trait
  which separate during gamete formation so that
  each gamete contains only one factor. When
  fertilization occurs, the new organism will have
  two factors for each trait, one from each
  parent". Remember, Mendel knew virtually nothing
  about cytology (chromosomes) and meiosis!! We now
  know his factors represent genes that occur on
  homologous chromosomes that segregate during
  meiosis I.

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• Abbreviations
• P parental generation,
• F1 first filial generation,
• F2 second filial generation
• Allele A, a
• Locus
• Dominant alleles
• Recessive alleles
• Homozygous AA, aa
• Heterozygous At

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• Mendel's Crossing Experiments
• A. Monohybrid cross follow the results of a
  cross between two parents that differ for a
  true-breeding trait.

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Modern Genetic Terminology to Explain Mendel's
Results

• A. Genes and homologous chromosomes
• 1. As we have learned, diploid organisms (like
  peas) have two sets of chromosomes (onefrom the
  male, one from the female) and these are called
  homologous chromosomes

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• 2. At equivalent positions on these chromosomes
  exist genes. The location of these "gene pairs"
  is called the locus.
• 3. Alternate forms of the gene (ones that differ
  by mutations in the DNA) are called alleles.
• 4. Dominant traits we now know are the same as
  dominant alleles, recessive traits are the same
  as recessive alleles. Dominant usually capital
  letter (here G, R, S, Z) and recessive lower case
  letters (here g, r, s, z)

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• B. True-breeding traits exist when the organism
  has two dominant alleles (e.g. TT) or two
  recessive alleles (tt). These organisms are
  termed homozygous for that gene or those alleles.
• C. The F1 individuals above that resulted from a
  cross between tall and short pea plants have both
  a dominant (T) allele and a recessive (t) allele.
  These organisms are termed heterozygous for that
  gene or those alleles.
• D. Phenotype the outward appearance of the
  organisms (i.e. the physical trait that we see,
  e.g. tallness). Genotype the allelic complement
  of the organism, often represented by letters,
  e.g. the genotype TT is called the homozygous
  dominant condition, tt is the homozygous
  recessive, and Tt is the heterozygous condition

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• Law of independent assortment

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• Punnett square
• Y(RY) Y(rY)
• R r
• y (Ry) y (ry)
• 1/4(RY), 1/4(Ry), 1/4(rY), 1/4(ry)

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• Probability

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Multiple alleles

• b black ( mutant) recessive
• b brown (wild type) dominant
• B bar eye (mutant) dominant
• B normal eye (wild type) recessive

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Agouti (brown)
Chinchilla (light brown)

Himalayan (white)
Albino (all white)
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• Ag x C Ag x H Ag x Al
• F1 Ag Ag Ag
• F2 Ag 3 Ag 3 Ag 3
• C 1 H 1 Al 1

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C x H C x Al H x Al F1
C C H F2 C 3
C 3 H 3 H 1
Al 1 Al 1
CgtCchgtChgtC
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Phenotype Genotype
Agouti Chinchilla Himalayan albino C C, CCch, CCh, CC CchCch, Cch Ch, CchC Ch Ch, ChC CC
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Blood type Antigen Antibody
A B AB O A B A and B - Anti-B Anti-A - Anti-A and B
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SEX-LINKAGE
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