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Warm up: Definitions. Dominant – trait expressed, Capital letter (ex “B”) Recessive – trait masked, lowercase letter (ex “b”) Heterozygous – 1 dominant allele, 1 recessive allele, called Hybrids or Carriers (ex “Bb”)

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warm up definitions
Warm up: Definitions
  • Dominant – trait expressed, Capital letter (ex “B”)
  • Recessive – trait masked, lowercase letter (ex “b”)
  • Heterozygous – 1 dominant allele, 1 recessive allele, called Hybrids or Carriers (ex “Bb”)
    • Hybrids – offspring that are the result of two genetically different parents
  • Homozygous – 2 identical alleles, called true breeding (ex “BB” or “bb”)
    • Purebreds – offspring that are the result of two genetically similar parents
  • Phenotype – physical characteristic or trait (ex tall)
  • Genotype – genetic makeup of the chromosome itself.

(BB, Bb, or bb)

  • Test Cross - The crossing of

an organism, with an unknown

genotype, to a homozygous

recessive organism (tester).

mendel and genetics
Mendel and Genetics
  • Genetics – study of heredity.
  • Heredity – set of characteristics passed from parent to child.
  • Gregor Mendel – Austrian monk (1822), high school teacher and gardener.
  • Bred pea plants and observed their patterns of inheritance.
  • “Father of Genetics”


what mendel did
What Mendel did…
  • Took plants true breeding (if self-pollinated, would make identical offspring) and prevented self-pollination.
  • Crossed parent plants (P generation) with different characteristics to look at the offspring (F1 generation)
    • Seed shape, seed color, seed coat color, pod shape, pod color, flower position, and plant height.
  • These offspring were called hybrids.
what mendel found out 4 conclusions
What Mendel found out… 4 conclusions
  • 1st conclusion: Biological inheritance is determined by factors passed from 1 generation to the next (genes) and had contrasting forms (alleles).

The Principle of Unit Characters states that individuals pass information on as individual traits.

  • 2nd Conclusion: Principle of Dominance – some alleles are dominant and others are recessive. An organism with a dominant allele for a particular form of a trait will always exhibit that form of the trait.
law of segregation
Law of Segregation
  • Mendel wondered: “Where did the recessive alleles go?”
  • Crossed the F1 plants with each other and looked at their offspring.
  • F2 generation showed ¼ of the plants had recessive alleles.
  • States that each unit character separates into a different sex cell.
remember meiosis
Remember Meiosis?
  • In Meiosis, alleles separate into eggs and sperm. (23)
  • Alleles: Versions of a particular trait.
  • 3rd Conclusion: Law of Segregation- When flowers produce gametes, the two alleles segregate from each other so that each gamete carries only 1 copy of each gene.
  • The reunite in offspring in homologous chromosomes. (46)
  • HC: Chromosomes that contain the same genes for the same traits.
    • One from mother (23)
    • One from father (23)
genetics is probability
Genetics is Probability
  • Alleles segregate at random.
  • 4th Conclusion: The Principle of Independent Assortment - states that genes segregate according to chance.
  • Gene combinations that might result from a genetic cross can be determined by a Punnett Square.
  • PS: Predict and compare the genetic variation that result from a genetic cross.
    • One trait: Monohybrid cross
    • Two traits: Dihybrid Cross
    • Three traits: Trihybrid cross
test crosses
Test Crosses
  • With homozygous recessive phenotypes, you can easily determine the genotype (e.g. both small letters like bb or zz)
  • However, the phenotype for Homozygous Dominant (BB and ZZ) and Heterozygous Dominant (Bb and Zz) are the same.
  • How do you determine this genotype?
  • A test cross is performed when you cross an unknown dominant phenotype with a known recessive phenotype and study the offspring.
    • If all offspring show the dominant phenotype, then it is homozygous dominant genotype.
    • If any (just one!) of the offspring show the dominant phenotype, then it is heterozygous dominant genotype.
your turn
Your Turn!
  • In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have spherical seeds? ___________
  • To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______