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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

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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”)

    • 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

  • 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…

  • 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

  • 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.

Why are there fewer white hamsters?(Which color is dominant? Which is recessive?)

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?

  • 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

  • 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

  • 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!

  • 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 _______

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