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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.

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

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 _______