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Mendelian Genetics

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Describes the Punnett Square and Mendel's Laws

Describes the Punnett Square and Mendel's Laws

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  • 1. Mechanisms of Evolution: Mendelian Genetics Anthropology 101 Hybrid Course
  • 2. Mendelian Genetics
    • The appearance of an individual is derived from the cell divisions we just described
    • Traits are maintained in the organism through mitosis
    • They are transmitted from parent to offspring through meiosis
    • When haploid cells are united between couples to become diploid
    • Each parent contributes exactly half their makeup to their offspring
  • 3. Individual Genetics; :Concepts
    • Traits are inherited through the chromosomes and their constituent genes
    • The genetic composition of a trait is known as a genotype
    • A phenotype is a trait of a genotype that is visible or otherwise observable and can be measured
    • Homozygous traits are those with two identical alleles in a gene pair
    • Heterozygous traits are those with two different alleles in a gene pair
  • 4. Individual Genetics: When Genotypes Become Phenotypes
    • A dominant allele is one whose trait appears in both homozygous heterozygous combination
    • A recessive allele is one whose trait appears only in homozygous combination.
    • A codominant allele is one whose trait reflects the genetic combination of two different alleles
    • Punnett squares illustrate how these principles work
  • 5. Case Study of Monogenic Trait: Tasters vs. Nontasters
    • Most of us can taste the bitterness of Brussels sprouts
    • This is the ability to taste phenylthiocarbamide (PTC)
    • Tasters are dominant; nontasters are recessive
    • A Punnett Square allows us to determine the proportion of tasters vs. nontasters of PTC
    • This is a table that gives us a visual count of the allele for each trait.
  • 6. PTC Tasters and Nontasters: Generation I
    • Suppose one parent is a taster and the other is a nontaster in the first generation
    • All the offspring (Generation II) will be tasters in the second generation, as shown in the next panel.
    • To simplify, we use only a 2 X 2 table
  • 7. Punnett Square of Tasters and Non-Tasters: Generation II
    • t (nontaster) t (nontaster)
    • T (taster) Tt Tt
    • T (taster) Tt Tt
  • 8. PHC Tasters and Nontasters: Generation II
    • The second generation generate a new combination of phenotypes
    • The proportion is now 1 homozygote for PTC tasters, 1 homozygote for nontasters, and 2 heterozygotes for tasters/nontasters.
  • 9. Punnett Square of Tasters and Non-Tasters: Generation III
    • T (tasters t (nontasters)
    • T TT Tt
    • t Tt tt
  • 10. Codominant Genes
    • Some alleles are codominant: one trait does not trump the other
    • A species of flower, four-o’clocks, may come in red and white
    • Their hybrids thus come in pink in Generation II
    • In the Punnett Square, in Generation III, the proportion is 1:2:1 (one red, two pinks, and one white)
  • 11. Mendel’s Laws: Law of Segregation
    • Mendel found that there were three principles of inheritance resulting from the study of pea plants for seven characteristics.
    • In so doing, he found that traits of the parent generation do not blend in those of their offspring.
    • Rather, one gene for each trait is segregated from other genes for other traits.
    • Segregation : Separation of alleles in the formation of gametes (sex cells)
  • 12. Mendel’s Law: Law of Independent Assortment
    • Independent Assortment: differing traits are inherited independently of each other (genes on separate chromosomes)
    • For example whether a pea plant flower is violet or white is separated from smooth or wrinkled peas
    • In other words, a white flowering plant can yield either a wrinkled or a smooth pea; so can a violet flowering plant
    • So flower color is independent from smoothness of peas
    • The PHC assortments in the Punnett Squares are another example of this law.
  • 13. Mendel’s Laws: Law of Recombination
    • Though independent, genes can recombined to allow further genetic variety
    • In meiosis, some genes cross over, allowing even further variety.
  • 14. Linkages
    • Nevertheless, if alleles occur on the same chromosome, they will be inherited together.
    • Sex-linked traits are one example: secondary characteristic are linked to the primary characteristics (organs of reproduction
  • 15. Conclusion
    • Genes are ultimately responsible for our traits
    • Cells are maintained by cell division called mitosis
    • Our traits are passed down from one generation to the next by meiosis
    • Through meiosis, we can predict what traits will be passed down and in what proportion.