Mendelian Genetics


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Describes the Punnett Square and Mendel's Laws

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Mendelian Genetics

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