Punnett squares


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Punnett squares

  1. 1. Punnett Squares<br />
  2. 2. Gregor Mendel<br />Austrian Monk<br />Famous for his work with pea plants<br />He is known as the father of genetics<br />
  3. 3. Mendel’s Work<br />Mendel used true-breeding plants which means if they were left to breed with themselves they would produce offspring identical to themselves.<br />Mendel studied 7 different traits in pea plants.<br />
  4. 4. The Principle of Dominance<br />The principle of dominance states that some alleles are dominant and others are recessive.<br />Dominant alleles are always expressed.<br />Recessive alleles are only expressed if both alleles are recessive.<br />
  5. 5. Segregation & Independent Assortment<br />The separation of alleles during meiosis to form gamates (sex cells).<br />The Law of Independent Assortment states that genes for different traits can segregate independently during the formation of gamates.<br />
  6. 6. Probability & Genetics<br />Probability is the likelihood that an event will happen.<br />The principle of probability can be used to predict the outcomes of genetic crosses.<br />
  7. 7. Punnett Squares<br />Diagram used to predict genetic crosses.<br />
  8. 8. How to use Punnett Squares<br />Choose a letter to represent the alleles in the cross.<br />Write the genotypes of the parents.<br />Determine the possible gametes (reproductive cells) that the parent can produce.<br />Enter the possible gamete at the top and side of the Punnett square.<br />
  9. 9. More Punnett Squares<br />Complete the Punnett square by writing the alleles from the gametes in the appropriate boxes.<br />Determine the phenotypes of the offspring.<br />Using the results of step E and F write down the genotypic and phenotypic ratios.<br />
  10. 10. Punnett Squares<br />
  11. 11. Mendel’s Principles<br />The inheritance of biological characteristics are determined by genes.<br />For two or more forms of a gene, dominance and recessive forms may exist.<br />Most sexually reproductive organisms have two sets of genes that separate during gamete formation.<br />Alleles segregate independently.<br />
  12. 12. Punnett Squares Monohybrid Crosses<br />Ratios and percentages<br />Genotype<br />1:2:1<br />25%: 50%: 25%<br />Phenotype<br />3:1<br />75%:25%<br />
  13. 13. Punnett Squares Dihybrid Crosses<br />Ratios<br />Genotype<br />1:2:2:1:4:2:1:2:1<br />Phenotye<br />9:3:3:1<br />
  14. 14. BeyondDominantand RecessiveNon-mendelian genetics<br />Incomplete Dominance – One allele is not completely dominant over the other. White flower crosses with a red = pink flower.<br />
  15. 15. Incomplete dominance<br />With incomplete dominance, a cross between organisms with two different phenotypes produces offspring with a third phenotype that is a blending of the parental traits. <br />It's like mixing paints, red + white will make pink.  Red doesn't totally block (dominate) the pink, instead there is incomplete dominance, and we end up with something in-between.<br />
  16. 16. We can still use a Punnett square<br />You just have to remember that the heterozygous genotype produces the “blended” phenotype.<br />
  17. 17. Codominance – Both alleles contribute to the phenotype. Red cow X White Cow = Roan Cow.<br />
  18. 18. Codominance <br />Similar to incomplete dominance because a hybrid organism shows a third phenotype --- not the usual "dominant" one & not the "recessive" one ... but a third, different phenotype.  <br />BUT instead of a blending of the dominant & recessive traits BOTH traits appear together<br />
  19. 19. You can still use Punnett squares<br />There are a couple of different ways of representing the genes – it is not really important which one you choose.<br />
  20. 20. Sex-linked - genes that occur on the X or Y chromosome. (there are not as many genes on the Y chromosome)<br />
  21. 21. Multiple Alleles – More than two alleles control the phenotype. Blood type.<br />
  22. 22. Polygenic traits – Several genes control the trait. Skin color in humans.<br />