7th grade life science genetics and probability


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Using Punnet Squares to predict genetic crosses

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7th grade life science genetics and probability

  1. 1. Genetics Predicting the probable offspring of selected parents
  2. 2. Mendel’s Laws <ul><li>We can use the laws Gregor Mendel developed from his observations of pea plant heredity to predict the offspring of selected parents. </li></ul><ul><li>Mendel’s laws are the direct result of probability. Remember that he used statistical analysis to come up with his ideas. </li></ul>
  3. 3. Probability <ul><li>Probability is the likelihood or the chance that something will happen. If we flip a coin five times, the result may be all heads or all tails. If we flip the coin a hundred times it is more likely to be close to 50 heads and 50 tails. </li></ul><ul><li>How many pea plants do you think Mendel would have to grow to show the 3:1 ratio of the F 2 generation? </li></ul>
  4. 4. Mendel’s Laws <ul><li>There are two hereditary factors (genes) for each characteristic. </li></ul><ul><li>Each parent gives one factor (gene) for each characteristic to each offspring. </li></ul><ul><li>The Law of Segregation: One allele for each trait is passed down to the offspring by each parent. </li></ul><ul><li>The Law of Independent Assortment: Factors (genes) for different characteristics are inherited independently of each other. </li></ul><ul><li>Some factors (genes) are dominant and hide the expression of the recessive form ( allele) </li></ul>
  5. 5. Mendel’s Experiments <ul><li>We have already described in detail how Mendel conducted his experiments in the garden of the Monastery using pea plants. </li></ul><ul><li>How can we apply his laws and the laws of probability to explain his work and the inheritance of other characteristics in other organisms? </li></ul><ul><li>We need some more definitions! </li></ul>
  6. 6. What is a Pure Bred Plant? Pure Bred Plants: consistently have offspring with the same trait as the parent.
  7. 7. Terms we need to further our discussion <ul><li>Alleles: The different forms of a gene </li></ul><ul><li>Dominant Allele: The form that always shows in the organism when it is present </li></ul><ul><li>Recessive Allele: The form that is hidden or masked by the dominant form when it is present. </li></ul><ul><li>Independence of Events: Each event is a separate entity to which the laws of probability apply. </li></ul>
  8. 8. More Terms <ul><li>Punnett Square : A device for predicting the probable offspring of particular parents. </li></ul><ul><li>Gametes: Sex cells that contain the genes the parents are providing for the offspring. </li></ul><ul><li>Cross: The mating of two parents </li></ul><ul><li>Homozygous: Having two alleles with the same expression. </li></ul><ul><li>Heterozygous: Having two alleles with different expressions. (hybrid) </li></ul>
  9. 9. Punnett square <ul><li>A Punnett square is used to show the possible combinations of gametes . </li></ul><ul><li>For a single trait we need to draw 4 boxes to show the probable results for the offspring of two parents. </li></ul>
  10. 10. How can we represent the alleles in a particular cross? <ul><li>A capital (upper case) letter is selected to represent the dominant allele. </li></ul><ul><li>The corresponding lower case letter is selected to represent the recessive allele </li></ul><ul><li>Be careful to make them easily distinguishable! </li></ul><ul><li>T= tall stem t = short stem </li></ul><ul><li>P = purple flower p = white flower </li></ul>TT X tt
  11. 11. Showing a cross with Pea Plants <ul><li>Pure bred tall x Pure bred short </li></ul><ul><li>TT X tt </li></ul>t t T T
  12. 12. Pure tall X Pure short TT X tt Phenotype: All Tall Genotype: All Heterozygous (Tt) t t T T Tt Tt Tt Tt
  13. 13. Cross the F 1 generation <ul><li>Tt X Tt </li></ul>T t T t
  14. 14. The cross of 2 heterozygous parents Tt x Tt Genotype 1:2:1 ratio 1 homozygous dominant 2 heterozygous 1 homozygous recessive Phenotype 3:1 ratio 3 tall stemmed plants 1 short stemmed plant F 2 TT Tt Tt tt T t T t
  15. 15. Wait a minute! More terms <ul><li>Phenotype : The appearance of the organism. The expression of the genotype </li></ul><ul><li>Genotype: the genetic makeup of the organism. </li></ul><ul><li>If I have a purple pea flower the phenotype is purple flower. What are the possible genotypes? </li></ul><ul><li>Since purple is dominant it could be PP or P p ! </li></ul><ul><li>What if the flower is white? </li></ul><ul><li>It has to be pp because white is recessive! </li></ul>
  16. 16. These are the classic Mendelian crosses! <ul><li>TT x tt homozygous dominant x homozygous recessive </li></ul><ul><li>Tt x Tt both parents are heterozygous </li></ul><ul><li>TT x Tt homozygous dominant x heterozygous </li></ul><ul><li>Tt x tt heterozygous x homozygous recessive </li></ul>
  17. 17. See if you can figure these out! <ul><li>A heterozygous purple flowered plant with a white flowered plant </li></ul><ul><li>A black Labrador Retriever with a yellow Labrador Retriever where black is dominant and the black Lab had a yellow mom. </li></ul>