Successfully reported this slideshow.
We use your LinkedIn profile and activity data to personalize ads and to show you more relevant ads. You can change your ad preferences anytime.

7th grade life science genetics and probability


Published on

Using Punnet Squares to predict genetic crosses

Published in: Education
  • Be the first to comment

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>