Mendelian Genetics


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Mendelian Genetics

  1. 1. Mendelian Genetics <ul><li>Unit I - Genetic Continuity </li></ul><ul><li>Sections 11-1, 11-2, 11-3, 14-1 </li></ul><ul><li>Mr. Connors </li></ul><ul><li>Biology 12 </li></ul>
  2. 2. Gregor Mendel’s Peas <ul><ul><ul><li>Genetics is the scientific study of heredity . </li></ul></ul></ul><ul><ul><ul><li>Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity. </li></ul></ul></ul><ul><ul><ul><li>Mendel carried out his work with ordinary garden peas. </li></ul></ul></ul>
  3. 3. Mendel’s Pea Plants <ul><ul><li>Mendel knew that: </li></ul></ul><ul><ul><li>the male part of each flower produces pollen, (containing sperm). </li></ul></ul><ul><ul><li>the female part of the flower produces egg cells. </li></ul></ul>
  4. 4. Mendel’s Peas <ul><li>During sexual reproduction, sperm and egg cells join in a process called fertilization. </li></ul><ul><li>Fertilization produces a new cell. </li></ul><ul><li>Pea flowers are self-pollinating. </li></ul><ul><li>Sperm cells in pollen fertilize the egg cells in the same flower. </li></ul><ul><li>The seeds that are produced by self-pollination inherit all of their characteristics from the single plant that bore them. </li></ul>
  5. 5. Mendel’s Peas <ul><li>Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves. </li></ul><ul><li>Mendel wanted to produce seeds by joining male and female reproductive cells from two different plants. </li></ul><ul><li>He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower with pollen from another plant. </li></ul>
  6. 6. Cross-pollination <ul><li>This process is called cross-pollination. </li></ul><ul><li>Mendel was able to produce seeds that had two different parents. </li></ul>
  7. 7. Genetic vocabulary……. <ul><li>Punnett square: predicts the results of a genetic cross between individuals of known genotype </li></ul><ul><li>Homozygous: pair of identical alleles for a character </li></ul><ul><li>Heterozygous: two different alleles for a gene </li></ul><ul><li>Phenotype: an organism’s traits </li></ul><ul><li>Genotype: an organism’s genetic makeup </li></ul><ul><li>Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype) </li></ul>
  8. 8. Mendelian genetics <ul><li>Character </li></ul><ul><li>(heritable feature, i.e., fur colour) </li></ul><ul><li>Trait (variant for a character, i.e., brown) </li></ul><ul><li>True-bred (all offspring of same variety) </li></ul><ul><li>Hybridization </li></ul><ul><li>(crossing of 2 different true-breds) </li></ul><ul><li>P generation (parents) </li></ul><ul><li>F 1 generation (first filial generation) </li></ul>
  9. 9. Genes and dominance <ul><li>A trait is a specific characteristic that varies from one individual to another. </li></ul><ul><li>Mendel studied seven pea plant traits, each with two contrasting characters. </li></ul><ul><li>He crossed plants with each of the seven contrasting characters and studied their offspring. </li></ul>
  10. 10. Genes and dominance <ul><li>Each original pair of plants is the P (parental) generation. </li></ul><ul><li>The offspring are called the F 1 , or “first filial,” generation. </li></ul><ul><li>The offspring of crosses between parents with different traits are called hybrids . </li></ul><ul><li>The F 1 hybrid plants all had the character of only one of the parents. </li></ul>
  11. 13. Genes and dominance <ul><li>Mendel's first conclusion was that biological inheritance is determined by factors that are passed from one generation to the next. </li></ul><ul><li>Today, scientists call the factors that determine traits genes . </li></ul><ul><li>Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait. </li></ul><ul><li>The different forms of a gene are called alleles. </li></ul><ul><li>Mendel’s second conclusion is called the principle of dominance. </li></ul>
  12. 14. The principle of dominance <ul><li>The principle of dominance states that some alleles are dominant and others are recessive. </li></ul><ul><li>An organism with a dominant allele for a trait will always exhibit that form of the trait. </li></ul><ul><li>An organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present. </li></ul>
  13. 15. Leading to the Law of Segregation <ul><li>Alternative versions of genes (alleles) account for variations in inherited characteristics </li></ul><ul><li>For each character, an organism inherits 2 alleles, one from each parent </li></ul><ul><li>If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance </li></ul><ul><li>The alleles for each character segregate (separate) during gamete production (meiosis). </li></ul><ul><li>Mendel’s Law of Segregation </li></ul>
  14. 16. Segregation <ul><ul><li>Mendel crossed the F 1 generation with itself to produce the F 2 (second filial) generation. </li></ul></ul><ul><ul><li>The traits controlled by recessive alleles reappeared in one fourth of the F 2 plants. </li></ul></ul>
  15. 18. Segregation <ul><li>Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F 1 generation. </li></ul><ul><li>The trait controlled by the recessive allele showed up in some of the F 2 plants. </li></ul><ul><li>The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated , from the allele for tallness. </li></ul>
  16. 19. Segregation <ul><li>Mendel suggested that the alleles for tallness and shortness in the F 1 plants segregated from each other during the formation of the sex cells, or gametes. </li></ul><ul><li>When each F 1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene. </li></ul><ul><li>Therefore, each F 1 plant produces two types of gametes — those with the allele for tallness, and those with the allele for shortness. </li></ul>
  17. 20. Alleles separate during gamete formation
  18. 21. Punnett Squares <ul><ul><li>The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square. </li></ul></ul><ul><ul><li>Punnett squares can be used to predict and compare the genetic variations that will result from a cross. </li></ul></ul>
  19. 22. Punnett Squares <ul><li>A capital letter represents the dominant allele for tall. </li></ul><ul><li>A lowercase letter represents the recessive allele for short. </li></ul><ul><li>In this example, </li></ul><ul><li>T = tall </li></ul><ul><li>t = short </li></ul>
  20. 23. Punnett Squares <ul><li>Gametes produced by each F1 parent are shown along the top and left side. </li></ul><ul><li>Possible gene combinations for the F2 offspring appear in the four boxes. </li></ul>
  21. 24. Punnett Squares <ul><li>Organisms that have two identical alleles for a particular trait are said to be homozygous. </li></ul><ul><li>Organisms that have two different alleles for the same trait are heterozygous . </li></ul><ul><li>Homozygous organisms are true-breeding for a particular trait. </li></ul><ul><li>Heterozygous organisms are hybrid for a particular trait. </li></ul>
  22. 25. Punnett Squares <ul><li>All of the tall plants have the same phenotype , or physical characteristics. </li></ul><ul><li>The tall plants do not have the same genotype , or genetic makeup. </li></ul><ul><li>One third of the tall plants are TT , while two thirds of the tall plants are Tt. </li></ul>
  23. 26. Punnett Squares <ul><ul><ul><li>The plants have different genotypes ( TT and Tt ), but they have the same phenotype (tall). </li></ul></ul></ul>
  24. 27. Probability and Segregation <ul><ul><ul><li>One fourth (1/4) of the F2 plants have two alleles for tallness ( TT ). </li></ul></ul></ul><ul><ul><ul><li>2/4 or 1/2 have one allele for tall ( T ), and one for short ( t ). </li></ul></ul></ul><ul><ul><ul><li>One fourth (1/4) of the F2 have two alleles for short (tt). </li></ul></ul></ul>
  25. 28. Probability and Segregation <ul><li>Because the allele for tallness ( T ) is dominant over the allele for shortness (t), 3/4 of the F 2 plants should be tall. </li></ul><ul><li>The ratio of tall plants ( TT or Tt ) to short (tt) plants is 3:1. </li></ul><ul><li>The predicted ratio showed up in Mendel’s experiments indicating that segregation did occur. </li></ul>
  26. 29. Probabilities predict averages <ul><ul><ul><li>Probabilities predict the average outcome of a large number of events. </li></ul></ul></ul><ul><ul><ul><li>Probability cannot predict the precise outcome of an individual event. </li></ul></ul></ul><ul><ul><ul><li>In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values. </li></ul></ul></ul>
  27. 30. The Law of Independent Assortment <ul><li>Law of Segregation involves 1 character. What about 2 (or more) characters? </li></ul><ul><li>Monohybrid cross vs. dihybrid cross </li></ul><ul><li>The two pairs of alleles segregate independently of each other. </li></ul><ul><li>Mendel’s Law of Independent Assortment </li></ul>
  28. 31. The Two-Factor Cross: F 1 <ul><ul><ul><li>Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY ) with true-breeding plants that produced wrinkled green peas (genotype rryy ). </li></ul></ul></ul><ul><ul><ul><li>All of the F 1 offspring produced round yellow peas ( RrYy ). </li></ul></ul></ul>
  29. 32. Independent Assortment <ul><li>The alleles for round ( R ) and yellow ( Y ) are dominant over the alleles for wrinkled ( r ) and green ( y ). </li></ul>
  30. 33. The Two-Factor Cross: F 2 <ul><ul><ul><li>Mendel crossed the heterozygous F 1 plants ( RrYy) with each other to determine if the alleles would segregate from each other in the F 2 generation. </li></ul></ul></ul><ul><ul><ul><li>RrYy × RrYy </li></ul></ul></ul>
  31. 34. Independent Assortment <ul><li>The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation. </li></ul>
  32. 35. Independent Assortment <ul><ul><li>In Mendel’s experiment, the F 2 generation produced the following: </li></ul></ul><ul><ul><ul><li>some seeds that were round and yellow </li></ul></ul></ul><ul><ul><ul><li>some seeds that were wrinkled and green </li></ul></ul></ul><ul><ul><ul><li>some seeds that were round and green </li></ul></ul></ul><ul><ul><ul><li>some seeds that were wrinkled and yellow </li></ul></ul></ul><ul><li>The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment . </li></ul><ul><li>Genes that segregate independently do not influence each other's inheritance. </li></ul>
  33. 36. Independent Assortment <ul><li>Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio predicted by the Punnett square. </li></ul><ul><li>Mendel had discovered the principle of independent assortment. </li></ul><ul><li>The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. </li></ul><ul><li>Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms. </li></ul>
  34. 37. Non-single gene genetics, I <ul><li>Incomplete dominance : appearance between the phenotypes of the 2 parents. Ex: snapdragons </li></ul><ul><li>Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: Tay-Sachs disease </li></ul><ul><li>Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types </li></ul>
  35. 38. Non-single gene genetics, II <ul><li>Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia </li></ul><ul><li>Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color </li></ul><ul><li>Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height </li></ul>
  36. 39. Human disorders <ul><li>The family pedigree </li></ul><ul><li>Recessive disorders: •Cystic fibrosis •Tay-Sachs •Sickle-cell </li></ul><ul><li>Dominant disorders: •Huntington’s </li></ul><ul><li>Testing: •amniocentesis •chorionic villus sampling (CVS) </li></ul>
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