Nonmendelian genetics


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Nonmendelian genetics

  1. 1. Patterns of Inheritance
  2. 2. What causes recessive traits to appear?
  3. 3. Early Ideas of Heredity <ul><li>Before the 20 th century, 2 concepts were the basis for ideas about heredity: </li></ul><ul><li>-heredity occurs within species </li></ul><ul><li>-traits are transmitted directly from parent to offspring </li></ul><ul><li>This led to the belief that inheritance is a matter of blending traits from the parents. </li></ul>
  4. 4. Early Ideas of Heredity <ul><li>Botanists in the 18 th and 19 th centuries produced hybrid plants. </li></ul><ul><li>When the hybrids were crossed with each other, some of the offspring resembled the original strains, rather than the hybrid strains. </li></ul><ul><li>This evidence contradicted the idea that traits are directly passed from parent to offspring. </li></ul>
  5. 5. Early Ideas of Heredity <ul><li>Gregor Mendel </li></ul><ul><li>-chose to study pea plants because: </li></ul><ul><li>1. other research showed that pea hybrids could be produced </li></ul><ul><li>2. many pea varieties were available </li></ul><ul><li>3. peas are small plants and easy to grow </li></ul><ul><li>4. peas can self-fertilize or be cross-fertilized </li></ul>
  6. 6. Early Ideas of Heredity <ul><li>Mendel’s experimental method: </li></ul><ul><li>1. produce true-breeding strains for each trait he was studying </li></ul><ul><li>2. cross-fertilize true-breeding strains having alternate forms of a trait </li></ul><ul><li>3. allow the hybrid offspring to self-fertilize and count the number of offspring showing each form of the trait </li></ul>
  7. 7. Monohybrid Crosses <ul><li>Monohybrid cross : a cross to study only 2 variations of a single trait </li></ul><ul><li>Mendel produced true-breeding pea strains for 7 different traits </li></ul><ul><li>-each trait had 2 alternate forms (variations) </li></ul><ul><li>-Mendel cross-fertilized the 2 true-breeding strains for each trait </li></ul>
  8. 8. Monohybrid Crosses <ul><li>F 1 generation (1 st filial generation) : offspring produced by crossing 2 true-breeding strains </li></ul><ul><li>For every trait Mendel studied, all F 1 plants resembled only 1 parent </li></ul><ul><li>-no plants with characteristics intermediate between the 2 parents were produced </li></ul>
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  10. 10. Monohybrid Crosses <ul><li>F 1 generation: offspring resulting from a cross of true-breeding parents </li></ul><ul><li>F 2 generation : offspring resulting from the self-fertilization of F 1 plants </li></ul><ul><li>dominant : the form of each trait expressed in the F 1 plants </li></ul><ul><li>recessive : the form of the trait not seen in the F 1 plants </li></ul>
  11. 11. Monohybrid Crosses <ul><li>F 2 plants exhibited both forms of the trait in a very specific pattern: </li></ul><ul><li>¾ plants with the dominant form </li></ul><ul><li>¼ plant with the recessive form </li></ul><ul><li>The dominant to recessive ratio was 3 : 1. </li></ul><ul><li>Mendel discovered the ratio is actually: </li></ul><ul><li>1 true-breeding dominant plant </li></ul><ul><li>2 not-true-breeding dominant plants </li></ul><ul><li>1 true-breeding recessive plant </li></ul>
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  13. 13. Monohybrid Crosses <ul><li>gene : information for a trait passed from parent to offspring </li></ul><ul><li>alleles : alternate forms of a gene </li></ul><ul><li>homozygous : having 2 of the same allele </li></ul><ul><li>heterozygous : having 2 different alleles </li></ul>
  14. 14. Monohybrid Crosses <ul><li>genotype : total set of alleles of an individual </li></ul><ul><li>PP = homozygous dominant </li></ul><ul><li>Pp = heterozygous </li></ul><ul><li>pp = homozygous recessive </li></ul><ul><li>phenotype : outward appearance of an individual </li></ul>
  15. 15. Monohybrid Crosses <ul><li>Principle of Segregation </li></ul><ul><li>Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization. </li></ul>
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  18. 18. Monohybrid Crosses <ul><li>Some human traits are controlled by a single gene. </li></ul><ul><li>-some of these exhibit dominant inheritance </li></ul><ul><li>-some of these exhibit recessive inheritance </li></ul><ul><li>Pedigree analysis is used to track inheritance patterns in families. </li></ul>
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  21. 21. Dihybrid Crosses <ul><li>Dihybrid cross : examination of 2 separate traits in a single cross </li></ul><ul><li>-for example: RR YY x rryy </li></ul><ul><li>The F 1 generation of a dihybrid cross (RrYy) shows only the dominant phenotypes for each trait. </li></ul>
  22. 22. Dihybrid Crosses <ul><li>The F 2 generation is produced by crossing members of the F 1 generation with each other or allowing self-fertilization of the F 1 . </li></ul><ul><li>-for example RrYy x RrYy </li></ul><ul><li>The F 2 generation shows all four possible phenotypes in a set ratio: </li></ul><ul><li>9 : 3 : 3 : 1 </li></ul>
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  24. 24. Dihybrid Crosses <ul><li>Principle of Independent Assortment </li></ul><ul><li>In a dihybrid cross, the alleles of each gene assort independently. </li></ul>
  25. 25. Testcross <ul><li>Testcross : a cross used to determine the genotype of an individual with dominant phenotype </li></ul><ul><li>-cross the individual with unknown genotype (e.g. P_) with a homozygous recessive (pp) </li></ul><ul><li>-the phenotypic ratios among offspring are different, depending on the genotype of the unknown parent </li></ul>
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  27. 27. Extensions to Mendel <ul><li>Mendel’s model of inheritance assumes that: </li></ul><ul><li>-each trait is controlled by a single gene </li></ul><ul><li>-each gene has only 2 alleles </li></ul><ul><li>-there is a clear dominant-recessive relationship between the alleles </li></ul><ul><li>Most genes do not meet these criteria. </li></ul>
  28. 28. Extensions to Mendel <ul><li>Polygenic inheritance occurs when multiple genes are involved in controlling the phenotype of a trait. </li></ul><ul><li>The phenotype is an accumulation of contributions by multiple genes. </li></ul><ul><li>These traits show continuous variation and are referred to as quantitative traits . </li></ul><ul><li>For example – human height </li></ul>
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  30. 30. Extensions to Mendel <ul><li>Pleiotropy refers to an allele which has more than one effect on the phenotype. </li></ul><ul><li>This can be seen in human diseases such as cystic fibrosis or sickle cell anemia. </li></ul><ul><li>In these diseases, multiple symptoms can be traced back to one defective allele. </li></ul>
  31. 31. Extensions to Mendel <ul><li>Incomplete dominance : the heterozygote is intermediate in phenotype between the 2 homozygotes. </li></ul><ul><li>Codominance : the heterozygote shows some aspect of the phenotypes of both homozygotes. </li></ul>
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  33. 33. Extensions to Mendel <ul><li>The human ABO blood group system demonstrates: </li></ul><ul><li>-multiple alleles: there are 3 alleles of the I gene ( I A , I B , and i) </li></ul><ul><li>-codominance: I A and I B are dominant to i but codominant to each other </li></ul>
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  35. 35. Extensions to Mendel <ul><li>The expression of some genes can be influenced by the environment. </li></ul><ul><li>for example: coat color in Himalayan rabbits and Siamese cats </li></ul><ul><li>-an allele produces an enzyme that allows pigment production only at temperatures below 30 o C </li></ul>
  36. 36. Extensions to Mendel
  37. 37. Extensions to Mendel <ul><li>The products of some genes interact with each other and influence the phenotype of the individual. </li></ul><ul><li>Epistasis : one gene can interfere with the expression of another gene </li></ul>