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  1. 1. The Genetic Basis of Inheritance From Chromosomes to Genes
  2. 2. Chromosomal Basis of Inheritance <ul><li>Traits are transmitted by chromosomes which contain units of heredity called genes </li></ul><ul><li>Genes are formed from DNA </li></ul>
  3. 3. Mendel and the History of Genetics <ul><li>Gregor Mendel </li></ul><ul><li>Born 1822 </li></ul><ul><li>Began his work at age 21 </li></ul><ul><li>A monk and school teacher interested in plant breeding </li></ul><ul><li>Studied pea plants </li></ul><ul><li>Discovered the basic principles of heredity </li></ul>
  4. 4. Mendel’s Studies <ul><li>Pea plants were an ideal choice for study </li></ul><ul><li>Displayed seven traits in one of two contrasting forms: </li></ul><ul><ul><li>seed shape, seed color, seed coat color, pod shape, pod color, flower position, stem length </li></ul></ul><ul><li>Mendel studied 30,000 plants in 7 years </li></ul><ul><ul><li>Kept careful records & looked for mathematical patterns </li></ul></ul>
  5. 5. Pure Lines <ul><li>Mendel recognized the 2 contrasting forms as distinct varieties, or pure lines </li></ul><ul><li>In a pure line, the offspring have same trait as the parents </li></ul><ul><li>‘ Tall’ always produced ‘tall,’ etc. </li></ul><ul><li>Called parental pure lines &quot;P1&quot; generation </li></ul><ul><li>First generation of offspring are F1 (for filial), then F2 and so on </li></ul><ul><li>Hybrid = An offspring produced by breeding 2 pure lines </li></ul><ul><li>What happens if you cross 2 pure lines? </li></ul>
  6. 6. Dominance <ul><li>Mendel crossed the two pure lines for each trait. </li></ul><ul><li>These were the P 1 generation.  </li></ul><ul><li>All offspring of these crosses (F 1 ) had the trait of only one parent </li></ul><ul><li>The trait of the other parent disappeared in the F1 generation (but reappeared in F2) </li></ul><ul><li>Mendel hypothesized that there were 2 factors for each trait </li></ul><ul><li>Mendel called 1 factor dominant because it prevailed </li></ul>
  7. 7. Dominant vs. Recessive Traits <ul><li>Mendel crossed F 1 plants with other F 1 plants, producing the F 2 generation </li></ul><ul><li>Not only did the recessive trait reappear in the F 2 generation, but in a consistent proportion: </li></ul><ul><ul><li>1/4 showed the recessive trait (ratio 3:1) </li></ul></ul><ul><li>Mendel referred to the factor that was hidden in the F 1 as recessive </li></ul><ul><li>If an offspring carries 2 dominant or one dominant and one recessive factor, the offspring will appear to have the dominant trait. </li></ul><ul><li>If the offspring carries 2 recessive factors, it will appear to have the recessive trait </li></ul>
  8. 8. The Experiment <ul><li>P Generation: </li></ul><ul><ul><li>Cross 2 pure lines – </li></ul></ul><ul><ul><li>One white, one purple </li></ul></ul><ul><li>F1 are all purple </li></ul><ul><ul><li>Purple is dominant </li></ul></ul><ul><li>Cross F1 x F1 (all purple) </li></ul><ul><ul><li>¼ F2 offspring are white </li></ul></ul><ul><ul><li>White is recessive </li></ul></ul>
  9. 9. Principle of Segregation <ul><li>Members of each pair of genes separate, or segregate , when gametes are formed. </li></ul><ul><ul><li>Recall what we now know happens to chromosomes during meiosis </li></ul></ul><ul><li>From Mendel: </li></ul><ul><ul><li>For each characteristic an individual carries 2 ‘factors’ </li></ul></ul><ul><ul><li>Each parent contributes one of its 2 factors to each offspring. </li></ul></ul><ul><ul><li>Chances of contributing either factor are equal. </li></ul></ul>
  10. 10. Genes and Alleles <ul><li>We now know that the heredity units are genes . </li></ul><ul><li>The separate forms of a gene that Mendel called ‘factors’ = alleles </li></ul><ul><li>One allele in a pair may prevent the other from being expressed </li></ul><ul><li>If an offspring carries 2 dominant, or one dominant and one recessive allele , the offspring will appear to have the dominant trait. </li></ul><ul><li>If an offspring carries 2 recessive alleles , it will appear to have the recessive trait. </li></ul>
  11. 11. Each individual carries one copy (allele) of a gene on the chromosome from their mother, and a second copy on the homologous chromosome from their father.
  12. 12. Representing Genes & Alleles <ul><li>Generally the capitalized first letter of the dominant trait is used to represent the dominant allele </li></ul><ul><li>The recessive allele is then lower case of the same letter (e.g. T for tall, so t for short) </li></ul><ul><li>Each characteristic can be represented by a pair of letters representing the genes </li></ul><ul><li>Some multi-allele systems (more than 2 possible forms of the trait) use different conventions </li></ul><ul><li>Traits carried on sex chromosomes are written as a superscript of the X or Y chromosome  </li></ul>
  13. 13. Hereditary Terminology <ul><li>Genes – The units of heredity </li></ul><ul><li>Alleles – The separate forms of a gene that Mendel called factors </li></ul><ul><li>Genotype - The set of alleles for a characteristic possessed by an organism </li></ul><ul><li>Phenotype - The appearance of the organism; the trait that is actually expressed </li></ul><ul><li>Homozygous - The 2 alleles in a pair are identical </li></ul><ul><ul><li>An individual can be either homozygous dominant (2 identical dominant alleles) or homozygous recessive (2 identical recessive alleles) </li></ul></ul><ul><li>Heterozygous – The 2 alleles in a pair are different </li></ul>
  14. 14. Genotype vs. Phenotype
  15. 15. Punnett Squares <ul><li>A way to visualize crosses </li></ul><ul><li>Punnett square can be used to determine probability of different genotypes or phenotypes </li></ul><ul><li>Each box contains a possible combination of alleles for offspring </li></ul><ul><li>Punnett square can be used to determine probability of different genotypes or phenotypes </li></ul>
  16. 16. <ul><li>Mendel’s F1 cross can be shown as a punnett square </li></ul><ul><li>Shows 3:1 ratio </li></ul>
  17. 17. Determining Genotype <ul><li>If you know the phenotype, is it possible to determine the genotype? </li></ul><ul><li>If an organism shows the recessive trait, you know the genotype. </li></ul><ul><ul><li>It must be homozygous recessive (tt) </li></ul></ul><ul><li>What if it shows the dominant trait? </li></ul><ul><li>If the dominant trait is expressed there are 2 possibilities: </li></ul><ul><li>homozygous dominant ( TT) </li></ul><ul><li>or heterozygous  (Tt) </li></ul>
  18. 18. Test Crosses <ul><li>Cross the unknown with a homozygous recessive (tt) x ? </li></ul><ul><ul><li>This is a test cross (or backcross) </li></ul></ul><ul><li>If the unknown is TT: </li></ul><ul><ul><li>then crossing with tt yields all Tt </li></ul></ul><ul><ul><li>All offspring are tall </li></ul></ul><ul><ul><li>all have same phenotype and the same genotype </li></ul></ul><ul><li>If the unknown is genotype Tt, when crossed with tt </li></ul><ul><ul><li>1/2 of the offspring will be Tt and 1/2 will be tt </li></ul></ul><ul><ul><li>so 1/2 will show a different phenotype , making it easy to tell </li></ul></ul>
  19. 19. Demonstrating a Test Cross
  20. 20. Incomplete Dominance <ul><li>In all of Mendel's cases, 1 allele was clearly dominant </li></ul><ul><ul><li>This is not always the case </li></ul></ul><ul><li>Some alleles show incomplete dominance </li></ul><ul><li>Heterozygotes express traits that are a blend of the phenotypes of the 2 alleles </li></ul><ul><ul><li>red and white flowers make pink </li></ul></ul><ul><li>Still obey the law of segregation </li></ul><ul><li>Only difference is phenotype of homozygous dominant individual is different from phenotype of heterozygous individual </li></ul>
  21. 21. Demonstrating Incomplete Dominance
  22. 22. Co-Dominance <ul><li>Occurs when both alleles for a gene are expressed in a heterozygote </li></ul><ul><li>Example – Red & white produce roan color in some animals because both genes are expressed </li></ul>
  23. 23. Multi-allele Systems <ul><li>Some traits are the result of more than 2 possible alleles at a locus </li></ul><ul><li>The ABO blood group is an important example </li></ul><ul><li>3 possible alleles: </li></ul><ul><ul><li>I A - produces A antigen; </li></ul></ul><ul><ul><li> anti-B antibodies in blood serum </li></ul></ul><ul><ul><li>I B - produces B antigen; </li></ul></ul><ul><ul><li>anti-A antibodies in serum </li></ul></ul><ul><ul><li>i - produces no antigen; </li></ul></ul><ul><ul><li>anti-A & anti-B antibodies in serum </li></ul></ul><ul><li>I A & I B are co-dominant alleles </li></ul><ul><li>i is recessive </li></ul>
  24. 24. The ABO Blood System
  25. 25. Working with 2 Alleles <ul><li>Crosses so far have examined only 1 characteristic = monohybrid crosses </li></ul><ul><li>It is possible to consider more than 1 characteristic at a time </li></ul><ul><ul><li>Crosses involving 2 genes = dihybrid crosses </li></ul></ul><ul><ul><li>Three genes = trihybrid </li></ul></ul><ul><li>Mendel crossed plants that differed in 2 characteristics </li></ul><ul><li>This led to Law of Independent Assortment </li></ul>
  26. 26. Law of Independent Assortment <ul><li>Mendel showed that traits produced by dominant factors do not necessarily appear together </li></ul><ul><li>2 or more pairs of genes segregate independently of one another during the formation of gametes </li></ul><ul><ul><li>Genes are distributed to gametes independently </li></ul></ul><ul><li>We now know this is true: </li></ul><ul><ul><li>recall independent assortment of chromosomes from meiosis </li></ul></ul><ul><li>If 2 traits are carried on the same chromosome independent assortment does not hold </li></ul><ul><ul><li>If 2 traits are located close together on the same chromosome they are linked </li></ul></ul>
  27. 27. Dihybrid Crosses <ul><li>Involve 2 genes </li></ul><ul><li>Can demonstrate independent assortment </li></ul><ul><li>Cross homzygous dominant for 2 alleles with homozygous recessive for 2 alleles: </li></ul><ul><ul><li>Y = yellow </li></ul></ul><ul><ul><li>R = round </li></ul></ul><ul><li>YYRR x yyrr can produce: </li></ul><ul><ul><li>Yellow/ round </li></ul></ul><ul><ul><li>Yellow/ wrinkled </li></ul></ul><ul><ul><li>Green/ round </li></ul></ul><ul><ul><li>Green wrinkled </li></ul></ul><ul><li>Produces 9:3:3:1 ratio </li></ul>
  28. 28. Diagramming a Dihybrid Cross
  29. 29. Sex Linkage <ul><li>Autosomal trait </li></ul><ul><ul><li>a gene carried on a non-sex chromosome & present in 2 copies </li></ul></ul><ul><li>Sex linked trait </li></ul><ul><ul><li>a gene carried on a sex chromosome that is present in both sexes; one copy in one sex, 2 in the other </li></ul></ul><ul><ul><li>X chromosome in mammals </li></ul></ul><ul><ul><li>Z chromosome in birds (one copy in females, 2 in males) </li></ul></ul><ul><li>Y- linked trait </li></ul><ul><ul><li>a gene carried on the Y chromosome in humans; very rare </li></ul></ul>
  30. 30. Showing Sex Linkage <ul><li>Symbols are written as superscript of the sex chromosome: </li></ul><ul><ul><li>X a - X chromosome carrying the recessive allele </li></ul></ul><ul><ul><li>X A – X chromosome carrying the dominant allele </li></ul></ul><ul><ul><li>No superscript is used for the normal or wild type allele </li></ul></ul><ul><li>Suspect sex linkage when the ratios of phenotypes are different in males and females </li></ul>
  31. 31. Pedigrees <ul><li>Used to determine mode of inheritance when few individuals, but several generations are involved </li></ul><ul><li>Assume genetic trait discussed is rare, so individuals marrying into the family are not assumed to carry the trait </li></ul><ul><li>Symbols: </li></ul><ul><ul><li>O female shaded = affected </li></ul></ul><ul><ul><li> male partial shading = carrier </li></ul></ul>
  32. 32. Pedigree Analysis
  33. 33. Polygenic Inheritance <ul><li>Most traits are not limited to 2 possibilities, yes or no </li></ul><ul><li>Most traits are a continuum (quantitative) </li></ul><ul><li>Examples: height, skin color </li></ul><ul><li>2 or more genes act additively on a trait </li></ul>
  34. 34. Pleitropy <ul><li>Genes have multiple phenotypic effects </li></ul><ul><li>The same gene that produces dark pigment in skin might produce dark pigment in fur </li></ul><ul><li>Pleiotropic Advantage: </li></ul><ul><li>Some genes that have one negative effect, also convey a benefit </li></ul><ul><ul><li>Example: sickle cell trait may have provided the heterozygote an advantage against malaria </li></ul></ul>
  35. 35. Sickle Cell Trait & Pleiotropy
  36. 36. Epistasis <ul><li>A gene at one locus alters expression of a second gene </li></ul><ul><li>Example: If black (B) is dominant over brown (b) in mice, but . . . </li></ul><ul><li>A second gene (C) determines if fur pigment is made at all, </li></ul><ul><li>Then you must be (CC) or (Cc) before the mice can be either black or brown </li></ul>
  37. 37. Diagramming Epistasis