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How does chromosome behavior account

Chapter 15 - Chromosomal Basis of Inheritance. AIM: How does chromosome behavior relate to Mendel?. How does chromosome behavior account. for Mendel’s Principles ?. (The chromosomal theory of inheritance). The chromosomal theory of inheritance:. The chromosomal theory of inheritance:.

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How does chromosome behavior account

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  1. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? How does chromosome behavior account for Mendel’s Principles ? (The chromosomal theory of inheritance)

  2. The chromosomal theory of inheritance:

  3. The chromosomal theory of inheritance:

  4. The chromosomal theory of inheritance:

  5. The Chromosome Theory of Inheritance: genes (allele pairs) are on chromosomes and homologous chromosomes segregate during meiosis (principle of segregation) and reunite during fertilization. If allele pairs are on different chromosomes they will sort independently (principle of independent assortment) due to independent orientation of the tetrads at metaphase I. If the allele pairs are on the same chromosome, they will sort dependently and travel together into the gametes.

  6. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? Draw out meiosis and indicate how this process accounts for the laws of segregation and independent assortment.

  7. Meiosis accounts for the laws of segregation and independent assortment. Make sure you can quickly draw this and explain how the laws of segregation and independent assortment are described by meiosis and specifically where in meiosis. Alleles of genes on non-homologous chomosomes assort independently during gamete formation. The two alleles of each gene separate.

  8. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? What have we been ignoring thus far during dependent assortment? Crossing-Over

  9. You perform an experiment where you cross a double heterozygous plant with purple flower and long pollen with another of the same genotype. P = purple flower L = long pollen p = red flower l = round pollen PpLl x PpLl 1. Determine the expected phenotypic ratio if the allele pairs were on different chromosomes – independent assortment. This is identical to Mendel’s classic F1 dihybrid cross resulting in a 9 purple,long : 3 purple, round : 3 red, long : 1 red, round 2. Determine the expected phenotypic ratio if the allele pairs were linked (P is linked to L). Possible gametes: PL or pl for both Making a Punnett square or just putting together the possible fertilization events gives you the following offspring: PPLL, PpLl, PpLl, ppll or a 3 purple,long : 1 red, round phenotypic ratio

  10. Take a look at the actual offspring: Neither prediction fits this observation… How can you account for this observation? Come up with a model for how this can occur.

  11. If the alleles did sort independently, you would expect to see the 9:3:3:1 phenotypic ratio. Therefore they are likely sorting dependently, but how do we account for the purple-round and red-long offspring?... CROSSING OVER Parental Type Gametes Synapsis during Prophase I P L p l P L p L P l p l Recombinant Gametes

  12. If the alleles did sort independently, you would expect to see the 9:3:3:1 phenotypic ratio. Therefore they are likely sorting dependently, but how do we account for the purple-round and red-long offspring?... CROSSING OVER Parental Type Gametes P L p l P L p L P l p l Recombinant Gametes Predict the resulting gametes…

  13. If the alleles did sort independently, you would expect to see the 9:3:3:1 phenotypic ratio. Therefore they are likely sorting dependently, but how do we account for the purple-round and red-long offspring?... CROSSING OVER Parental Type Gametes P L p l P L p L P l p l Recombinant Gametes You get the same gametes as if it were independent assortment because of crossing over, but what is going to be different?

  14. If the alleles did sort independently, you would expect to see the 9:3:3:1 phenotypic ratio. Therefore they are likely sorting dependently, but how do we account for the purple-round and red-long offspring?... CROSSING OVER Parental Type Gametes P L p l P L p L P l p l Recombinant Gametes They are not made in a 1:1:1:1 ratio since crossing over doesn’t always occur between the allele pairs… The majority will be parental type…

  15. Which gametes need to fuse in order to get the recombinant offspring? Parental Type Gametes P L p l P L p L P l p l Recombinant Gametes Whatever the PL gamete fertilizes, the outcome is the same… the purple, long phenotype.

  16. What do you think determines the number of recombinant offspring? Parental Type Gametes P L p l P L p L P l p l Recombinant Gametes The number of gametes with the recombinant chromosome of course…

  17. What do you think determines how often crossing over occurs between the P/p and L/l genes? Parental Type Gametes P L p l P L p L P l p l Recombinant Gametes The number of crossing over events between the allele pairs is directly related to how far apart the allele pairs are on the chromosome… The further apart they are, the more likely it is that crossing over will occur BETWEEN them.

  18. Parental Type Gametes P L p l P L p L P l p l Recombinant Gametes Therefore, the more recombinant offspring you get, the_________________ apart the allele pairs are. further

  19. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? If I have three or more pairs of alleles, I want to know where they are on a chromosome relative to each other. How can we do that?

  20. Thomas Hunt (T. H.) Morgan Drosophila melanogaster (The modern day pea plant) The Fly Room

  21. Fruit fly experiments demonstrating the role of crossing-over in inheritance and recombination frequency Predict the offspring phenotypic ratio for dependent assortment where G is linked to L. Wing shape Body Color Characteristics: G = gray body (dominant) L = long wings (dominant) Traits: g = black body l = vestigial,short wings GgLl (wild type female) x ggll (male) Gametes: GL, gl gl Offspring: GgLl or ggll (both are parental type) 1 Gray, long wings : 1 black, vestigial/short wings Fig. 9.19C

  22. Fruit fly experiments demonstrating the role of crossing-over in inheritance and recombination frequency 1 Gray, long wings : 1 black, short wings IT DOESN’T MATCH…WHY? CROSSING OVER… Draw the possible gametes in each parent (show chromosomes). Indicate the parental-type and recombinant chromosomes. Fig. 9.19C

  23. Fruit fly experiments demonstrating the role of crossing-over in inheritance and recombination frequency CROSSING OVER… Fig. 9.19C

  24. Fruit fly experiments demonstrating the role of crossing-over in inheritance and recombination frequency What do the number of recombinant phenotypes relative to the total offspring tell you? 1. They tell you the number of times crossing over occurred between the G/g and L/l allele pairs in the FEMALE ONLY…why? The male was chosen to be double homozygous recessive on purpose so that crossing over does not matter because the same gametes form either way. Thus, from this experiment one can determine the frequency of crossing over in a SINGLE INDIVIDUAL.

  25. Fruit fly experiments demonstrating the role of crossing-over in inheritance and recombination frequency That’s great, but who cares…what is this useful for? 1. Remember, the further apart the allele pairs are, the more recombinant offspring you should get. 2. Therefore, one can do this with another pair of alleles like G/g and P/p. If the result is fewer recombinants than with G and L, P is closer to G… 3. You can determine where allele pairs are relative to each other on a chromosome = chromosome mapping (linkage mapping)!!

  26. Fruit fly experiments demonstrating the role of crossing-over in inheritance and recombination frequency The recombinant frequency can be determined by taking the number of recombinants and dividing by the total offspring and multiplying by 100. Fig. 9.19C

  27. Fruit fly experiments demonstrating the role of crossing-over in inheritance and recombination frequency Fig. 9.19C

  28. Fruit fly (Drosophila) nomenclature 1. Genes are typically named after the mutant (non-wild type) allele. In this case black (b) and vestigial (vg). 2. The wild type is then indicated with a “+”. Therefore grey would be b+ and long wings would be vg+.

  29. Fruit fly (Drosophila) nomenclature SUMMARY:

  30. Fruit fly (Drosophila) nomenclature This is the same figure as shown previously just using Drosophila gene nomenclature.

  31. Independent Assortment 9:3:3:1 phenotypic ratio

  32. Independent Assortment Linked Genes (no recombination) 9:3:3:1 phenotypic ratio

  33. Independent Assortment Linked Genes (WITH recombination) 9:3:3:1 phenotypic ratio

  34. Independent Assortment Linked Genes (WITH recombination) 9:3:3:1 phenotypic ratio

  35. Independent Assortment Linked Genes (WITH recombination) 9:3:3:1 phenotypic ratio

  36. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? How can we use recombination frequency to determine the position of genes on a chromosome (gene mapping)? You have determined the frequency between G/g and L/l (17%). There is a third gene on the same chromosome (E/e). How can you determine the frequency between G and E/e? By doing this testcross: GgEe x ggee

  37. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? How can we use recombination frequency to determine the position of genes on a chromosome (gene mapping)? Let’s say you determine the recombinant frequency to be 9%. What does that tell you? It tells you that recombination occurred less frequently between G and E and therefore they are closer together than G and L. Let’s try to map this on a chromosome… (convince yourself that it is not possible yet) G E L 9% 17% or E G L 9%

  38. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? How can we use recombination frequency to determine the position of genes on a chromosome (gene mapping)? What else do we need to do to figure out the order on the chromosome of G, E and L? 9% We need to figure out the recombinant freq between L and E…is it ~8% or ~26%: G E L 17% E G L LlEe x llee 9%

  39. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? How can we use recombination frequency to determine the position of genes on a chromosome (gene mapping)? You determine the recombinant offspring frequency to be 9.5%. Now can you map the genes?

  40. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? How can we use recombination frequency to determine the position of genes on a chromosome (gene mapping)? e This is the only possible combination that fits the data (or l,e,g – e must be between them)…

  41. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel?

  42. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? A geneticist wants to map the position of three dominant/recessive allele pairs (A/a, B/b, and F/f) relative to each other in D. melanogaster (fruit flies). For simplicity let’s say all the dominant alleles are on the same chromosome. Where should she begin? 1. Do AaBb x aabb and look for recombinant phenotypes to determine the frequency of crossing-over Her cross generates 1000 offspring of which 200 are recombinant. What is the recombination frequency? 200/1000 * 100 = 20% What should she do next?

  43. Chapter 15 - Chromosomal Basis of Inheritance AIM: How does chromosome behavior relate to Mendel? A geneticist wants to map the position of three dominant/recessive allele pairs (A/a, B/b, and F/f) relative to each other in D. melanogaster (fruit flies). For simplicity let’s say all the dominant alleles are on the same chromosome. Where should she begin? 2. Do AaFf x aaff and BbFf x bbff Find recombinant phenotypes to determine the frequency of crossing-over The AaFf cross results in a 7% recombinant frequency The BbFf cross results in a 28% recombinant frequency The AaBb cross resulted in a 20% recombinant frequency Map the genes on the chromosome: F B A

  44. Chapter 15 - Chromosomal Basis of Inheritance AIM: Sex-linked gene inheritance. Sex-Linked Gene Inheritance

  45. Chapter 15 - Chromosomal Basis of Inheritance AIM: Sex-linked gene inheritance. Sex determination in humans obviously also uses the XY system where the Y chromosome determine if the individual is male. Do all sexually reproducing animals work the same…?

  46. Chapter 15 - Chromosomal Basis of Inheritance AIM: Sex-linked gene inheritance. Turns out the answer is no… Your book has a very brief explanation of these systems (Figure 15.9).

  47. Chapter 15 - Chromosomal Basis of Inheritance AIM: Sex-linked gene inheritance. Eye color is determined by a dominant/recessive single gene locus on the X chromosome. R = red-eye allele r = white-eye allele Q. Determine the possible offspring from a cross between a homozygous dominant red eye female and a white eye male.

  48. Chapter 15 - Chromosomal Basis of Inheritance AIM: What are sex-linked genes and what makes them different from autosomal genes? AIM: Sex-linked gene inheritance. Notice that now because we have two variables to follow…allele pair and sex chromosome…we no longer just write the allele pair. Therefore the female in this case is homozygous dominant RR, each on an X chromosome = XRXR), while the white eye male is just r on one X (Xr) as the Y chromosome does not have the same loci as X (X and Y are not homologous) = XrY. Continue as normal- possible gametes, fertilize, etc…

  49. Chapter 15 - Chromosomal Basis of Inheritance AIM: What are sex-linked genes and what makes them different from autosomal genes? AIM: Sex-linked gene inheritance. There are two possible offspring: Red eye females Red eye males

  50. Chapter 15 - Chromosomal Basis of Inheritance AIM: What are sex-linked genes and what makes them different from autosomal genes? AIM: Sex-linked gene inheritance. Q. Determine the possible offspring from cross between a red eye female heterozygous for the gene locus and a red eye male.

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