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1. Meiosis And Dihybrid Cross


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1. Meiosis And Dihybrid Cross

  1. 1. Meiosis and the Dihybrid Cross Genetics & Adaptation SQA HIGHER BIOLOGY
  2. 2. Learning Content <ul><li>Meiosis and the dihybrid cross. </li></ul><ul><ul><li>Sexual reproduction as a means of enabling genetic variation to be maintained in the population and its importance in long-term evolutionary change. </li></ul></ul><ul><ul><li>Outline of meiosis: haploid gamete production. </li></ul></ul><ul><ul><ul><li>Crossing over and independent assortment of chromosomes during meiosis: a means of producing new phenotypes. </li></ul></ul></ul><ul><ul><li>The dihybrid cross: expected F 2 phenotypic ratios. </li></ul></ul>
  3. 3. Revision <ul><li>What do you remember about mitosis? </li></ul>
  4. 4. Sex <ul><li>What is the role of sexual reproduction? </li></ul><ul><li>What is the alternative to sexual reproduction? </li></ul><ul><li>What benefits do sexual reproduction give to a species? </li></ul><ul><li>What is the chromosome complement? </li></ul><ul><li>What must a sexually reproducing species do with its chromosome complement? </li></ul><ul><li>Define diploid & haploid. </li></ul>
  5. 5. Meiosis I
  6. 6. Meiosis II
  7. 7. Meiosis <ul><li>Summarise the process of meiosis as a flow chart. E.g. </li></ul>Diploid Cell with uncoiled replicating chromosomes. Chromosomes coil and become visible.
  8. 8. Crossing Over <ul><li>What is crossing over? </li></ul><ul><li>Define chiasma(ta). </li></ul><ul><li>What is the significance of crossing over? </li></ul><ul><li>What is the significance of the independent assortment of chromosomes? </li></ul><ul><li>What benefits does variation give to a species? </li></ul>
  9. 9. Monohybrid Cross <ul><li>A plant’s seeds can either be round or wrinkled. </li></ul><ul><li>Round is dominant to wrinkled. </li></ul><ul><li>What would be the ratio of phenotypes from a cross between a true-breeding round seed plant and a true-breeding wrinkled seed plant? </li></ul><ul><li>What would the ratio of phenotypes be for a cross between two of the offspring from the previous cross? </li></ul>
  10. 10. Monohybrid Cross <ul><li>Let the allele for round seeds be: R (dominant allele) </li></ul><ul><li>Let the allele for wrinkled seeds be: r (recessive allele) </li></ul><ul><li>Parents </li></ul><ul><li>Phenotype round seeds x wrinkled seeds </li></ul><ul><li>Genotype RR rr </li></ul><ul><li>Gametes </li></ul>R r
  11. 11. Monohybrid Cross <ul><li>F1 Generation </li></ul><ul><li>Genotypes </li></ul>R r Phenotype ratio 100% plants producing round seeds Genotype ratio 100% heterozygotes Rr Rr
  12. 12. Monohybrid Cross <ul><li>F1 Intercross </li></ul><ul><li>Parents </li></ul><ul><li>Phenotype round seeds x round seeds </li></ul><ul><li>Genotype Rr Rr </li></ul><ul><li>Gametes </li></ul>R r R r
  13. 13. Monohybrid Cross <ul><li>F2 Generation </li></ul><ul><li>Genotypes </li></ul>R R r r Genotype ratio 25% RR 50% Rr 25% rr Phenotype 3:1 round seeds: wrinkled seeds or ratio 75% plants producing round seeds 25% plants producing wrinkled seeds rr Rr Rr RR
  14. 14. Dihybrid Cross <ul><li>Dihybrid inheritance refers to the simultaneous inheritance of two characters </li></ul><ul><li>Mendel investigated the inheritance of seed shape (round v wrinkled) and seed colour (green v yellow) at the same time. </li></ul><ul><li>From his monohybrid crosses he knew that round seeds were dominant to wrinkled seeds and yellow seeds were dominant to green seeds. </li></ul><ul><li>He chose to cross plants that were pure breeding for both dominant features (round and yellow seeds) with plants that were pure breeding for both recessive features (wrinkled and green seeds). </li></ul>
  15. 15. Mendel’s Results <ul><li>Parents plants with round x plants with wrinkled </li></ul><ul><li>and yellow seeds and green seeds </li></ul>F1 offspring 100% plants with round and yellow seeds F1 intercross plants with round x plants with round and yellow seeds and yellow seeds F2 offspring Total 556 seeds collected from F2 plants Ratio 315 9 round and yellow seeds 108 3 round and green seeds 101 3 wrinkled and yellow seeds 32 1 wrinkled and green seeds However, the ratio of dominant to recessive features is still 3:1
  16. 16. Explanation of the results <ul><li>Let the allele for round seeds be R and the allele for wrinkled seeds be r </li></ul><ul><li>Let the allele for yellow seeds be Y and the allele for green seeds be y </li></ul><ul><li>Parents Phenotype round x wrinkled </li></ul><ul><li> yellow seeds green seeds </li></ul><ul><li> Genotype RR YY rryy </li></ul><ul><li>Gametes </li></ul><ul><li>F1 offspring </li></ul>RY ry RY ry RrYy
  17. 17. Explanation of the results <ul><li>F1 Intercross </li></ul><ul><li>Parents </li></ul><ul><li>phenotype round, yellow seeds x round, yellow seeds </li></ul><ul><li>genotype Rr Yy Rr Yy </li></ul><ul><li>Gametes </li></ul><ul><li>What are the genotypes and phenotypes of the F2 offspring and in what ratio? </li></ul>RY Ry rY ry ry rY Ry RY
  18. 18. Explanation of the results <ul><li>F2 offspring </li></ul>RY Ry rY ry RY Ry rY ry rryy rrYy Rryy RrYy rrYy rrYY RrYy RrYY Rryy RrYy RRyy RRYy RrYy RrYY RRYy RRYY
  19. 19. Explanation of the results <ul><li>genotypes phenotypes expected observed </li></ul><ul><li>RRYY (1) RrYy (4) yellow, round </li></ul><ul><li>RRYy (2) RrYY (2) yellow, round 9 315 </li></ul><ul><li>RRyy (1) Rryy (2) round, green 3 108 </li></ul><ul><li>rrYY (1) rrYy (2) wrinkled, yellow 3 101 </li></ul><ul><li>rryy (1) wrinkled, green 1 32 </li></ul><ul><li>Allowing for statistical error, Mendel’s results were a reasonable approximation to the expected 9:3:3:1 ratio. </li></ul><ul><li>Answer AYK Q1 page 101. </li></ul>
  20. 20. Learning Content <ul><li>Linkage and crossing over. </li></ul><ul><ul><li>The existence of linked genes and their effect on the F2 generation. </li></ul></ul><ul><ul><li>Comparison of the distance between linked genes and the frequency of recombination. </li></ul></ul><ul><li>Crossing over of genes at chiasmata during meiosis resulting in recombinant gametes. </li></ul><ul><ul><li>Separation of linked genes as a source of variation. </li></ul></ul><ul><li>Sex linkage. </li></ul>
  21. 21. Recombination <ul><li>What is recombination? </li></ul><ul><li>What are recombinants? </li></ul><ul><li>Identify the recombinants for the previous cross. </li></ul>
  22. 22. Linkage <ul><li>The 9:3:3:1 ratio derived from Mendel's principle of independent assortment does not always occur for dihybrid crosses. </li></ul><ul><li>In some of the F2 generations the parental phenotypes appear more frequently than expected. </li></ul><ul><li>Sometimes recombinant phenotypes do not appear in the F2 generation. </li></ul><ul><li>If two (or more) genes are located on the same chromosome they are said to be linked. </li></ul><ul><li>If there is gene linkage then there is a lower probability that the alleles will be separated during meiosis. </li></ul>
  23. 23. Linkage <ul><li>Consider a dihybrid cross between two Drosophila fruit flies. </li></ul><ul><li>A hetereozygous long-winged, thin-legged fly is crossed with a vestigial-winged, thick-legged fly. </li></ul><ul><ul><li>Wing length (long = L, vestigial = l) </li></ul></ul><ul><ul><li>Leg thickness (thin = T, thick = t) </li></ul></ul><ul><li>Take it that the two genes are completely linked together. </li></ul><ul><li>What would be the expected F1 phenotype ratio? </li></ul>
  24. 24. Linkage <ul><li>The actual ratios are given in figure 14.5 on page 98. </li></ul><ul><ul><li>How do these differ from your expected ratio? </li></ul></ul><ul><ul><li>How can this be explained? </li></ul></ul>
  25. 25. Frequency of Recombination <ul><li>The frequency of crossing over between two linked genes will increase with the distance between the two genes. </li></ul><ul><ul><li>The frequency of crossing over between two genes is proportional to the distance between them. </li></ul></ul><ul><li>The recombination frequency for two genes, expressed as a percentage, is a measure of crossing over between them, and it is defined as follows: </li></ul>Number of F 2 recombinants Total number of F 2 offspring x 100
  26. 26. Chromosome Maps <ul><li>A recombination frequency of 1% represents one unit of measure on the genetic map. </li></ul><ul><li>If genes A/a and B/b show a recombination frequency of 9%, then they will be 9 units apart from each other on the genetic map. </li></ul><ul><li>To construct a genetic map for a particular chromosome, a line is drawn and one gene is then placed on the map. </li></ul><ul><li>This acts as a reference point from which the positions or loci of all the other genes on the same chromosome are determined. </li></ul>
  27. 27. Chromosome Maps <ul><li>Draw a map for the following values: </li></ul>10 C/c x D/d 8 B/b x D/d 18 B/b x C/c 3 A/a x D/d 13 A/a x C/c 5 A/a x B/b Recombination Frequency (%) Genes
  28. 28. Chromosome Maps <ul><li>Identify the two genes that are most distantly linked </li></ul><ul><li>Identify the gene that is closest to the one that you have placed on the left of the map </li></ul><ul><li>Now place the remaining gene on the map. </li></ul><ul><li>Finally, complete the map by indicating the map distance between each gene. </li></ul>
  29. 29. Sex Linkage <ul><li>In addition to their role in determining sex, the sex chromosomes have genes for many characters. </li></ul><ul><li>Genes located on a sex chromosome are called sex linked genes. </li></ul><ul><li>In humans the term usually refers to X-linked characters: genes located only on X chromosomes. </li></ul><ul><li>Fathers can pass X-linked alleles to their daughters, but not sons. </li></ul><ul><li>Mothers can pass sex-linked alleles to both sons and daughters. </li></ul>
  30. 30. Sex Linkage <ul><li>If a sex linked trait is due to a recessive allele: </li></ul><ul><ul><li>A female will express the phenotype only if she is homozygous recessive. </li></ul></ul><ul><ul><li>If a male receives the recessive allele from his mother he will express the phenotype. </li></ul></ul><ul><li>Far more males have disorders that are inherited as sex linked recessives than females. </li></ul><ul><ul><li>Examples in humans: Colour blindness & Haemophilia </li></ul></ul>
  31. 31. Red-green colour blindness <ul><li>X chromosome has a locus for colour vision with two alleles: </li></ul><ul><ul><li>X N = Normal colour vision </li></ul></ul><ul><ul><li>X n = Red-green colour blindness </li></ul></ul><ul><li>Y chromosome does not have a colour vision locus. </li></ul><ul><li>If a male receives the X n allele he will have impaired colour vision, whereas a female with X N X n will not. </li></ul>
  32. 32. Red-green colour blindness <ul><li>Parental </li></ul><ul><li>Phenotypes Carrier Female x Normal Male </li></ul><ul><li>Genotypes X N X n X N Y </li></ul><ul><li>Gametes </li></ul><ul><li>Offspring 1 </li></ul><ul><li>Genotypes </li></ul>Phenotypes Normal Female : Carrier Female : Normal Male : Colour blind Male 1 : 1 : 1 : 1 X N X n X N Y X n Y X N Y Y X N X n X N X N X N Male Gametes X n X N Female Gametes
  33. 33. Showing genetic pedigree <ul><li>Unaffected female: </li></ul><ul><li>Affected female: </li></ul><ul><li>Unaffected male: </li></ul><ul><li>Affected male: </li></ul>1 2 I 1 2 3 4 5 II 1 2 3 4 5 III
  34. 34. Haemophilia <ul><li>What is haemophilia? </li></ul><ul><li>Why is haemophilia described as a sex-linked condition? </li></ul><ul><ul><li>Use the symbols X H , X h and Y to draw a diagram to show the F 1 generation that would result from a cross between a normal mother and a haemophiliac father. </li></ul></ul><ul><ul><li>One of the sons of this couple marries a carrier woman. What proportion of their sons are likely to be haemophiliacs? </li></ul></ul>
  35. 35. Practice Questions <ul><li>Page 90 Q5 </li></ul><ul><li>Page 101 Q2 </li></ul><ul><li>Page 103 Q6 & 7 </li></ul><ul><li>Page 108 Q5 & 6 </li></ul>