Your SlideShare is downloading. ×
  • Like
  • Save
1. Meiosis And Dihybrid Cross
Upcoming SlideShare
Loading in...5
×

Thanks for flagging this SlideShare!

Oops! An error has occurred.

×

Now you can save presentations on your phone or tablet

Available for both IPhone and Android

Text the download link to your phone

Standard text messaging rates apply

1. Meiosis And Dihybrid Cross

  • 12,969 views
Published

 

  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Be the first to comment
No Downloads

Views

Total Views
12,969
On SlideShare
0
From Embeds
0
Number of Embeds
0

Actions

Shares
Downloads
0
Comments
0
Likes
4

Embeds 0

No embeds

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
    No notes for slide

Transcript

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