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Heredity – passing traits to offspring

Heredity – passing traits to offspring. Chapters 11, 12. Kevin Bleier Milton HS, GA. How produce offspring?. Two major modes of reproduction Asexual reproduction Sexual reproduction. section 11.1. Asexual reproduction.

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Heredity – passing traits to offspring

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  1. Heredity – passing traits to offspring Chapters 11, 12 Kevin Bleier Milton HS, GA

  2. How produce offspring? • Two major modes of reproduction • Asexual reproduction • Sexual reproduction section 11.1

  3. Asexual reproduction • One parent making exact genetic copy of itself (offspring are clone of parent) • Advantages: quick, no need for mate, both males and females can produce offspring • Disadvantages: no genetic diversity in offspring

  4. Asexual reproduction • Many organisms can do this – plants, fungi, protists, some animals, bacteria • What type of eukaryotic cell division creates exact copies of cells? • What type of prokaryotic cell division? mitosis binary fission

  5. Sexual reproduction • Two parents both contribute half of their genes to offspring • Which half they contribute can be different each time = genetically DIFFERENT offspring

  6. Sexual reproduction • Advantages: genetic diversity • Why is this important? • Disadvantages: finding a mate, only females bear children, more energy expense more to come in evolution unit next

  7. Sexual reproduction • Who reproduces sexually? • Most multicellular eukaryotes can reproduce sexually (protists, fungi, plants, animals)

  8. Sexual reproduction • What type of cell division produces sperm and egg cells needed for sex? meiosis (focus of section 11.2) germ line cells gametes (sperm / egg) (beginning of meiosis) (end of meiosis)

  9. Sexual vs. asexual • Bacteria can ONLY reproduce asexually • Some organisms can ONLY reproduce sexually (like us humans) • MANY organisms can reproduce both ways … so when might they use one method?

  10. Sexual vs. asexual • Sexual reproduction: generating genetic diversity needed to overcome a challenge(threatening, dangerous, unstable environments) • Asexual reproduction: producing many offspring when conditions are stable (safe, stable environment with lots of resources)

  11. Preparation for meiosis discussion • Our goal: how do we make cells in preparation for sexual reproduction? • How does this process generate the genetic diversity important for sexual reproduction?

  12. Some vocabulary review

  13. Some vocabulary review • DNA – chemical code (order of letters) • gene – small segment of DNA letters that codes for a specific protein (that leads to a specific trait) • chromosome – entire strand of DNA that is packed up for cell division (carries 100s to 1000s of genes)

  14. Some vocabulary review T A C G G T A A T G C C R gene DNA code chromosome

  15. Some new vocabulary gene = seed shape 2 alleles: R = round seed r = wrinkled seed • Organisms’ body cells (somatic cells) have chromosomes that come in pairs • Pairs called homologous chromosomes(carry same genes, may carry different versions of gene = alleles) R R r r R r

  16. Some new vocabulary • Cells that contain homologous chromosome pairs = diploid • Somatic cells are diploid, as are germ-line cells that start meiosis • Gamete cells (sperm and egg) are haploid (only one of each chromosome, not pairs)

  17. Why haploid gametes? r R r R parent 1’s diploid germ-line cell parent 2’s diploid germ-line cell meiosis meiosis sexual intercourse so sexual reproduction overall = meiosis + sexual intercourse parent 1’s haploid gamete (sperm) parent 2’s haploid gamete (egg) R results in diploid zygote (which grows into new offspring) r

  18. Different species = different chromosome # • Diploid = 2n • Haploid = n • 2n for humans is 46 • n for dogs is 39

  19. Types of chromosomes • Human karyotype • 22 of 23 pairsall sexes have samegenes (autosomes) • 23rd pair determinessex (sex chromosomes) Males = XY Females = XX

  20. Meiosis – creating gametes • Two questions to answer while studying meiosis: • How does a diploid germ-line cell eventually become haploid gametes? • How does the process generate genetic variety? (making every gamete different) section 11.2

  21. Meiosis simulation • We will work with a diploid germ-line cell where 2n = 8 (or 4 pairs of homologous chromosomes) • So haploid number (n = ___ ) 4

  22. Meiosis simulation meiosis haploid gamete (n = 4) diploid germ-line cell (2n = 8)

  23. Meiosis • Copies all DNA at the beginning (like all cell divisions) • Two cell divisions and DNA divisions yields haploid cells

  24. 2 divisions in meiosis exact copies beginning of meiosis – diploid germ-line cell (here, 2n = 8) homologous pairs exist, just not organized or close together yet first step of any cell division – copy the DNA

  25. First meiotic cell division exact copies in meiosis I, put homologous pairs together homologous pairs

  26. First meiotic cell division in meiosis I, put homologous pairs together now, cell lines up pairs together (2 lines of Xs … DIFFERENT than mitosis!)

  27. First meiotic cell division in meiosis I, put homologous pairs together now, cell lines up pairs together (2 lines of Xs … DIFFERENT than mitosis!) any division of DNA must be symmetrical – here, meiosis I splits the homologouspairs

  28. Finishing meiosis I this cell has divided its DNA up equally, and now splits into 2 cells but meiosis is not finished yet, as both of these cells will divide again

  29. Second meiotic cell division Looks disorganized because chromosomes unpacked at end of meiosis I, then repacked for meiosis II Line up into 1 line of Xs (just like mitosis)

  30. Second meiotic cell division Meiosis II must reorganize chromosomes Line up into 1 line of Xs (just like mitosis) Meiosis II splits DNA evenly by splitting exact copies Each cell splits into 2, creating 4 haploid gametes at the end

  31. Summary of first question • How does a diploid germ-line cell eventually become haploid gametes? • Diploid germ-line cell copies all DNA • Then divides DNA in two separate divisions • Meiosis I separates homologous pairs • Meiosis II separates exact copies

  32. Meiosis – creating gametes • Two questions to answer while studying meiosis: • How does a diploid germ-line cell eventually become haploid gametes? • How does the process generate genetic variety? (making every gamete different) independent assortment and crossover

  33. Back to early meiosis I gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele B = yellow seed p = white allele t = short allele r = wrinkled allele b = green seed P P P Let’s label one gene on each homologous pair t R R R Remember, there are actually 100s / 1000s of genes on each pair t t T T T p p p b b b We also assume all heterozygous here –organisms can be homozygous (RR or rr) B B B r r r

  34. Independent assortment gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele B = yellow seed p = white allele t = short allele r = wrinkled allele b = green seed P P When homologous chromosomes pair up, they do so randomly R R t t Ultimate lineup will be different in every instance of meiosis T T p p b b B B r r

  35. Independent assortment gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele B = yellow seed p = white allele t = short allele r = wrinkled allele b = green seed P P p p Assuming this particular lineup … T T t t We will get gametes carrying these particular alleles B B b b R R r r gamete 1: pTbr gamete 2: PtBR

  36. Independent assortment gene = flower color gene = height gene = seed shape gene = seed color P = purple allele T = tall allele R = round allele B = yellow seed p = white allele t = short allele r = wrinkled allele b = green seed P P p p What if another round of meiosis had the “R” chromosomes lineup differently? T T t t We will get gametes carrying these particular alleles B B b b R R r r gamete 1: pTbr gamete 2: PtBR gamete 3: PtBr gamete 4: pTbR

  37. Independent assortment P P p p How many different gametes are possible if every pair can line up two different ways? 2 x 2 T T t t x 2 B B b b R R x 2 r r 16

  38. Independent assortment • How many homologous chromosome pairs do human germ line cells have? • So how many different gametes can every human make by lining them up differently every time? 23 pairs = 223 ~ 8,000,000 different gametes

  39. Independent assortment • Recall though that sexual reproduction requires 2 parents both making gametes • And we cannot choose which gametes fertilize – that’s also random ~ 8,000,000 possible sperm x 8,000,000 possible eggs ~ 64,000,000,000,000 possible zygotes for 2 parents ~ 6,500,000,000 people on Earth

  40. Crossing over Before meiosis I lineup of homologous pairs P P p p Inner, non-sister chromatids can get so close that tips of chromosomes exchange T T Whether or not this occurs in each pair is random every time meiosis occurs t t b b B B R R r r

  41. Results of crossover event PtBR PTBR ptbr pTbr P P p p All four gametes will be genetically different T T t t B B b b R R r r

  42. Meiosis – creating gametes • Two questions to answer while studying meiosis: • How does a diploid germ-line cell eventually become haploid gametes? • How does the process generate genetic variety? (making every gamete different) independent assortment and crossover

  43. Meiosis summary • Starts with 1 diploid germ-line cell, ends with 4 haploid gametes • Each gamete has one of the chromosome pairs, all genetically different in alleles that they carry • Sperm and egg gametes must combine to complete sexual reproduction

  44. Errors in meiosis = nondisjunction Diploid germ line cell 2n = 4 Haploid gametes should be n = 2 Some gametes have 1 extra (offspring would have too many chromosomes) Some gametes are missing 1 (offspring would have too few chromosomes)

  45. Nondisjunction effects • Example: Down syndrome (trisomy 21) • Extra or missing chromosome in all somatic cells has large effect on phenotype • Many trisomies / monosomies result in inviable embryo

  46. Overall human life cycle haploid gametes (n = 23) egg(ovum) sperm meiosis fertilization diploid germ-line cells (within sex organs) diploid zygote(single-cell)(2n = 46) diploid somatic cells(multicellular human) (2n = 46) mitosis and development section 11.3

  47. Where we are headed … • Punnett squares show all the possibilities of gametes that can be made in meiosis P P P P P P P P P p p p parent 1 parent 2 possible gametes: P, p possible gametes: P Pp Pp original diploid germ-line cells P P p p possible haploid gametes PP P PP Pp P PP Pp PP or possible diploid zygotes formed by fertilization of specific sperm and egg P PP Pp chapter 12

  48. Two – trait heredity analysis • We will assume that genes are on different chromosome pairs Parent 1: PpTt Parent 2: PPtt P P P P p p p p P P P P or T T T T t t t t t t t t possible gametes: PT, pt , Pt, pT possible gamete: Pt rules for making gametes: 1) half of what you started with 2) one of each chromosome pair (one of each letter)

  49. Two – trait Punnett square Parent 1: PpTt Parent 2: PPtt possible gametes: PT, pt, Pt, pT possible gamete: Pt PpTt PT Pt pT pt PT Pt pT pt Pt PPTt PPtt PpTt Pptt Pt PPTt PPtt PpTt Pptt Pt PPTt PPtt PpTt Pptt PPtt or PPTt PPtt PpTt Pptt Pt PPTt PPtt PpTt Pt Pptt

  50. Two – trait Punnett squares • Please don’t do this PpTt P T p t P PP Pp PT Pt P PP Pp PT Pt PPtt Pt pt Tt t tt t Pt Tt tt pt

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