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Meiosis - Why have sex?

Meiosis - Why have sex?. Causes an increase in genetic variation within a population. Independent assortment of homologues Crossovers (recombination) Random fertilization. Why not sex? 1. High energy cost to produce gametes that are not used. 2. Must find a mate.

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Meiosis - Why have sex?

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  1. Meiosis - Why have sex? Causes an increase in genetic variation within a population Independent assortment of homologues Crossovers (recombination) Random fertilization

  2. Why not sex? 1. High energy cost to produce gametes that are not used. 2. Must find a mate. Asexual reproduction 1. Preserves genotype 2. No mate required

  3. Gregor Mendel Peas, please • Segregation of alleles Shown by monohybrid crosses • Independent assortment of alleles shown by dihybrid crosses

  4. Basic ideas: Genetic elements come in pairs Elements do not change over generations Pairs separate when gametes form • Gregor Mendel - pea research done 1856-1863 Figure 9.2Ax

  5. White 1 Removed stamensfrom purple flower • Pea plants: • Self- or cross-pollinate • Rapid life cycle • Variety of traits Stamens Carpel 2 Transferred pollen from stamens of white flower to carpel of purple flower PARENTS(P) Purple 3 Pollinated carpel matured into pod 4 Planted seeds from pod • Shown here: cross-fertilization OFF-SPRING(F1) Figure 9.2C

  6. Phenotype - appearance or function of body Genotype - genes that determine the phenotype

  7. P = parental generation • F1= first progeny generation (filial) • F2= second progeny generation • Monohybrid cross = parents differ in one gene

  8. female male yy YY yy YY yy YY yy YY P generation y y Y Y y y Y Y female gametes male gametes y Y Yy y Y Yy Yy possible outcomes in fertilization Yy Yy Yy Yy Yy

  9. Y Y YY y y Yy Yy yy three genotypes YY Yy Yy yy two phenotypes yellow green

  10. 1st law - segregation of alleles • Cells contain 2 copies of each gene (alleles) • Alleles do not blend (dominant, recessive) • Alleles separate during gamete formation (meiosis)

  11. F1 generation Yy self-pollination YY Yy Yy Yy Yy F2 generation yy yy Yy YY “pure” yellow “pure” green mixed yy YY YY YY yy Yy yy yy YY YY Yy Yy yy yy YY F3 generation yy YY yy Yy YY

  12. What happens in dihybrid crosses? - parents differ in genes for 2 traits

  13. HYPOTHESIS: DEPENDENT ASSORTMENT HYPOTHESIS: INDEPENDENT ASSORTMENT RRYY rryy PGENERATION RRYY rryy ry ry Gametes RY Gametes RY F1GENERATION RrYy RrYy Eggs 1/2 RY 1/2 RY Sperm Eggs 1/4 RY 1/4 RY 1/2 ry 1/2 ry 1/4 rY 1/4 rY RRYY 1/4 Ry 1/4 Ry RrYY RrYY F2GENERATION 1/4 ry 1/4 ry RRYy rrYY RrYy Yellow round RrYy RrYy RrYy RrYy 9/16 Actual resultscontradict hypothesis Green round rrYy RRyy rrYy 3/16 ACTUAL RESULTSSUPPORT HYPOTHESIS Yellow wrinkled Rryy Rryy 3/16 Green wrinkled rryy 1/16 Figure 9.5A

  14. Law of Independent Assortment • During gamete formation, genes for different traits separate independently into gametes • Why? random alignment of homologues at Meiosis I • A sperm or egg carries only one allele of each pair

  15. Independent assortment of two genes in the Labrador retriever Blind Blind Black coat, normal visionB_N_ Black coat, blind (PRA)B_nn Chocolate coat, normal visionbbN_ Chocolate coat, blind (PRA)bbnn PHENOTYPES GENOTYPES MATING OF HETEROZYOTES(black, normal vision) BbNn BbNn 9 black coat,normal vision 3 black coat,blind (PRA) 3 chocolate coat,normal vision 1 chocolate coat,blind (PRA) PHENOTYPIC RATIO OF OFFSPRING Figure 9.5B

  16. The offspring of a testcross can reveal the genotype of a parent. TESTCROSS: GENOTYPES B_ bb Two possibilities for the black dog: BB or Bb B B b GAMETES b Bb b Bb bb OFFSPRING Figure 9.6 All black 1 black : 1 chocolate

  17. Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel’s principles and the rules of probability Figure 9.8A

  18. Family pedigrees are used to determine patterns of inheritance and individual genotypes Dd Joshua Lambert Dd Abigail Linnell D_ JohnEddy ? D_ HepzibahDaggett ? D_ Abigail Lambert ? dd JonathanLambert Dd Elizabeth Eddy Dd Dd dd Dd Dd Dd dd Female Male Deaf Hearing Figure 9.8B

  19. Connection: Many inherited disorders in humans are controlled by a single gene Normal Dd Normal Dd PARENTS • Most such disorders are caused by autosomal recessive alleles • Examples: cystic fibrosis, sickle-cell disease D D Eggs Sperm DD Normal d d Dd Normal (carrier) Dd Normal (carrier) OFFSPRING dd Deaf Figure 9.9A

  20. Examples: achondroplasia, Huntington’s disease • A few are caused by dominant alleles Figure 9.9B

  21. Table 9.9

  22. Connection: Fetal testing can spot many inherited disorders early in pregnancy • Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions • Fetal cells can be obtained through amniocentesis Amnioticfluidwithdrawn Centrifugation Amnioticfluid Fluid Fetalcells Fetus(14-20weeks) Biochemicaltests Placenta Severalweeks later Figure 9.10A Uterus Cervix Karyotyping Cell culture

  23. Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping Fetus(10-12weeks) Several hourslater Placenta Suction Karyotyping Fetal cells(from chorionic villi) Some biochemical tests Chorionic villi Figure 9.10B

  24. Examination of the fetus with ultrasound is another helpful technique Figure 9.10C, D

  25. Chromosome behavior accounts for Mendel’s principles Figure 9.17

  26. A B a b B A • Genes on the same chromosome tend to be inherited together = linked genes • Crossing over produces gametes with recombinant chromosomes a b A b a B Tetrad Crossing over Gametes

  27. Geneticists use crossover data to map genes • Crossing over is more likely to occur between genes that are farther apart • Recombination frequencies Chromosome g c l 17% 9% 9.5% Figure 9.20B

  28. VARIATIONS ON MENDEL’S PRINCIPLES Incomplete dominance P GENERATION Whiterr Red RR • an offspring’s phenotype is intermediate between the phenotypes of its parents Gametes R r PinkRr F1 GENERATION 1/2 R 1/2 r 1/2 R 1/2 R Eggs Sperm RedRR 1/2 r 1/2 r PinkRr PinkrR F2 GENERATION Whiterr Figure 9.12A

  29. GENOTYPES: • Incomplete dominance in human hypercholesterolemia HH Homozygousfor ability to makeLDL receptors Hh Heterozygous hh Homozygousfor inability to makeLDL receptors PHENOTYPES: LDL LDLreceptor Cell Normal Mild disease Severe disease Figure 9.12B

  30. Many genes have more than two alleles in the population The three alleles for ABO blood type in humans is an example

  31. The alleles for A and B blood types are codominant, and both are expressed in the phenotype. The O allele is recessive. BloodGroup(Phenotype) AntibodiesPresent in Blood Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Genotypes O A B AB Anti-A Anti-B O ii IA IA or IA i A Anti-B IB IB or IB i B Anti-A AB IA IB Figure 9.13

  32. ABO blood types Figure 9.13x

  33. pleiotropy is when a single gene affects phenotype in many ways • Ex. sickle-cell disease - hemoglobin • Ex. Marfan syndrome - fibrillin The gene’s effects may be dependent on environment, and not be simultaneous.

  34. Individual homozygousfor sickle-cell allele Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped Sickle cells Clumping of cells and clogging of small blood vessels Accumulation ofsickled cells in spleen Breakdown of red blood cells Physical weakness Heart failure Pain and fever Brain damage Damage to other organs Spleen damage Anemia Figure 9.14 Pneumonia and other infections Impaired mental function Kidney failure Rheumatism Paralysis

  35. A single characteristic may be influenced by many genes • This situation creates a continuum of phenotypes • Quantitative traits • Example: skin color, height

  36. P GENERATION aabbcc(very light) AABBCC(very dark) F1 GENERATION AaBbCc AaBbCc Eggs Sperm Fraction of population Skin pigmentation F2 GENERATION Figure 9.16

  37. The X-O system • Chromosomes determine sex in many species • The Z-W system • Chromosome number Figure 9.21B-D

  38. Sex-linked genes exhibit a unique pattern of inheritance • All genes on the sex chromosomes are said to be sex-linked • the X chromosome carries many genes unrelated to sex • Ex. Fruit fly eye color Figure 9.22A

  39. Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles • Ex: hemophilia, red-green color blindness • mostly in males • A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected Figure 9.23A

  40. A high incidence of hemophilia has plagued the royal families of Europe QueenVictoria Albert Alice Louis Alexandra CzarNicholas IIof Russia Alexis Figure 9.23B

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