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Understanding Gregor Mendel's Pea Experiments: Laws of Inheritance & Genetic Variations

Explore Gregor Mendel's groundbreaking pea experiments from 1856-1863 that revealed the principles of genetics including segregation, independent assortment, mono- and dihybrid crosses. Discover genetic terms, laws, phenotypes, genotypes, and variations like incomplete dominance and pleiotropy.

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Understanding Gregor Mendel's Pea Experiments: Laws of Inheritance & Genetic Variations

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

  2. 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

  3. 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

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

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

  6. 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

  7. Y Y F1 generation: YY y y Punnett square shows parental gametes and genotypes of next generation Yy Yy yy F2 generation three genotypes YY Yy Yy yy two phenotypes yellow green

  8. 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)

  9. 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

  10. Monohybrid crosses in Mendel’s peas What happens in dihybrid crosses? - parents differ in genes for 2 traits

  11. 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

  12. 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

  13. 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 BbNn BbNn MATING OF HETEROZYOTES(black, normal vision) 3 black coat,blind (PRA) 1 chocolate coat,blind (PRA) 3 chocolate coat,normal vision 9 black coat,normal vision PHENOTYPIC RATIO OF OFFSPRING Figure 9.5B

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

  15. 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

  16. 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

  17. 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

  18. 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

  19. 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

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

  21. Blood contains genetically determined proteins • A foreign protein (antigen) may be attacked by the immune system Blood is “typed” by using antibodies that will cause blood with certain proteins to clump (agglutination)

  22. ABO blood types Figure 9.13x

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

  24. 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

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

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

  27. Environmental Effects on Phenotype • Genotype and environment can interact to affect phenotype • Himalayan rabbit ice pack experiment • Transplantation of plant cuttings to different elevations • Human depression

  28. 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

  29. 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 • If a male receives a single X-linked recessive allele from his mother, he will have the disorder; while a female has to receive the allele from both parents to be affected Figure 9.23A

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

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

  32. Variations on Mendel’s Principles • Codominance, multiple alleles • Pleiotropy • Polygenic traits • Sex-linked genes • Environmental effects

  33. 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 Uterus Cervix Karyotyping Cell culture Figure 9.10A

  34. 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

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

  36. 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

  37. 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

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

  39. Table 9.9

  40. 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

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