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Mendel and the Gene

Mendel and the Gene. Chapter 13. Mendel. Mendel. Discovered the basic principles of heredity bred garden peas in carefully planned experiments Because they are available in many varieties Because he could strictly control which plants mated with which

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Mendel and the Gene

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  1. Mendel and the Gene Chapter 13

  2. Mendel

  3. Mendel • Discovered the basic principles of heredity • bred garden peas in carefully planned experiments • Because they are available in many varieties • Because he could strictly control which plants mated with which • Looked for specific characters and variants in that character

  4. Mendel • Peas self-pollinate • Removed the male reproductive organs from the flower • Cross-pollinated with a different plant

  5. Mendel • Chose seven easily recognizable traits: • seed shape, seed color, pod shape, pod color, flower color, flower and pod position, and stem length • Each trait had two forms, or phenotypes

  6. Mendel • Worked with pure lines • Created hybrids—a mix of two different pure lines • Did quantitative and qualitative analysis

  7. Mendel • Followed the traits for at least 3 generations • The true-breeding parents • Are called the P generation • The hybrid offspring of the P generation • Are called the F1 generation • When F1 individuals self-pollinate • The F2 generation is produced

  8. P Generation (true-breeding parents)  Purple flowers White flowers F1 Generation (hybrids) All plants had purple flowers F2 Generation Mendel’s Experiments EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F1 hybrids were allowed to self-pollinate or were cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation. RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.

  9. Mendel’s Experiments • When Mendel crossed contrasting, true-breeding white and purple flowered pea plants • All of the offspring were purple • When Mendel crossed the F1 plants • Many of the plants had purple flowers, but some had white flowers • A ratio of about three to one, purple to white flowers, in the F2 generation

  10. Mendel’s Experiments • Mendel reasoned that • In the F1 plants, only the purple flower factor was affecting flower color in these hybrids • Purple flower color was dominant, and white flower color was recessive • White flowers were evidence that the white factor was still inherited it was just not shown • Observed the same pattern in many other pea characteristics

  11. Relation Between Genotype and Phenotype • Dominant and recessive alleles • Do not really “interact” • Lead to synthesis of different proteins that produce a phenotype • The recessive form may be the lack of an enzyme • Wrinkled peas or round peas

  12. Heredity

  13. Heredity • Until the 20th century, many biologists erroneously believed that • Particles called pangenes traveled from each part of an organisms body to the eggs and sperm and are then passed on to the offspring • Characteristics acquired during lifetime could be passed on to offspring • Characteristics of both parents blended irreversibly in their offspring

  14. Mendel’s Theories of Inheritance • Did not know what genes were • Did not know what DNA was • Proposed that hereditary determinants maintain their integrity from generation to generation • Contradicted the blending inheritance and inheritance by acquired characters hypotheses

  15. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Mendel’s Concepts • First, alternative versions of genes • Account for variations in inherited characters, which are now called alleles

  16. Mendel’s Concepts • Second, for each character an organism inherits two alleles, one from each parent • A genetic locus is actually represented twice • May have identical alleles or different alleles

  17. Mendel’s Concepts • Third, if the two alleles at a locus differ • Then one, the dominant allele, determines the organism’s appearance • The other allele, the recessive allele, has no noticeable effect on the organism’s appearance • Recessive allele is still inherited

  18. The Principle of Segregation • Alleles segregate into different gametes during gamete formation (haploid) • Diploid condition is restored when an egg is fertilized by a sperm to form a zygote • Two alleles per organism, can be homozygous or heterozygous • Punnett square is used to predict the genotypes and phenotypes

  19. Monohybrid Cross

  20. Monohybrid Cross

  21. Phenotype Genotype Purple PP (homozygous) 1 Pp (heterozygous) 3 Purple 2 Pp (heterozygous) Purple pp (homozygous) White 1 1 Ratio 3:1 Ratio 1:2:1 Phenotype vs. Genotype

  22. Law of Independent Assortment • Mendel identified his second law of inheritance by following two characters at the same time • Crossing two, true-breeding parents differing in two characters • Produces dihybrids in the F1 generation, heterozygous for both characters • Shows that the two genes are inherited independently

  23. TESTCROSS: GENOTYPES B_ bb Two possibilities for the black dog: BB or Bb B B b GAMETES b Bb b Bb bb OFFSPRING All black 1 black : 1 chocolate Test Cross • If the genotype of is not known, scientists can use controlled breeding to reveal the genotype • The offspring reveal the genotype of an individual

  24. Chromosome Theory of Inheritance • States that Mendel's rules can be explained by independent alignment and separation of homologous chromosomes at meiosis I

  25. Exceptions to Mendel’s Rules

  26. Sex Chromosomes • The X and Y chromosomes are the sex chromosomes • They determine the sex of the offspring (XX = female; XY = male)

  27. Sex-Linked Genes • Sex chromosomes also carry genes, any gene located on the sex chromosome is called a sex-linked gene • Most are found on X chromosomes

  28. QueenVictoria Albert Alice Louis Alexandra CzarNicholas IIof Russia Alexis Sex-linked Disorders • A high incidence of hemophilia has plagued the royal families of Europe

  29. Sex-linked Disorders • Most sex-linked human disorders are due to recessive alleles • Examples: hemophilia, red-green color blindness • These are mostly seen 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

  30. Linked Genes • Genes that are located close together on the chromosome and tend to be inherited together are called linked genes • Do not follow Mendel’s principles

  31. New Alleles Produced by Crossing Over • Crossing over recombines linked genes into assortments of alleles that are not found in the parent • Produces new recombinant gametes

  32. Linkage Map • Crossing over is more likely to occur between genes that are farther apart

  33. Linkage Map • Recombination frequencies can be used to map the relative positions of genes on chromosomes • Distance between two genes is designated by centiMorgans (cM) • One cM and represents the physical distance that produces 1% recombinant offspring

  34. Mutant phenotypes Shortaristae Black body (g) Cinnabar eyes (c) Vestigial wings (l) Browneyes Long aristae(appendageson head) Gray body (G) Red eyes (C) Normal wings (L) Redeyes Wild-type phenotypes

  35. Incomplete Dominance • Alleles of a gene are not always clearly dominant or recessive • incomplete dominance • the heterozygotes have an intermediate phenotype

  36. Codominance • A heterozygous organism that displays the phenotype of both alleles of a single gene • Neither allele is dominant or recessive to the other

  37. Multiple Alleles and Polymorphic Traits • Some genes have more than two alleles, a situation known as multiple allelism. • Polymorphic traits- When different combinations of alleles produce more than two distinct phenotypes • ABO blood types in humans

  38. 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 Polymorphic Traits

  39. Pleiotropy • A gene that influences many traits rather than just one is pleiotropic. • Marfan syndrome • Genetic disorder of the connective tissue • increased height • disproportionately long limbs and fingers • abnormally shaped chest • potentially severe heart problems

  40. 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 Brain damage Damage to other organs Spleen damage Anemia Heart failure Pain and fever Impaired mental function Pneumonia and other infections Kidney failure Rheumatism Paralysis Pleiotropy

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