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Genetics

Explore the study of heredity and genetics, including Gregor Mendel's experiments with garden peas and the principles of dominance and segregation. Learn how to predict genetic outcomes using Punnett squares.

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Genetics

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  1. Genetics

  2. What makes each species unique? Heredity: The passing of genetic traits from parent to offspring. The scientific study of heredity is known as: genetics

  3. The History: • Until the nineteenth century the explanation for why offspring resemble their parents was the blending inheritance model. • Blending inheritance: • The hypothesis that factors from both parents mix in the offspring. Like mixing yellow and blue to make green

  4. Gregor Mendel • (1822-1884): • The founder of modern genetics. • Worked with garden peas because: • They have easily distinguished traits & • self-fertilize in nature

  5. Worked with garden peas because: • All were true-breeding: • self-fertilization produces offspring identical to the parent.

  6. Mendel cross-pollinated the plants • by mating his different true breeding varieties with each other • Mendel’s Experiments: • Studied traits: 7 characteristics that all had two distinct forms • Trait:a specific characteristic • that varies from one individual to another

  7. Original pair of plants were called P (Parental) • Offspring were called F1 (first Filial) • Hybrids: the offspring • that result from crosses • between parents with different traits

  8. When crossing two different parents, what were the offspring like? P: tall plant X short plant F1: All tall offspring

  9. Mendel’s Conclusions: • 1. Inherited traits are determined by factors that are passed from one generation to the next. • Today these factors are called genes.

  10. Mendel’s Conclusions: Alleles:The different forms of a gene – each is given a letter. Ex: The gene for height has a tall plant allele (T) and a short plant allele (t).

  11. Mendel’s Conclusions: • 2. The principle of dominance: Some alleles are dominant and others are recessive • If an organism has adominant formof an allele for a trait, the organism will always show that form.Dominant alleles are written with a capital letter.

  12. If an organism has a recessive form of a trait, it will only show if the dominant allele is NOT present.Recessive alleles are written with a lower-case letter.

  13. Let’s look back at Mendel’s experiment. The cross of tall and short plants resulted in all tall hybrid offspring. The recessive allele reappeared in the F2 generation! parental F1 F2

  14. Mendel’s Conclusions: • 3. Principle of Segregation: • During gamete formation, alleles aresegregatedfrom each other so that each gamete carries only asingle copy of each gene. • The alleles are paired up again when the gametes fuse during fertilization.

  15. Principle of Segregation: X Tt F1 Tt Gametes T t t T F2 TT Tt Tt tt

  16. Probability and Punnett Squares: • Mendel realized that the principle of probability could be used to explain the results of his genetic crosses. • Probability: The likelihood that a particular event will occur • What is the probability of flipping heads twice? • The principles of probability can be used • to predict the outcomes of genetic crosses • We can determine the probable results • of a genetic cross by drawing a • Punnett square ½ X ½ = ¼

  17. Let’s cross the F1 generation plants from Mendel’s garden F1 Parents: Tt x Tt Two types of gametes produced by each parent T t T TT Tt t Tt tt

  18. Homozygous: having two of the same allele. • Ex: TT or tt • Heterozygous: having two different alleles. • Ex: Tt • Phenotype: the physical characteristics of the organism • Ex: “Tall plant” or “short plant” • Genotype: The genetic makeup of an individual (the actual alleles) • Ex: TT or Tt

  19. Note: If a plant has a tallphenotype, it can have one of two genotypes. Phenotype: tall plant; Genotype: TT OR Tt

  20. Fill in the table below with the appropriate phenotype or genotype for the given trait tall TT or Tt tt short BB or Bb black red bb

  21. Problem: Cross a heterozygous tall plant with a heterozygous tall plant. Use the letter “T.” P: ___ X ___ Gametes: x Tt Tt T t T t T t Tt T TT t Tt tt TT, Tt & tt Genotypes of offspring: Phenotypes of offspring: Genotypic ratio: (example 1 TT : 1 tt) Phenotypic ratio: (example 1 tall : 1 short) Tall & short 1TT: 2Tt: 1tt 3 tall: 1 short

  22. Note: The larger the number of individuals in the sample, the closer the results will be to the expected ratio.

  23. Try a monohybrid (single-trait cross) problem: In rabbits, brown fur (B) is dominant to white fur (b). Cross a homozygous brown furred rabbit with a white furred rabbit and determine the genotypic and phenotypic ratios resulting from the cross. Use a Punnett Square to show your work. P: __________________X_______________ BB bb b b Bb Bb Phenotypic ratio: Genotypic ratio: B all brown B Bb Bb all Bb

  24. If one of the parent rabbits has white fur and the other has brown fur, is there anyway they could get an offspring with white fur? Do a cross to support your answer. P: __________________X_______________ B b Phenotypic ratio: Genotypic ratio: 1 brown: 1 white bb b Bb 1Bb: 1bb Bb b bb

  25. Test cross: a cross with a recessive individual and a dominant individual to see if a dominant individual is heterozygous or homozygous.

  26. When two traits are inherited, are the two characteristics inherited as a package or are they inherited independently of each other? • The Two-Factor Cross: • Mendel crossed true-breeding plants with round yellow peas with plants that had wrinkled green peas. (Use “R” for shape and “Y” for color) • P cross: RRYY x rryy • F1 cross:RrYy x RrYy • F2 generation: Produced 556 seeds • 315 round and yellow • 32 were wrinkled and green • 209 had combinations of traits different from both original parents.

  27. 4. Principle of Independent Assortment: Each pair of alleles segregates independently of the other pairs of alleles during gamete formation. Let’s show how this occurs: Gametes: RY RyrYry Ry rY ry RY RrYy RY RRYY RRYy RrYY RRYy RRyy RrYy Ry Rryy rY RrYY RrYy rrYY rrYy ry Rryy rrYy rryy RrYy

  28. The expected phenotypic ratio in the F2 generation: 9 Round Yellow: 3 Round Green: 3 Wrinkled Yellow: 1 Wrinkled Green Mendel’s results were very close to this expected ratio.

  29. Let’s try another Dihybrid Cross: Brown eyes (B) and tongue rolling (R) are dominant traits in humans. These traits are dominant over blue eyes and non-tongue rolling. A heterozygous brown eyed, non-tongue roller female marries a blue eyed, heterozygous tongue roller male and they have children. What is the genotype of the wife? What is the genotype of the husband?

  30. Exceptions to Mendel: So far all of our problems have dealt with complete dominance. However, some alleles are neither dominant nor recessive, and many are controlled by multiple alleles or multiple genes. 1. Incomplete dominance: one allele is not completely dominant over another The heterozygous phenotype is in between the two homozygous phenotypes Example: In the Japanese 4 o’clock flower , a red flower (R) crossed with a white flower (W) will produce offspring with pink flowers (a blending of the phenotypes) R R RW RW W Genotypic ratio: Phenotypic ratio: All RW RW W RW All pink

  31. Exceptions to Mendel: 2. Co-dominance: both alleles contribute to the phenotype Example: In cattle, red hair and white hair are codominant. Cattle with both alleles are roan. (both alleles appear in the phenotype: red hairs & white hairs) Cross a Roan cow with a Red Bull: HR HR Genotypic ratio: Phenotypic ratio: HR HR HR HR HR 1HRHR: 1HRHW HR HW HW HR HW 1Red: 1Roan

  32. Exceptions to Mendel: 3. Multiple alleles: Some genes have more than just 2 alleles Example: human blood has three alleles: A, B, and O. Human Blood Types (Phenotype:Genotype) A blood: IAIA OR IAi B blood: IBIB OR IBi AB blood: IAIB O blood: ii

  33. As long as we are talking about blood types…. Rh Blood Group (not a case of multiple alleles) Phenotype: Genotype Rh+: Rh+ Rh+ or Rh+ Rh- Rh-: Rh- Rh-

  34. Exceptions to Mendel: 4. Polygenic traits: One trait is controlled by 2 or more genes Example: Human height, skin color

  35. Exceptions to Mendel: 5. Pleiotropy: One gene influences multiple characteristics Example: Sickle-cell anemia

  36. Sex-linkage: genes located on one of the sex chromosomes are called sex-linked. Example: hemophilia & red green colorblindness There are two types of chromosomes in animals: Autosomes: The chromosomes that are not involved in determining gender. Sex chromosomes: The “mismatched” chromosomes that determine the gender of the organism (Female = XX ; Male = XY)

  37. Colorblindness is a recessive sex-linked trait Genotypes of sex-linked traits are written with a superscript: XC, Xc Write the following genotypes: Colorblind male: ___________ Female carrying the colorblind gene: ___________ Male with normal vision: ___________ XcY XCXc XCY

  38. Example: The gene for eye color in fruit flies is located on the X chromosome. Red eye color is dominant to white eye color. A white eyed male is crossed with a homozygous red eyed female. Determine the genotypes and phenotypes of the F1 generation. XrY X XRXR Genotypic Ratio: Phenotypic Ratio: Xr Y 1XRXr: 1XRY XR XRXr XRY XR XRY 1 red eyed female: 1 red eyed male XRXr

  39. The Chromosome Theory of Heredity Where in the cell are the factors that control heredity? Where are the genes? Chromosomal theory of heredity: Genes occupy specific positions on chromosomes. • Chromosomes undergo independent assortment & segregation

  40. Linked Genes: • Genes are linked together in chromosomes • Linkage groups: Genes that are close together are likely to be inherited together a chromosome

  41. Thomas Hunt Morgan: (1900’s) Worked with fruit flies, In fruit flies Gray bodies (G) is dominant over black (g) and Normal wings (N) are dominant over vestigial wings (n). He first crossed a homozygous gray, normal winged fly (GGNN) with a black, vestigial winged fly (ggnn). He expected the F1 flies to be: He then performed a test cross with the F1 generation. Drosophila melangaster gray with normal wings • All F1 flies were gray w/ normal wings GgNn X ggnn

  42. GgNn X ggnn GN Gn gN gn gn GgNn Ggnn ggNn ggnn Phenotypic ratio: 1 gray normal: 1 gray vestigial: 1 black normal: 1 black vestigial

  43. Morgan “scored” 2300 offspring from the matings: 575 575 575 575 965 949 206 180 Genes are inherited together

  44. The results indicate that the genes for body color and wing size • Why don’t all of the offspring have the same genotypes as their parents? are linked.

  45. Crossing over: exchange of information between homologous chromosomes during meiosis • Crossing over breaks linkages of genes

  46. Gene map: A diagram of chromosomes showing relative locations of genes • Genes close together are likely to be inherited together • Genes that are far apart are more likely to be separated by crossing over

  47. We can calculate the relative distance between genes using the following method: • 1. Perform a test cross • 2. Count the total number of offspring. • 3. Count the number of recombinants • 4. Determine the recombination frequency • Recombination frequency = • # of recombinants/total offspring x 100 = % • 5. Every 1% indicates a single map unit (mu) • Calculate the distance between the body color and wing shape genes. 386/2300 X 100 = 16.8 % 16.8 mu

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