4.5 Theoretical Genetics


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Mendelian genetics, Punnet square, sex-linkage

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4.5 Theoretical Genetics

  2. 2. Vocabulary preview genetics:  homozygous: fertilization:  heterozygous: true-breeding:  dominant: trait:  recessive: hybrid: gene: allele: segregation: gamete:
  3. 3.  genetics: The study of inheritance fertilization: true-breeding: trait: an observable physical characteristic hybrid: gene: a heritable factor that determines a specific characteristic allele: one of a number of different forms of a gene segregation: separation of the genes gamete: sex cells, i.e: ovules and sperm cells
  4. 4.  homozygous: having two identical alleles of a gene heterozygous: having two identical alleles of a gene dominant: an allele that has the same effect on the phenotype whether it is present in the homozygous or heterozygous state recessive: an allele that only has an effect on the phenotype when it is present in the homozygous state genotype: the alleles of an organism
  5. 5. Prior knowledge: traits
  6. 6. The work of Gregor Mendel In the 19th century, most biologists worked by observing and describing nature. Gregor Mendel was one of the first to apply an experimental approach to the question of inheritance. His work eventually gave rise to genetics, the study of heredity.
  7. 7. The blending hypothesis ofinheritance In the early 1800s, biologists proposed the blending hypothesis to explain how offspring inherit traits from both parents. For example, a red-flowered plant crossed with a yellow-flowered plant of the same species. According to the blending hypothesis, the red and yellow hereditary material in the offspring would blend, producing orange-flowered plants—like blending red and yellow paint to make orange paint
  8. 8. Mendel´s plant breedingexperiments For seven years, Mendel bred pea plants and recorded inheritance patterns in the offspring. Based on his results, he developed a particulate hypothesis of inheritance. This hypothesis states that parents pass on to their offspring separate and distinct factors (today called genes) that are responsible for inherited traits
  9. 9. Mendel´s Principle ofsegregation Hybrids: the offspring of two different true- breeding varieties (white flower and purple flower) The parental plants are called the P generation (P for parental), and the hybrid offspring are the F1 generation (F for filial). When the F1 plants self-fertilize or fertilize each other, their offspring are the F2 generation
  10. 10. Example of a monohybrid crossMonohybrid: Different in only one characteristic
  11. 11. Class activity Pick a pea pod Open it carefully Draw and describe the outside and the inside of your pod, what did you find?
  12. 12. Compare your pod to the chartIf you had to take a guess, would you say your pod is truebreeding or a hybrid? What evidence do you have?
  13. 13. Mendel´s hypothesis 1. There are alternative forms of genes. For example, the gene for flower color in pea plants exists in one form for purple and in another form for white = Alleles 2. For each inherited character, an organism has two alleles for the gene controlling that character, one from each parent. Alleles are the same = homozygous (AA). Alleles are different = heterozygous (Aa)
  14. 14. Mendel’s hypotheses 3. When only one of the two different alleles in a heterozygous individual appears to affect the trait = dominant allele (Aa). The other allele that does not appear to affect the trait = recessive allele (Aa). 4. The two alleles for a character segregate during the formation of gametes, so that each gamete carries only one allele for each character = Mendels principle of segregation.
  15. 15. Probability and Punnet Squares For a monohybrid cross = only one trait: P generation: both true breeding: AA x aa F1 generation: all hybrid: ½ Aa x ½ Aa F2 generation: ¼ AA, ½ Aa, ¼ aa AA x aa Aa x Aa AA Aa Aa aa
  16. 16. Probability Remember that during fertilization gametes combine randomly, so Mendel’s hypotheses represent a probability
  17. 17. Hands-on activity 2 coins, work in pairs Predict the outcome of 36 coin tosses. Write down the results for each flip combination: heads/heads, heads/tails, tails/heads, tails/tails Calculate the fraction of the total tosses for each combination Tally up the results for all student pairs Calculate the fraction of the tosses for the class total
  18. 18. Punnet’s square: F1 generation A punnet’s grid helps to explain the segregation of alleles and their effect on the organism’s phenotype 1. Draw the grid T T 2. Place the alleles 3. Fill the squares t T t T t 4. Compare dominantand recessive traits t T t T t
  19. 19. Punnet’s square: F2 generation A punnet’s grid helps to explain the segregation of alleles and their effect on the organism’s phenotype 1. Draw the grid T t 2. Place the alleles 3. Fill the squares T T T T t 4. Compare dominantand recessive traits t T t t t
  20. 20.  Remember dominant alleles are written with capital letters : T Recessive alleles are written with smallletters: tPhenotype: The traits expressed (seen) are dictated by the dominant alleles in the case of heterozygous individualsRecessive alleles influence the phenotype only when we have homozygous individuals: ttOnline activity: 10.2
  21. 21. Data-based questions: coat color in the house mouse In the early years of the 20th century, many crossing experiments were done in a similar way to those of Mendel. The French geneticist Lucien Cuénot used the house mouse, Mus musculus, to see whether the principles that Mendel had discovered also operated in animals. He crossed normal grey-colored mice with albino mice. The hybrid mice that were produced were all grey. These grey hybrids were crossed together and produced 198 grey and 72 albino offspring. 1. Calculate the ratio between grey and albino offspring, showing your working. 2. Deduce the color of coat that is due to a recessive allele, with two reasons for your answer. 3. Choose suitable symbols for the alleles for grey and albino coat and list the possible genotypes of mice, using your symbols, together with the phenotype for each genotype. 4. Suggest how one gene can determine whether the mice had grey fur and black eyes or white fur and red eyes.
  22. 22. Variations of inheritancepatterns Co-dominance: or intermediate inheritance There is no dominant-recessive, rather both dominants: CBCB and CWCW
  23. 23.  Both alleles affect the phenotype, however this does NOT support the blending hypothesis. Each allele codes for a particular characteristic, and both are then shown physically (like coding for the production for the production of a protein that gives color) The ratio in this case is 1:2:1 Homozygous dominant: hybrids: homozygous recessive
  24. 24. Multiple alleles Some genes have more than one allele. Example: blood type in humans. Blood groups: A, B, AB and O
  25. 25.  The four blood types result from various combinations of 3 alleles, symbolized as IA (for carbohydrate A), IB (for carbohydrate B), and i (for neither A nor B).
  26. 26.  Note that blood type O is recessive. The alleles for carbohydrate A and B are co- dominant, meaning they are both expressed in the phenotype (hence the group AB). Polygenic inheritance refers to many genes affecting a single characteristic, such as height and skin color in humans. Influence of the environment: the product of a genotype is generally not a single, rigidly defined phenotype, but a range of possibilities influenced by the environment.
  27. 27. Chromosome Theory ofinheritance  “Genes are located on chromosomes (locus), and the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns.”
  28. 28.  Chromosomes undergo segregation and independent assortment during meiosis. Every diploid organism has two sets of homologous chromosomes (one from mom and one from dad) The alleles of a gene reside in the same locus on both homologous chromosomes. Crossing over can recombine gene loci on homologous chromosomes. This is unlikely when the genes are very close together. A crossover is more likely to recombine the alleles when the genes are far apart
  29. 29. Genetic linkage and crossingover It’s the tendency for the alleles on one chromosome to be inherited together. The closer two genes are on a chromosome, the greater the genetic linkage. The farther apart the genes are, the more likely it is that a crossover event will separate them.
  30. 30. Sex-linkage Many species have sex chromosomes, X and Y, that are associated with determining the organism’s sex. Any gene located in a sex chromosome is a sex-linked gene. Discovered by Thomas Hunt Morgan while he was studying the inheritance of white eye color in fruit flies.
  31. 31. Thomas Hunt Morgan was studying theinheritance of white eye color in fruit flies.White eyes are very rare. Normally, fruitflies have red eyes.When he mated a white-eyed male fly witha red-eyed female fly, all the F1 offspringhad red eyes. The allele for red eyes wasdominant. When Morgan bred F1 offspringwith each other, he got the classical 3 : 1ratio of red-eyed to white-eyed flies in theF2 generation.A surprising twist: none of the flies withwhite eyes was female. Morgan realizedthat in these flies, eye color must
  32. 32.  Morgan deduced that the gene involved in this inheritance pattern is located only on the X chromosome. There is no corresponding eye color locus on the Y. Thus, females (XX) carry two copies of the gene for this character, while males (XY) carry XR Xr XR Xr only one.XR XRXR XRXr Xr XX XXY XRY XrY Y XY XY
  33. 33. Hemophilia: a famous example ofsex-linkage Sex linkage: The pattern of inheritance where there are differences in genotypes and phenotypic ratios between males and females. Hemophilia: the ability of the blood to clot is severely reduced.