Bft1033 3 mendelian_genetics_2012

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Bft1033 3 mendelian_genetics_2012

  1. 1. Genetics Inheritance Mendelian Genetics
  2. 2. Introduction ۞ Genetics is the branch of biology that studies heredity ۞ Genetics is the branch of biology that studies the storage, duplication, and transfer of information ۞ Organisms inherit characteristics from their parents ۞ The information for these characteristics is contained in an organism’s DNA.
  3. 3. The History of Modern Genetics ۞ Modern genetics began early in the 20th century with the pioneering work of Gregor Mendel ۞ Mendel worked on the garden pea (Pisum sativum), and he established that certain traits could be passed from generation to generation Gregor Mendel ۞ Prior to Mendel, heredity was regarded as a "blending" process and the offspring were essentially a "dilution"of the different parental characteristics.
  4. 4. Gregor Mendel’s Peas
  5. 5. Gregor Mendel’s Peas ۞ Mendel studied a number of characteristics in pea plants:        Plant height - short or tall Seed color - green or yellow Seed shape - wrinkled or round Flower color - white or purple Pod shape - constricted or smooth Pod color - yellow or green Flower position - terminal or axial
  6. 6. Gregor Mendel’s Peas 7 True-breeding phenotypes in pea
  7. 7. Gregor Mendel’s Peas
  8. 8. Gregor Mendel’s Peas Cross Pollinating Peas Instead of self pollination, the pollen from one plant is used to pollinate the egg cells of another plant ۞ Cut away anthers ۞ Hand pollinated using pollen from a chosen plant. ۞
  9. 9. Gregor Mendel’s Peas 1. Self-fertilization 2. Cross-pollination
  10. 10. Genetics terms you need to know: Gene – a unit of heredity; a section of DNA sequence encoding a single protein ۞ Genome – the entire set of genes in an organism ۞ Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait (Different forms of a specific gene) (A or a) ۞ Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located. ۞
  11. 11. Genetics terms you need to know: Homozygous – having identical alleles (one from each parent) for a particular characteristic (AA or aa) ۞ Heterozygous – having two different alleles for a particular characteristic (Aa) ۞ Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition (A) ۞ Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous. (a) ۞
  12. 12. Genetics terms you need to know: ۞ ۞ ۞ ۞ ۞ ۞ Genotype – the genetic makeup of an organisms (AA, Aa or aa) Phenotype – the physical appearance of an organism (Genotype + environment) Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait P = Parental generation F1 = First filial generation; offspring from a genetic cross F2 = Second filial generation of a genetic cross.
  13. 13. Mendel's data: Monohybrid Cross X P: round wrinkled Mendel's insight F1: All round selfing F2: wrinkled round 5474 : 1850 2.96 : 1 "round" F1 is actually heterozygous: Aa A = round - dominant a = wrinkled - recessive Parents are AA and aa
  14. 14. Mendel's data: Monohybrid Cross X P Mendel's insight “purple" F1 is actually heterozygous: Bb F1 selfing F2 705 : 224 3.15 : 1 B = purple - dominant b = white - recessive Parents are BB and bb Law of segregation: alleles segregate from each other in gametes, 1/2 get one, 1/2 get the other.
  15. 15. Gamete Formation: Parents purple parent PP P white parent P all P sperm and eggs pp p p all p sperm and eggs
  16. 16. Gamete Formation: F1 purple F1 p P Pp P p P and p in sperm and eggs
  17. 17. Use of Punnet Square for F2 Determining the outcome of a single-trait cross ♂ ♀ p PP Pp Pp P P pp p Genotypic ratio: 1 PP : 2 Pp : 1pp Phenotypic ratio: 3 purple:1white
  18. 18. Monohybrid Cross
  19. 19. P Homozygous P P Heterozygous p
  20. 20. Monohybrid Crosses Yielded Consistent Results Therefore, the Principle of Segregation indeed is a general principle of genetics.
  21. 21. Consistency is Good No matter what the character, Mendel observed a 3:1 ratio of characters in the F2.
  22. 22. Dihybrid Cross ۞ ۞ ۞ Mendel investigated the inheritance of seed shape (smooth v wrinkled) and seed color (green v yellow) at the same time From his monohybrid crosses he knew that smooth seeds were dominant to wrinkled seeds and yellow seeds were dominant to green seeds He chose to cross plants that were pure breeding for both dominant features (smooth and yellow seeds) with plants that were pure breeding for both recessive features (wrinkled and green seeds).
  23. 23. Dihybrid Cross Parents plants with round x plants with wrinkled and yellow seeds and green seeds F1 offspring 100% plants with round and yellow seeds F1 intercross plants with round x plants with round and yellow seeds and yellow seeds F2 offspring Total 556 seeds collected from F2 plants Ratio 315 108 101 32 9 3 3 1 round and yellow seeds round and green seeds wrinkled and yellow seeds wrinkled and green seeds However, the ratio of dominant to recessive features is still 3:1
  24. 24. Dihybrid Cross Law of independent assortment: Alleles of separate genes assort independently into the gametes SY Sy sY sy Genotypic ratio = 1SSYY : 2SSYy : 1SSyy : 2SsYY : 4SsYy : 2Ssyy : 1ssYY : 2ssYy : 1ssyy Phenoypic ratio = 9 :3 :3 :1
  25. 25. Dihybrid Cross ۞ A dihybrid cross can be treated as two separate monohybrid crosses ۞ The expected probability of each type of seed can be calculated: Probability of an F2 seed being round = 75% or ¾  Probability of an F2 seed being wrinkled =25% or ¼  Probability of an F2 seed being yellow = 75% or ¾  Probability of an F2 seed being green = 25% or ¼ 
  26. 26. Dihybrid Cross Therefore:  Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25%  Probability of an F2 seed being round and green = ¾ x ¼ = 3/16 = 18.75%  Probability of an F2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 = 18.75%  Probability of an F2 seed being wrinkled and green = ¼ x ¼ = 1/16 = 6.25%
  27. 27. Dihybrid Cross We could expect What Mendel observed 556 x 9/16 round yellow 313 315 556 x 3/16 round green 104 108 556 x 3/16 wrinkled yellow 104 101 556 x 1/16 wrinkled green 35 32 Phenotype
  28. 28. Trihybrid crosses P: Gamete: F1: AABBCC x aabbcc ABC abc AaBbCc Gametes: ABC, ABc, AbC, Abc, aBC, aBc, abC, abc
  29. 29. Trihybrid Crosses AaBbCc B A a b B b C c C c C c C c ABC ABc AbC Abc aBC aBc abC abc
  30. 30. Trihybrid Cross - Phenotypes Forked-line Method - bb - aa bb - A-B-C- A-B-C- cc A-B-cc A-B-cc - A-bbC- A-bbC- cc A-bbcc A-bbcc - aaB-C- aaB-C- cc aaB-cc aaB-cc - aabbC- aabbC- cc aabbcc aabbcc Phenotypic ratio: 27:9:9:9:3:3:3:1
  31. 31. Mendel’s Principles ۞ Principle of Dominance: One allele masked another, one allele was dominant over the other in the F1 generation ۞ Principle of Segregation: When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene (allele).
  32. 32. Mendel’s Principles ۞ Principle of Independent Assortment: “Members of one gene pair segregate independently from other gene pairs during gamete formation” Genes get shuffled – these many combinations are one of the advantages of sexual reproduction
  33. 33. Exercises 1. For the following genotypes, tell how many gametes will be produced a) BB b) Ss c) DDRr Solution # gametes = 2n Use this formula: where n is number of heterozygous genes. a) BB, n=0, 20=1 gamete: B b) Ss, n=1, 21=2 gametes: S, s c) DDRr, n=1, 21=2 gametes: DR, Dr
  34. 34. Exercises 2. List all possible gametes produced by a) b) c) d) aabbcc kkLLMm PpQQRr RrSsTt Solution: a) aabbcc: 1 gametes: abc b) kkLLMm: 2 gametes: kLM and kLm c) PpQQRr: 4 gametes: PQR, PQr, pQR, pQr d) RrSsTt: 8 gametes: RST, RSt, RsT, Rst, rST, rSt, rsT, rst
  35. 35. Monohybrid Test Cross  How can you determine genotype from individual expressing dominant phenotype? - DD or Dd?  Cross individual with dominant phenotype to a homozygous recessive individual.
  36. 36. Dihybrid test cross ۞ In monohybrid crosses, to know if a dominant trait is homozygous (SS) or heterozygous (Ss) it is necessary to carry out a test cross ۞ This is done with a homozygous recessive (ss) individual ۞ The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (ssyy)
  37. 37. Dihybrid testcross Testcross results of four smooth round individuals SSYY SSYY SsYY SsYY SSYy SSYy SsYy SsYy SSYYs x ssyy x SsYY x ssyy SSYy x ssyy SsYy x ssyy sy Sy SsYy sy SY SSYy sY ssYy sy SY SsYy Sy Ssyy sy SY SsYy Sy Ssyy sY ssYy sy Phenotypic ratio All smooth yellow All smooth yellow Phenotypic ratio ½ smooth yellow ½ wrinkle yellow All yellow Phenotypic ratio ½ smooth yellow ½ smooth green All smooth ssyy Phenotypic ratio ¼ smooth yellow ¼ smooth green ¼ wrinkle yellow ¼ wrinkle green Mixed
  38. 38. Monohybrid Problems 1. In rabbits the allele for black coat color (B) is dominant over the allele for brown coat color (b). What is the genotypic ratio and phenotypic ratio be for a cross between an animal homozygous for black coat color and one homozygous for brown coat color? 2. White (W) hair in sheep is caused by the dominant gene while black (w) hair is recessive. A heterozygous white male and a black female are parents of a black lamb. What is the probability that their next lamb will be white? What are the genotypic and phenotypic ratios? 3. Albinism is recessive in humans. An albino man marries a woman who is not albino, but had an albino father. What is the probability of this couple having a child that is not an albino? What are the genotypic and phenotypic ratios?
  39. 39. Monohybrid Problems 4. 5. 6. Todd and Melissa are college students who are planning to get married. They are currently taking a genetics course and decided to determine the eye color of any possible children they might have. Blue eyes are recessive to brown eyes. Todd has brown eyes, like his three brothers. His mother and grandmother have blue eyes, but his father and all other grandparents have brown eyes. Brown eyed Melissa has one blue eyed sister and one brown eyed sister and a mother with blue eyes. Her father and all of her grandparents have brown eyes. Construct an accurate punnett square to determine the possible eye colors of their yet to be born children. What are the genotypic and phenotypic ratios? In Teenage Mutant Ninja Turtles, green shells are dominant over brown shells. Leonardo, who is heterozygous for a green shell, marries the lovely Mona Lisa, who has a brown shell. What are the genotypic and phenotypic ratios? In humans, polydactyly (an extra finger on each hand or toe on each foot) is due to a dominant gene. When one parent is polydactylous, but heterozygous, and the other parent is normal, what are the genotypic and phenotypic ratios of their children?
  40. 40. Monohybrid Problems 5. In guinea pigs, rough coat (R) is dominant over smooth coat (r). A rough coated guinea pig is bred to a smooth one, giving eight rough and seven smooth progeny in the F1. a) b) 6. What are the genotypes of the parents and their offspring? If one of the rough F1 animals is mated to its rough parent, what progeny would you expect? In summer squash, white fruit (W) is dominant over yellow (w), and disk-shaped fruit (D) is dominant over sphere-shaped fruit (d). The following problems give the phenotype of the parents and their offspring. Determine the genotypes of the parents in each case: a) b) c) White, disk x yellow, sphere gives 1/2 white, disk and 1/2 white, sphere. White, sphere x white, sphere gives 3/4 white, sphere and 1/4 yellow, sphere. Yellow, disk x white, sphere gives all white, disk progeny.
  41. 41. Dihybrid Problems 1. In human, aniridia, (a type of blindness resulting from absence of an iris) is due to a dominant gene. Migraine (a sickening headache) is due to a different dominant gene. A man with aniridia, whose mother was not blind, marries a woman who suffers from migraine. The woman’s father did not suffer from migraine. In what proportion of their children would both aniridia and migraine be expected to occur? 2. In watermelons, solid green color (G) is dominant over striped pattern (g), and short shape (S) is dominant over long shape (s). What is the probability of each phenotype of possible offspring if a heterozygous solid, long watermelon cross pollinates with a heterozygous solid, heterozygous short watermelon? 3. Having two eyebrows is dominant (E) over having one large eyebrow (e). Also having six fingers (F) is dominant over having five fingers (f). What is the probability of each phenotype if a man that has one eyebrow and twelve fingers total (heterozygous), has children with a woman that is heterozygous for both traits?
  42. 42. Any Questions? Thank You

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