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Chapter 16 Genetics 2 H.L. 1 Lesson Objectives At the end of this sub section you should be able to: 1. State the Law of Segregation 2.State the Law of Independent Assortment 3. Describe the experiments used to formulate these 2 laws. 4.Complete dihybrid crosses using punnett square 5.Define linkage 6.Explain outcome in results with linked genes 7. Explain sex linked traits 8.Discuss non nuclear inheritance 9.Describe DNA structure & protein synthesis 2 Gene • A segment of DNA that contains coding for a polypeptide or protein • A unit of hereditary information. 3 Heredity • The transmission of characteristics from parents to offspring. 4 5 Gregor Mendel • Father of genetics – Gregor Mendel, an Austrian monk • 1857 – began collecting pure lines of peas • Chose self-fertilizing peas so that all offspring look exactly like their parent • Mendel chose 7 traits for study 6 7 Pea Traits used by Mendel 8 9 Mendel’s Peas – Why Peas? 1.Pure lines with easily identifiable traits were available 2.Peas are self-fertilizing with a flower structure that minimizes accidental pollination 3.Peas can be artificially fertilized which allows specific crosses to be made 4.Peas have a short growth period 5.Peas produce large numbers of offspring 10 Mendel’s Experiments 1. He crossed pure plants with alternative phenotypes for a single trait 2. He recorded how many offspring were of each type – 1st generation results (F1 generation) 3. He allowed these offspring to self-fertilize 11 Mendel’s Findings 4. He again recorded the nature of the offspring – F2 generation 5. He did a mathematical analysis 6. He deduced several principles 7. He published paper in good scientific journal – 1866 12 Example of Results – Seed Coat (Smooth seeds vs Wrinkled seeds) Parents: • one parent had smooth seeds • the other wrinkled seeds Result: F1 all smooth 13 2nd Generation 2nd generation offspring – F2 Results Parents – both heterozygous smooth Offspring: ¾ of offspring were smooth ¼ of offspring were wrinkled 3:1 Ratio 14 Results For all seven traits he got a 3:1 ratio 15 16 Summary of Mendel’s Results • F1 showed only one of two parental traits • All crosses were the same; it did not matter which plant the pollen came from • Trait not shown in F1 reappeared in 25% of the plants in the F2 generation 17 Mendel’s Results • Traits are not blended as they are passed from parent to offspring • Each parent makes equal contribution • Genes can be carried but not expressed • Appearance may be similar but genetic make up may differ 18 Why was Mendel successful? 1) Good luck – he chose peas! 2) Naturally self-fertilizing – easy to have pure lines but it can be forced to cross with a different line and experimenter can completely control crosses 3) Analyzed his results quantitatively, large numbers yielded good statistical ratios 4) Started simply – worked from simple to complex one trait at a time then two at a time, etc 19 Learning Check 1. 2. 3. 4. 5. 6. 7. Who was Gregor Mendel? Why did he choose peas? Give examples of the pea traits used by Mendel What types of peas did he cross? What is meant by F1 generation What results did he get in the F2 generation? What conclusions can be drawn form his results? 20 Mendel’s Laws • The Law of Segregation • The Law of Independent Assortment 21 Mendel’s First Experiment Crossed Pure Tall x Pure Short (Dwarf) Predictions: The offspring would be either: 1. Some would be tall and some short 2. All intermediate 3. All short 4. All tall 22 Mendel’s Experiments 1st Experiment: C Pure Tall x Pure Short Results: All offspring (F1) tall 2nd Experiment: Bred F1 Plants Results: Ratio of 787 tall to 277 short (3:1) 1 2 23 Mendel’s First Principle • Mendel assumed that the two “Factors” for each trait must exist in the parental cells producing the gametes • These “Factors” came from the parents’ parents and were united in fertilization • In forming pollen and egg, the two”factors” for any trait must separate and go into different gametes • This became known as Mendel’s “Principle of Segregation” 24 The Law of Segregation “states that organisms contain 2 factors for each trait. These factors separate in gamete formation producing gametes with only one copy of each factor.” 25 F1 Generation 26 F2 generation 27 How the results are explained • Chromosome movement in meiosis explained Mendel’s results • Chromosomes occur in pairs – genes occur in pairs • Paired chromosomes separate in meiosis just as paired alleles separate in gamete formation 28 Law of Independent Assortment States that alleles of any one gene are transmitted independently of any other pair of alleles 29 Animation Explaining Independent Assortment of Genes 30 THE DIHYBRID CROSS Studying the inheritance of two characters simultaneously 31 Mendel’s peas Character Seed shape Seed colour 32 Trait Allele Round R Wrinkled r Yellow Y Green y Possible Combinations Phenotype Genotype Round Yellow Round Yellow Round Yellow Round Yellow Round Green Round Green Wrinkled Yellow Wrinkled Yellow Wrinkled Green RRYY RRYy RrYY RrYy RRyy Rryy rrYY rrYy rryy 33 Probability • Probability: the likelihood that a specific event will occur. • May be expressed as a decimal, %, fraction, or ratio. • Probability= # of actual times # of opportunities 34 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 35 THE LAW OF INDEPENDENT ASSORTMENT • It appears that the inheritance of seed shape has no influence over the inheritance of seed colour • The two characters are inherited INDEPENDENTLY • The pairs of alleles that control these two characters assort themselves independently 36 Learning Check 1. What are Mendel’s two Laws? 2. State the Law of Segregation 3. State the Law of Independent Assortment 4. What is the difference between a monohybrid and a dihybrid cross? 37 38 Homozygous x Homozygous 39 Homozygous x Heterozygous 40 Heterozygous x Heterozygous 41 Learning Check 1. If R is dominant to r, what the offspring of the cross of RR with rr be? 2. According to Mendel, what kind of genes "disappear" in F1 pea plants? 3. Assuming complete dominance, the F2 generation following the cross Aa x Aa will show a phenotypic ratio of _____ . 4. In meiosis what happens to chromosome from each homologous pair? 42 Dihybrid test cross In a dihybrid cross the test cross is made with an individual which is homozygous recessive for both characters RRYY and rryy 43 Parents Phenotypes: X Parents Genotypes: X Gametes: F1 Genotypes: F1 Phenotypes: Ratio 44 45 Phenotype Round, yellow Genotype RRYY Gametes all F1 X rryy RY all Wrinkled, green all ry RrYy All of the F1 generation would be heterozygous for both characteristics, meaning that they would all be round and yellow. Mendel then crossed two of the F1 generation together… 46 Phenotype Round, yellow Genotype RrYy Gametes RY, Ry, rY, ry X Round, yellow RrYy RY, Ry, rY, ry F2 Use a punnett square 47 RY Ry rY ry RY RRYY RRYy RrYY RrYy Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy 48 F2 9 : 3 : 3 : 1 Round Round Wrinkled Wrinkled Yellow Green Yellow Green This is the typical ratio expected in a dihybrid cross. 49 Parents Phenotypes: Parents Genotypes: X X Gametes Genotypes Phenotypes Ratio 50 Dihybrid test cross Phenotypes Round Yellow Genotypes RrYy rryy RY, Ry, rY, ry ry Gametes Genotypes ry X Wrinkled Green RY Ry rY ry RrYy Rryy rrYy rryy Phenotypes Round Yellow Round Green Wrinkled Yellow Wrinkled Green Proportions 25% 25% 25% 25% 51 Learning Check 1. What is a dihybrid test cross? 2. In pea plants, tall plant (T) is dominant over a short plant(t). Purple flower (P) is dominant over Short white(p) plant. A homozygous Tall purple flower (P) plant is crossed with a Short white(p) plant. State the genotypes of the parent plants and of the F1 generation by doing out a cross. 52 Parents Phenotypes: Tall purple X Short white Parents Genotypes: Gametes: TTPP X TP ttpp tp F1 Genotype: TtPp F1 Phenotype: Tall purple 53 4. If the F1 plants are self fertilised what will the genotypes of the F2 generation be? • Use a punnett square 54 Parents Phenotypes: Tall purple X Tall purple Parents Genotype: Gametes: TtPp X TtPp TP Tp tP tp X TP Tp tP tp TP TP TTPP Tp TTPp tP TtPP tp TtPp Tp TTPp TTpp TtPp Ttpp tP TtPP TtPp ttPP ttPp tp TtPp Ttpp ttPp ttpp Ratio 9:3:3:1 55 Linkage • Linked Genes are genes are contained in the same chromosome which tend to be inherited together. • In a dihybrid heterozygote, if the genes are not linked, 4 gamete types are produced in equal ratio. • If the genes are linked only 2 types of gametes are formed in equal ratio. 56 57 Linkage The genes A and B are linked as are the genes a and b A a B b 58 Locus The locus of a gene is its position on a chromosome Cell Locus of Gene Chromosome 59 Activity sheet 3 -Linkage In Drosophila Straight wing (S) is dominant to curled wing (s) and grey body (G) is dominant to ebony body (g). S and G are linked. Parents: Ss Gg x ssgg Gametes: SG, sg x sg F1 genotypes: SsGg and ssgg F1 phenotypes: Straight wing curled wing Grey body ebony body 60 Sex Linked Genes Are genes found on the X chromosome without a corresponding gene on theY chromosome. 61 Human Chromosomes • We have 46 chromosomes, or 23 pairs. • 44 of them are called autosomes and are numbered 1 through 22. Chromosome 1 is the longest, 22 is the shortest. • The other 2 chromosomes are the sex chromosomes: the X chromosome and the Y chromosome. • Males have and X and a Y; • females have 2 X’s 62 Male Karyotype 63 Female Karyotype 64 Sex-linked Genes • Genes on the X chromosome are called “sexlinked”, because they expressed more often in males than in females • There are very few genes on the Y chromosome. • Since males only have one X chromosome, all genes on it, whether dominant or recessive, are expressed. 65 Mutations on the X Chromosome • In contrast, a mutant gene on an X chromosome in a female is usually covered up by the normal allele on the other X. • Most mutations are recessive. So, most people with sex-linked genetic conditions are male. 66 Boy or Girl? Males produce sperm with X chromosome, and sperm with their Y chromosome. The X-bearing sperm lead to daughters The Y-bearing sperm lead to sons. 67 Sons or Daughters? • Sons get their only X chromosome from their mothers • The father’s X chromosome goes only to daughters. • The Y chromosome is passed from father to son. 68 • Historically, women have been held responsible (blamed!) for not producing the male heir. • King Henry VIII blamed his wife and subsequently had her beheaded for not producing a male heir. • Are women responsible for the sex of the child? • Which parent actually determines the gender of the child? 69 Learning Check 1. What is the difference between linkage and sex linkage? 2. What is a locus of a gene? 3. What is the sex of an individual determined by? 4. What can we predict about the phenotype of offspring when genes are linked? 70 Color blindness • We have 3 color receptors in the retinas of our eyes. • They respond best to red, green, and blue light. • Each receptor is made by a gene. • The blue receptor is on an autosome • The red and green receptors are on the X chromosome (sex-linked). 71 72 Inheritance of Colourblindness • A heterozygous female has normal color vision. • Sons get their only X from their mother. • So, ½ of the sons of a heterozygous mother are colorblind, and ½ are normal. 73 Colour-blindness • A colourblind male will give his X chromosome to his daughters only. • If the mother is homozygous normal, all of the children will be normal. • However, the daughters will be heterozygous carriers of the trait, and ½ of their sons will be colorblind. 74 Haemophilia • blood does not clot when exposed to air. • People with hemophilia can easily bleed to death from very minor wounds. • Hemophilia is another sex-linked trait. 75 Treatment of Haemophilia • Hemophilia is treated by injecting the proper clotting proteins • These are isolated from the blood of normal people. 76 77 Non Nuclear Inheritance 78 Non Nuclear Inheritance • Non nuclear DNA found in mitochondria and chloroplasts. • When these organelles replicate during cell division, they pass on their DNA. 79 Learning Check 1. What is the difference between linkage and sex linkage? 2. Give an example of a sex linked condition 3. What is meant by non nuclear inheritance? 80 Tutorials • • • • • • The Biology Project – Monohybrid Crosses The Biology Project – Dihybrid Crosses Slide Show Drag and Drop Genetics Java Genetics problems BBC Bitesize tutorial 81 This is a molecule of messenger RNA. It was made in the nucleus by transcription from a DNA molecule. codon A U G G G C U U AAA G C A G U G C A C G U U mRNA molecule 82 A ribosome attaches to the mRNA molecule. ribosome A U G G G C U U AAA G C A G U G C A C G U U 83 Amino acid tRNA molecule A transfer RNA molecule arrives. It brings an amino acid to the first three bases (codon) on the mRNA. anticodon The three unpaired bases (anticodon) on the tRNA link up with the codon. UAC A U G G G C U U AAA G C A G U G C A C G U U 84 Another tRNA molecule comes into place, bringing a second amino acid. Its anticodon links up with the second codon on the mRNA. UAC A U G G G C U U AAA G C A G U G C A C G U U 85 Another tRNA molecule brings the next amino acid into place. A U G G G C U U AAA G C A G U G C A C G U U 86 A peptide bond joins the second and third amino acids to form a polypeptide chain. A U G G G C U U AAA G C A G U G C A C G U U 87 The process continues. The polypeptide chain gets longer. This continues until a termination (stop) codon is reached. The polypeptide is then complete. A U G G G C U U AAA G C A G U G C A C G U U 88 End 89