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Transcript
Genetics Why do we look the way we do? Honors Biology Chapters 9 & 12 Inheritance of chromosomes Egg + sperm zygote meiosis egg zygote sperm fertilization mitosis & development Inheritance of genes Chromosomes passed from Mom & Dad to offspring are comprised of genes may be same information may be different information eye color (blue or brown?) eye color (blue or brown?) Effect of genes Gene = region of a chromosome that codes for a trait Genes come in different versions for each trait brown vs. blue eyes brown vs. blonde hair A version of a gene = an allele Genes affect what you look like X bb Bb BB Bb Bb Where did the blue eyes go?? Bb Genes affect what you look like… X bb Bb Bb Bb bb Why did the blue eyes stay?? bb Genes affect what you look like… X Bb BB or Bb BB or Bb BB or Bb Bb bb Where did the blue eyes come from?? What did we show here? Genes come in “versions” brown vs. blue eye color alleles Alleles are inherited separately from each parent brown & blue eye colors are separate & do not blend either have brown or blue eyes, not a blend Some alleles mask others brown eye color masked blue How does this work? Homologous chromosomes have same genes… …but maybe different alleles eye color (blue?) hair color eye color (brown?) hair color Traits are inherited as separate units For each trait, an organism inherits 2 copies of a gene (2 alleles), 1 from each parent 1 from Mom homologous chromosomes 1 from Dad Genetics vs. appearance There can be a difference between how an organism looks & its genetics Its expressed trait/s = phenotype brown eyes vs. blue eyes Its alleles, or genetic makeup = genotype BB, Bb, bb 2 people can have the same phenotype but have different genotypes: BB vs Bb Genetics vs. appearance How were these brown eyes made? eye color (brown) eye color (brown) eye color (brown) eye color (blue) vs. B BB B Bb B b Practice If G is the allele for pointy ears and g is the allele for floppy ears, what will be the ear shape phenotypes of the puppies with these genotypes? The dominant allele is _ for the trait ___________ The recessive allele is _ for the trait ___________ Genotype GG = Phenotype __________ Genotype Gg = Phenotype __________ Genotype gg = Phenotype ___________ Practice G is for pointy ears and g is for floppy ears. Also, H is for a pink nose and h is for a black nose. Genotype GGHH = Phenotype ______ and ______ Genotype GgHh = Phenotype ______ and ______ Genotype gghh = Phenotype ______ and _______ Genotype GGhh = Phenotype ______ and _______ Genotype Gghh = Phenotype ______ and _______ Genotype ggHH = Phenotype ______ and _______ Practice Which of these are traits and which are phenotypes? 1. Finger length 2. Blue eyes 3. Long hair 4. Number of leaves 5. Shape of tentacles 6. Warbling song Practice Which of these are alleles and which are traits? 1. Eye color 2. Bone integrity 3. i 4. Insulin shape 5. B 6. Na Practice Which of these are phenotypes and which are genotypes? 1. Curly hair 2. Jj 3. PP 4. Arthritic knees 5. Type B blood 6. Spotted fur and a pink nose 7. HHGg 8. Purple leaves and spiny stem Genetics & The Work of Mendel Gregor Mendel Modern genetics began in the mid1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas used good experimental design used mathematical analysis collected data & counted them excellent example of scientific method Mendel’s work Pollen transferred from white flower to stigma of purple flower Bred pea plants cross-pollinate true breeding parents allowed offspring to self-pollinate & observed next generation all purple flowers result self-pollinate ? Mendel collected data for 7 pea traits Looking closer at Mendel’s work Parents 1st F1 true-breeding true-breeding X purple-flower peas white-flower peas 100% purple-flower peas generation (hybrids) 100% self-pollinate 2nd F2 generation 75% purple-flower peas 25% white-flower peas 3:1 What did Mendel’s findings mean? Some traits mask others purple & white flower colors are separate traits that do not blend purple x white ≠ light purple purple masked white dominant allele functional protein affects characteristic masks other alleles recessive allele no noticeable effect allele makes a non-functioning protein allele producing functional protein mutant allele malfunctioning protein homologous chromosomes Mendel’s Results and Conclusions RESULT: Whenever Mendel crossed two P plants, one of the traits disappeared in the F1 plants. The missing trait reappeared in the F2 plants in a 3:1 ratio pattern CONCLUSION: LAW OF DOMINANCE One trait is dominant because it masked or dominated the other trait One trait is recessive because it “hid” behind the dominant one. It can only be seen when the plant has no dominant alleles. Mendel’s Results and Conclusions CONCLUSION: LAW OF SEGREGATION Pairs of alleles segregate (separate) during the formation of gametes (meiosis— homologous pairs separate) A parent only passes one allele for each gene onto a zygote Mendel’s Results and Conclusions CONCLUSION: LAW OF INDEPENDENT ASSORTMENT Factors for different characteristics are distributed to gametes independently or randomly. Which allele is passed for one one gene doesn’t affect which allele is passed down from other genes Mendel’s Legacy DNA and chromosomes weren’t discovered until many decades after Mendel’s death Today, we understand the genetic mechanisms that underlie his mathematical discoveries… Gamete Formation Suppose there’s a gene for eye color, with the alleles B for brown eyes or b for blue eyes. A man has the genotype Bb, which gives him the phenotype brown eyes. He can make Meiosis produces his gametes… gametes that are B BB Normal cell in G1 BB Cytokinesis bb bb 2nd b b 1st Cytokinesis S Phase b B B Four Gametes EITHER B or b. Half of his gametes will be one, half will be the other. We simplify, saying that he produces either B or b allele sperm. Equal chance of each. Practicing the Law of Segregation (Some gametes are written with more than one letter. If Dad’s genotype is LTLt, he will make a sperm that has the LT allele or a sperm that has the Lt allele.) Genotype YY makes what gamete/s? Genotype Tt makes what gamete/s? Genotype bb makes what gamete/s? Genotype Ii makes what gamete/s? Genotype K1K2 makes what gamete/s? B How do we say it? 2 of the same allele= Homozygous BB = brown eyes bb = blues eyes homozygous dominant homozygous recessive 2 different alleles= Heterozygous Bb = brown eyes BB B b bb b B Bb b Practice Identify each of these genotypes as being homozygous or heterozygous. GG ____________ Yy ___________ kk ____________ Ss ________ Vv ________ Practice Identify each of these genotypes as being homozygous dominant, homozygous recessive, or heterozygous. ee ____________ QQ ___________ Ll ____________ CC ________ pp ________ Practice Suppose that the I allele codes for orange fins and the i allele codes for yellow fins. The heterozygous genotype: __ The homozygous dominant genotype: __ The homozygous recessive genotype: __ A fish with yellow fins must have a _____________ genotype. A fish with orange fins could be either _____________ or ___________________. Punnett squares x Bb male / sperm X female / eggs Bb B b BB Bb Bb bb B b Genetics and Probability Figuring out offspring is a matter of chance. A Punnett Square provides the probabilities of two parents producing particular zygotes. An example, using coins: Punnett Squares Using the letter H to stand for heads… If you flip a coin that’s heads (H) on both sides, what are the chances that it will come up heads (H)? Punnett Squares If it’s a normal coin, heads (H) on one side and tails (h) on the other… What are the odds that it will come up heads (H) on a flip? Punnett Squares If you flip TWO normal coins, what are the odds that you will get heads (H) on both flips? Punnett Squares The first flip will be either heads (H) or tails (h): Punnett Squares The second flip will also be either heads (H) or tails (h): Punnett Squares These are the possible combinations that you could have produced: Punnett Squares These are the possible combinations that you could have produced: Punnett Squares These are the possible combinations that you could have produced: Punnett Squares These are the possible combinations that you could have produced: Punnett Squares These are the possible combinations that he could have produced: H H H H H H Punnett Squares 1 in 4 possible outcomes would be both heads (HH). The chance of getting heads on both flips = 1/4 = 25% H H h H H h H H h h h h Punnett Squares What are the odds of getting heads on one flip, tails on the other? h H H h H H H H h h h h Punnett Squares 2 of 4 possible outcomes = 1/2 = 50% h H H h H H H H h h h h Punnett Squares What if you flip two different coins: One coin has two heads The other is normal, one heads and one tails What are the odds of getting heads on both flips? Punnett Squares H H H H H H h h H H H h 2/4 = 1/2 = 50% This is called a Punnett Square. Punnett Squares display all possible gametes and possible offspring. Punnett Squares The top and side boxes show possible gametes. The middle boxes show possible zygotes (offspring) they would create. Step-By-Step Instructions Sample problem: What are the chances that a heterozygous browneyed father and a blue-eyed mother would have a blue-eyed child? (Use letters B/b) 1. Figure out Mom and Dad’s genotypes. In the example: Dad = ___, Mom = __ Punnett Squares 2. Figure out Mom and Dad’s gametes. Dad’s gametes = __ and __ Mom’s gametes = __ and __ 3. Set up a square. For a monohybrid cross (studying only one gene), make a normal tic-tac-toe board. Punnett Squares 4. Write Dad’s gametes on one side, and Mom’s on the other. • It doesn’t matter whether Mom or Dad is on the side vs top, just keep both eggs together and both sperm together. Punnett Squares 4. (continued) Like a genotype, if there’s a dominant allele, put it first. A A A a a A a a Punnett Squares 5. Complete zygote genotypes. Remember to put dominant allele first, if there is one. 6. Write out all the zygote genotypes as fractions. I.e. 1/4, 2/4, 3/4, 4/4 Punnett Squares 7. Reduce fractions if possible, and convert fractions to percentages. For instance, if two of the four zygotes are AA, the probability of genotype AA is 2/4 = 1/2 = 50% 8. If applicable, rewrite offspring genotype results as phenotype results. Punnett Squares b b If you did the problem correctly, it should EITHER look like this, OR… B b Bb bb Bb Genotype Probabilities BB = 0/4 = 0% Bb = 2/4 = 1/2 = 50% bb = 2/4 = 1/2 = 50% Phenotype Probabilities Brown eyes = 2/4 = 1/2 = 50% Blue eyes = 2/4 = 1/2 = 50% bb Final Answer 50% probability of a blue-eyed child Punnett Squares …OR like this. Notice that results are the same. b b B Bb Bb b bb bb Genotype Probabilities BB = 0/4 = 0% Bb = 2/4 = 1/2 = 50% bb = 2/4 = 1/2 = 50% Phenotype Probabilities Brown eyes = 2/4 = 1/2 = 50% Blue eyes = 2/4 = 1/2 = 50% Final Answer 50% probability of a blue-eyed child Practicing with Punnett Squares Parents – Tt and tt T t Parents – TT and Tt T T t Tt tt T TT TT t Tt tt t Tt Tt Offspring: 50% Tt – tall 50% tt – short Offspring: 100% TT or Tt – tall Practicing Punnett Squares Show a monohybrid cross, with all genotype and phenotype probabilities, for parents who are HH and hh. Practicing Punnett Squares Show a monohybrid cross, with all genotype and phenotype probabilities, for parents who are Ll and Ll. Practicing Punnett Squares If Dad’s genotype is Rr and Mom is homozygous recessive, what are the odds of having homozygous dominant offspring? Practicing Punnett Squares If both parents are heterozygous, and they have ten offspring, how many of those offspring would you predict will be homozygous recessive? Practicing Punnett Squares Suppose black fur is dominant and white fur is recessive. Two parents, one with black fur and one with white fur, have many offspring. Roughly half of their babies are black-furred, and half are whitefurred. What were the genotypes of the parents? Hint: when a question asks you to figure out parental genotypes, make test crosses, Punnett Squares for every possibility, then see which one gives you offspring results that fit. Testcross Dominant phenotypes can have either BB or Bb genotypes. How do you know? Perform a test cross, or a cross with a known recessive genotype (bb) If any recessive offspring appear, unknown must be heterozygous If no recessive offspring appear, unknown can be homozygous.