- Patterns of Inheritance Chapter 9. Overview Definitions Patterns of Mendelian Inheritance Non-Mendelian Inheritance.
Patterns of Inheritance Chapter 9. Overview Definitions Patterns of Mendelian Inheritance Non-Mendelian Inheritance.
Slide 1 Patterns of Inheritance Chapter 9 Slide 2 Overview Definitions Patterns of Mendelian Inheritance Non-Mendelian Inheritance Slide 3 Genes: Info in chromosomal DNA Heritable traits passed to offspring Diploid (2n): Pairs of genes on pairs of homologous chromosomes Slide 4 Alleles: Alternative forms of a gene One form usually dominant over other If pair is identical over many generations = true-breeding lineage Hybrid: Cross between 2 true-breeding individuals that have non-identical alleles for trait e.g. AA x aa = hybrid offspring Slide 5 Homozygous: Pair of identical alleles on pair of homologous chromosomes e.g. A & A Heterozygous: Pair of non-identical alleles on pair of homologous chromosomes e.g. A & a Slide 6 M locus: leaf colour Both alleles are the same = homozygous D locus: plant height Both alleles are the same = homozygous Bk locus: fruit shape Alleles are different = heterozygous chromosome 1 from tomato pair of homologous chromosomes M D Bk Slide 7 Dominant allele (e.g. A): Effect on trait masks effect of recessive allele (e.g. a) Note: dominant alleles are not necessarily more common or better Homozygous dominant genotype = AA Homozygous recessive genotype = aa Heterozygous genotype = Aa Slide 8 Genotype: Genes Individuals alleles e.g. Aa Phenotype: How genes are expressed Individuals observable traits e.g. green eyes Slide 9 P = true-breeding parents F 1 = 1 st -generation offspring F 2 = 2 nd -generation offspring of self-fertilized or crossed (mated) F 1 individuals Slide 10 Old Inheritance Theory Hereditary material from both parents mixed at fertilization e.g. red flowers + white flowers = pink flower offspring Couldnt explain obvious variation in traits + Slide 11 Gregor Mendel & His Peas Viennese monk who studied botany & math Pisum sativum: garden pea Self-fertilizing (flowers produce male & female gametes that fuse to form new plant so that parent & offspring = same traits) Can also be cross-fertilized Slide 12 Mendel tracked 7 traits over 2 generations Slide 13 Mendels Theory of Segregation Monohybrid cross: 2 homozygous parents that differ in trait dictated by alleles of 1 gene P F 1 AA x aa Aa Slide 14 After Mendel tracked 7 traits for 2 generations, he found that: F 2 : recessive forms & dominant forms of trait Slide 15 Fertilization is chance event with # of possible outcomes Can calculate probabilities of possible outcomes of genetic crosses Can determine all types of genetically different gametes that can be produced by male & female parents Genetics is a science of probability Slide 16 homozygous parent AAAA gametes Slide 17 heterozygous parent AaaA gametes Slide 18 The Punnett Square Method Allows prediction of both genotypes & phenotypes of genetic crosses A a aA Slide 19 Draw Punnett square with each row & column labelled with one of possible gametes of sperm & eggs respectively Fill in genotype of offspring in each box by combining male & female gametes A A A A a a a a AAAa aAaa Slide 20 Count # offspring with each genotype & convert to fraction of total # offspring To determine phenotype proportions, add fractions of genotypes that would produce given phenotype Phenotype I (dominant; AA & Aa) = + 2/4 = Phenotype II (recessive; aa) = AA = Aa = aA = 2/4 = aa = A A a a AAAa aAaa Slide 21 So, for Mendels cross of F 1 offspring from monohybrid cross, he predicted: F 2 = AA, Aa, aa Phenotypic ratio = 3:1 AA + Aa = dominant phenotype aa = recessive phenotype A A a a AAAa aAaa Slide 22 Since each gamete is equally likely, each of these offspring is equally likely Due to dominance we see a ratio of 3 purple:1 white Slide 23 An Example Imagine you are crossing a true breeding plant with yellow peas & a true breeding plant with green peas. If yellow color is dominant: What would the F 1 generation look like? What would the F 2 look like? Slide 24 PPPP PpPpPp These three all look the same! PP pPpP PpPp P ppp white spermeggs offspring genotypes genotypic ratio (1:2:1) phenotypic ratio (3:1) 1212 1212 1212 1212 1212 1212 1212 1212 1414 1414 1414 1414 1414 2424 1414 1414 Dominance creates some problems for scientists For example: How can I know which genotype I have if all I can see is phenotype? Slide 25 Test cross: Individual shows dominance for trait but genotype is unknown Cross with homozygous recessive individual to see if homozygous dominant or heterozygous Slide 26 If homozygous dominant:If heterozygous: Slide 27 Test crosses supported Mendels predictions Mendel found that crossing F 1 purple flowers with true- breeding white flowers: F 2 = purple (Aa), F 2 = white (aa) F 1 purple flowers were heterozygous sperm p p P pp PpPp all eggs PP or Pp sperm unknown if PP if Pp egg pollen p 1212 1212 1212 P p 1212 all Pp sperm Slide 28 An Example Imagine you have a plant with yellow peas but you dont know its genotype. Remember that yellow is dominant to green. What type of pea would you mate it with? Why? If the offspring are all yellow what does this tell you? Does it matter how many offspring there are? Slide 29 Mendels Big Ideas Genes have alternate versions (alleles) Organisms have two particles for each gene = diploid Some alleles are dominant to others (in organisms with two different alleles (heterozygous), the dominant allele masks the recessive allele) Alleles separate during gamete formation = the law of segregation Heterozygotes produce two different types of gametes Slide 30 Mendels Theory of Segregation 2n cells have pairs of genes on pairs of homologous chromosomes Members of each gene pair separate during meiosis & end up in different gametes Slide 31 Applying Mendels Ideas Imagine you have mated a black guinea pig with an albino guinea pig. They have 12 offspring & all are black. What alleles are dominant in this case? How do you know? What are the parents phenotypes? Genotypes? Slide 32 Now imagine a cross between a different pair of guinea pigs, one black & one albino. If they have 7 black & 5 albino offspring: What are the parents genotypes? How do you know? Slide 33 Mendel performed a lot of crosses & sometimes he was tracking more than one trait at a time This let him develop one more Big Idea Slide 34 Mendels Theory of Independent Assortment Dihybrid cross: True-breeding homozygous parents that differ in 2 traits dictated by alleles of 2 genes P F 1 AABB x aabb AaBb F 1 heterozygous for alleles of both genes Slide 35 ab AB F 1 = 100% AaBb AaBb For P (AABB), gametes = AB For P (aabb), gametes = ab Slide 36 With independent assortment, alleles for one trait are independent of alleles for another e.g. if you have A you are equally likely to have B or b This means that each of the four gametes are equally likely Slide 37 During meiosis of F 1 cells, there are 4 possible combos of alleles in sperm or eggs: 1/4 AB, Ab, aB, ab With 4 different sperm & egg types, F 2 offspring of hybrid cross = 16 possible combos of gametes Slide 38 AABBAABbAaBBAaBb AABbAAbbAaBbAabb AaBBAaBbaaBBaaBb AaBbAabbaaBbaabb AB Ab aB ab aBAbAB Slide 39 e.g. with A = purple, a = white B = tall, b = dwarf 9/16 tall, purple 3/16 dwarf, purple 3/16 tall, white 1/16 dwarf, white Phenotypic ratio = 9:3:3:1 abaBAbAB Ab aB ab AABBAABbAaBBAaBb AABbAAbbAaBbAabb AaBBAaBbaaBBaaBb AaBbAabbaaBbaabb Slide 40 An Example A true breeding plant with wrinkled green seeds was mated to a true breeding plant with smooth yellow seeds. In the first generation all the plants had smooth yellow seeds. What alleles are dominant in this case? How do you know? Slide 41 Taking these (dihybrid) F 1 plants, Mendel allowed them to self-fertilize We could write the F 1 genotypes like this: SsYy x SsYy What would their gametes look like? SY Sy sY sy What would the zygotes look like? Use a Punnett Square. Slide 42 SY SSYY SsYY ssYY ssyY SsyY SSYy SsYySsYy SsYySsYy ssyy Ssyy SSyy sSyYsSyY sSyy sSYY sSYy SSyY sYsY sYsY sy SySy SySy eggs self-fertilize ssYy 1 4 1 4 1 4 1 4 1 4 1 4 1 4 1 4 sperm 1 16 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 9/16 smooth yellow 3/16 smooth green 3/16 wrinkled yellow 1/16 wrinkled green Phenotypic ratio: 9:3:3:1 Slide 43 P p PP P p pp PpPp eggs Pp self-fertilize pPpP 1212 1212 1212 1212 1414 sperm 1414 1414 1414 Remember that a monohybrid cross will give you a 3:1 ratio The 9:3:3:1 ratio is actually just two 3:1 ratios stacked on top of each other Slide 44 seed shape (3:1) seed color (3:1) phenotypic ratio (9:3:3:1) smooth yellow smooth yellow smooth green wrinkled yellow wrinkled green yellow green smooth wrinkled 3 4 3 4 3 4 3 4 1 4 1 4 1 4 1 4 3 16 3 9 1 x x x x= = = = Slide 45 Independent Assortment Alleles for one trait are independent of alleles for another This happens because of events in metaphase of meiosis I Remember that chromosomes line up independently of non-homologous chromosomes Slide 46 S S ss s s Y S Y YY y yy S y Independent assortment produces four equally likely allele combinations during meiosis SYsySysY meiosis II meiosis I S S S S s s s s Y Y Y Y y y y y chromosomes replicate S S ss Y Y y y replicated homologues pair during metaphase of meiosis I, orienting like this or like this pairs of alleles on homologous chromosomes in diploid cells S s Y y Slide 47 Mendels Big Ideas Genes have alternate versions (alleles) Organisms have two particles for each gene = diploid Some alleles are dominant to others (In organisms with two different alleles (heterozygous) the dominant allele masks the recessive allele) Alleles separate during gamete formation (the law of segregation) (heterozygotes produce two different types of gametes) Alleles for one trait are independent of alleles for another = the law of independent assortment Slide 48 Mendels Theory of Independent Assortment After meiosis, genes on each pair of homologous chromosomes are sorted out, but independently of how genes on other pairs of homologous chromosomes are sorted out Slide 49 Independent assortment + segregation = genetic variation # genotypes = 3 n where n = # gene pairs More pairs = more genotypes Slide 50 AABBAABbAaBBAaBb AABbAAbbAaBbAabb AaBBAaBbaaBBaaBb AaBbAabbaaBbaabb AB Ab aB ab aBAbAB 3 n = 3 2 = 9 different genotypes Slide 51 In horses grey coat colour is dominant to chestnut. Imagine you own a grey horse & a chestnut horse & over the years they have several offspring, 2 chestnut & 1 grey. Given what you know about genetics, what is the genotype of each parent & of each offspring? How do you know? Using Mendels Big Ideas Slide 52 Applying Mendels Ideas Imagine you have mated a true-breeding tall plant with round seeds to a true-breeding dwarf plant with wrinkled seeds. In the F 1 generation, the plants are all tall with round seeds. What alleles are dominant in this case? Now imagine you have mistakenly mixed these F 1 plants with some true-breeding tall round plants. What kind of cross do you need to do to tell the plants apart? Slide 53 A Test Cross! You need to do a test cross on your tall round plants. What kind of plant will you mate your tall round plants with? Genotype? Phenotype? Now predict the two possible outcomes of your cross using a Punnett square. Slide 54 Crossing Over & Inheritance During meiosis, crossing-over occurs between non-sister chromatids on homologous chromosomes Get combos of alleles not seen in parents Slide 55 Some genes stay together more often than others because closer together chance that crossing over will separate ACDB Slide 56 2 genes are closely linked when distance between them is small = combos of alleles usually end up in same gamete When far apart, crossing over is very frequent = genes independently assort into different gametes ACDB Slide 57 Dependent Assortment Genes on the same chromosome are linked Their alleles tend to assort dependently Slide 58 flower color gene purple allele, P long allele, L red allele, p round allele, l pollen shape gene sister chromatids homologous chromosomes (duplicated) at meiosis I sister chromatids Copyright 2005 Pearson Prentice Hall, Inc. Slide 59 Alleles for genes on the same chromosome assort dependently = alleles tend to stay together during meiosis The 4 types of gametes are not equally likely: Two (called the parental type) are common Two (called the recombinant type) are rare Slide 60 How can you ever get recombinant gametes? Remember the events of Prophase I? Crossing-over generates recombinant gametes Dependent Assortment Slide 61 Slide 62 Detecting Linkage Imagine you have mated a true breeding black guinea pig with smooth hair to a true breeding white one with rough hair. All the offspring are black with rough hair. What are the dominant alleles here? What is the genotype of the F 1 ? Slide 63 You now mate one of your black rough F 1 guinea pigs to a white smooth one. What are the four types of offspring that should be produced? What ratio would you expect them to be in if: There isnt linkage? There is linkage? Slide 64 What are the four types of offspring that should be produced? Black rough Black smooth White rough White smooth Slide 65 Without linkage, all are equally likely: Black rough Black smooth White rough White smooth Slide 66 With linkage: Black smooth: more common (>25%) White rough: more common (>25%) Why are these the parental types? White smooth: less common (