Heredity. Biology 30i Cooper

Transcription

1 Heredity Biology 30i Cooper

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8 Early Theories of Inheritance Aristotle ( B.C.E.) l proposed the first widely accepted theory of inheritance called pangenesis egg and sperm consist of particles called pangenes that come from all parts of the body. upon fertilization the pangenes develop into the parts of the body from which they are derived.

9 egg and sperm consist of particles called pangenes that come from all parts of the body. upon fertilization the pangenes develop into the parts of the body from which they are derived. Antony van Leeuwenhoek ( ) l discovered sperm in semen. he believed he thought he saw a complete miniature person called a homunculus inside the head of the sperm. other people of Antony s time thought that the egg contained the entire person.

10 Mendelian Genetics Gregor Mendel ( ) l a Augustinian monk in Brunn (Czech Republic), Austria l his research laid the foundation for modern genetics and the science of inheritance. l for seven years he bred pea plants (Pisum sativum) and analyzed the results.

11 Mendelian Genetics l Mendel focuses on seven different traits of pea plants..

12 Mendelian Genetics l Mendel let plants self-pollinate to ensure they were true breeding. true breeding plants exhibit the same characteristics generation after generation. Mendel called true breeding plants the parental or P generation the first offspring first filial or F 1 generation If the F 1 generation were to pollinate the offspring would be called the second filial or F 2 generation

13 Mendelian Genetics Mendel called the first offspring first filial or F 1 generation If the F 1 generation were to pollinate the offspring would be called the second filial or F 2 generation because all of Mendel s initial crosses only involved one trait we call them monohybrid crosses. Mendel observed that: for every trait crossed the F 1 generation only showed one of the two parental traits. ie. if plants with round seeds were crossed with plants of wrinkled seeds the F 1 generation would only have plants of round seeds.

14 Mendelian Genetics Mendel observed that: for every trait crossed the F 1 generation only showed one of the two parental traits. ie. if plants with round seeds were crossed with plants of wrinkled seeds the F 1 generation would only have plants of round seeds. even though the F 1 generation had a copy of both genes only one was expressed. Mendel called this characteristic dominant. allele: one of alternative forms of a gene. the gene for wrinkled and the gene for round peas are alleles.

15 Mendelian Genetics even though the F1 generation had a copy of both genes only one was expressed. Mendel called this characteristic dominant. allele: one of alternative forms of a gene. the gene for wrinkled and the gene for round peas are alleles. dominant trait: a characteristic that is expressed when one or both alleles in an individual are the dominant form ~ dominant alleles are indicated by an uppercase letter (R)

16 Mendelian Genetics dominant trait: a characteristic that is expressed when one or both alleles in an individual are the dominant form ~ dominant alleles are indicated by an uppercase letter (R) Mendel called the characteristic that was not expressed recessive recessive trait: a characteristic that is expressed only when both alleles in an individual are the recessive form. Mendel concluded that one form showed complete dominance. an individual with one dominant and one recessive (Rr) had the same characteristics as one with two dominant forms (RR)

17 Mendelian Genetics Mendel concluded that one form showed complete dominance. an individual with one dominant and one recessive (Rr) had the same characteristics as one with two dominant forms (RR) Mendel s Traits Trait Dominant Recessive Stem Length Tall (T) Short (t) Pod Shape Inflated (I) pinched (i) Seed Colour Yellow (Y) Green (y) Flower Position Axial (A) Terminal (a) Flower Colour Purple (P) White (p) Seed Shape Round (R) Wrinkled (r) Pod Colour Green (G) Yellow (g)

18 Mendelian Genetics Important Definitions Homozygous: having identical alleles for the same gene Heterozygous: having different alleles for the same gene. Genotype: the genetic complement of an organism Phenotype: the observable characteristics of an organism Segregation: the separation of alleles during meiosis.

19 Mendelian Genetics Genotype: the genetic complement of an organism Phenotype: the observable characteristics of an organism Segregation: the separation of alleles during meiosis. Law of Segregation l Mendel s First Law All individuals have two copies of each factor (gene). These copies segregate (separate) randomly during gamete formation, and each gamete receives one copy of every gene. l in 1909 Danish Botanist Wilhem Ludwig Johannsen called Mendel s factors genes

20 Mendelian Genetics Analyzing Genetic Crosses Reginald Punnett ( ) devised a visual way to analyze the results of crosses, called a Punnett s square.

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22 Mendelian Genetics Trait Dominant Genotype(s) Recessive Genotype(s) Phenotype Phenotype Stem Length Tall TT (homozygous) Tt (heterozygous) Short tt (homozygous) Pod Shape Inflated II (Homozygous) Ii (hetorozygous) Pinched ii (homozygous) Seed Shape Round RR (Homozygous) Rr (Heterozygous) Wrinkled rr (homozygous) Flower Colour Purple PP (Homozygous) Pp (herozygous) White pp (homozygous) In order to see recessive phenotypes the genotype must be homozygous

23 Mendelian Genetics Punnett Squares are used to predict the genotype and phenotype of potential off-spring very useful when producing economically important cattle and plants. P Generation Phenotypic Ratio Genotypic Ratio

24 Mendelian Genetics Punnett Squares are used to predict the genotype and phenotype of potential off-spring very useful when producing economically important cattle and plants. P Generation F 1 Generation Phenotypic Ratio Genotypic Ratio Phenotypic Ratio Genotypic Ratio

25 Test Cross l a test cross of an individual of unknown genotype to an individual that is fully recessive l the phenotypes of the F 1 generation of the test cross reveals whether the unknown genotype is homozygous or heterozygous l example: you have a white ram (white is dominant W and black is recessive w ) and want to know if it is heterozygous or homozygous for breeding purposes.

26 l example: you have a white ram (white is dominant W and black is recessive w ) and want to know if it is heterozygous or homozygous for breeding purposes. l do a test cross by crossing your unknown ram with one showing a recessive phenotype. it must have a recessive genotype (ww)

27 Test 1 ww Test 2 ww Ww Ww Ww WW Ww Ww ww ww Ww Ww If the ram is Heterozygous it will produce: Phenotypic ratio: 50% white 50% black, or 2:2 or 1:1 Genotypic ratio: 2:2 or 1:1 hetero:homo recessive If the ram is Homozygous it will produce: Phenotypic ratio: 100 % White Genotypic ratio: 100% heterozygous

28 Analyze: Heterozygous Seed shape crossed with a recessive seed shape rr Test 1 Rr Rr Rr rr rr Analyze: What are the predicted phenotypes and genotypes? Phenotypic Ratio 50% round 50% wrinkled or 2:2 or 1:1, ½, 2/4, Genotypic Ratio 50% hetero: 50% homo recessive, 2:2 or 1:1, hetero 2/4 or ½ homo recessive 2/4 or ½

29 Example Problem l A horticulturist has seeds from a cross but does not know the genotype of the phenotype of the parents. Use the following information to figure out the parental phenotype and genotype Solution l Offspring Phenotype round-seed peas wrinkleseed peas Numbers because there is two different phenotypes one the parents must not be homozygous dominant

30 Offspring Phenotype round-seed peas wrinkleseed peas Solution Numbers l 5472/1850 = /1 or 2.96 : 1 ~ 3 : 1 to get a 3:1 ratio both parents must be heterozygous Solution l because there are two different phenotypes one the parents must not be homozygous dominant R R Tester?? R? R? R? R?

31 Mendelian Genetics Proof Rr Proof rr Rr RR Rr Rr rr Rr Rr rr Rr rr 3:1 Phenotypic ratio 1:1 (50%) Phenotypic ratio

32 Mendel s Second Law The Law of Independent Assortment l Mendel also crossed plants of two traits. because two traits are involved in these crosses they are called a dihybrid cross. l Mendel crossed true breeding tall plants that had green pods (TTGG) with true breeding short plants that had yellow pods (ttgg) to produce the F 1 generation

33 l because two traits are involved in these crosses they are called a dihybrid cross. Mendel crossed true breeding plants tall plants that had green pods (TTGG) with true breeding short plants that had yellow pods (ttgg) to produce the F 1 generation P 1 cross TTGG X ttgg predicted phenotypic ratio

34 TG tg tg TG

35 l in this case the true breeding plants will produce only one type of gametes TTGG will produce gametes with the TG genes ttgg will produce gametes with the tg genes tg tg TG TtGg TtGg TG TtGg TtGg l the phenotypic ratio of the F 1 generation: 100% tall and green pods l the genotypic ratio of the F 1 generation 100% heterozygous

36 l Mendel then crossed the F 1 generation to produce an F 2 generation l in this case the plants of the F 1 generation produce four different types of gametes TtGg will produce gametes with the: TG genes (tall, green) Tg genes (tall, yellow) tg genes (short, green) tg genes (short, yellow)

37 TtGg will produce gametes with the: TG genes Tg genes tg genes tg genes TG Tg tg tg TG TTGG TTGg TtGG TtGg Tg TTGg TTgg TtGg Ttgg tg TtGG TtGg ttgg ttgg tg TtGg Ttgg ttgg ttgg

38 TT = tall Tt = tall tt = short GG = green Gg = green gg = yellow TG Tg tg tg Phenotypes Tally TG TTGG TTGg TtGG TtGg Tall & Green Pods 9 Tg TTGg TTgg TtGg Ttgg tg TtGG TtGg ttgg ttgg tg TtGg Ttgg ttgg ttgg Tall & Yellow Pods Short & Green Pods Short & Yellow Pods 3 3 1

39 l for every dihybrid cross that Mendel carried he got the 9:3:3:1 ratio (when he crossed the F 1 generation). this ratio is what is expected if the segregation of alleles for one gene had no influence on the segregation of alleles of another gene. Law of Independent Assortment The two alleles of one gene segregate (assort) independently of the alleles for other genes during gamete formation

40 Law of Independent Assortment The two alleles of one gene segregate (assort) independently of the alleles for other genes during gamete formation Pleiotropic Genes a gene that affects more than one characteristic example: Sickle-cell anemia the normal hemoglobin is produced by the allele Hb A in sicke-cell anemia the individual has two copies of the mutated allele Hb s

41 Pleiotropic Genes a gene that affects more than one characteristic example: Sickle-cell anemia the normal hemoglobin is produced by the allele Hb A in sicke-cell anemia the individual has two copies of the mutated allele Hb s the mutation cause abnormally shaped hemoglobin that cannot deliver oxygen to the cells. causes fatigue, enlarged spleen, pneumonia and major organ damage. a heterozygous individual has resistance to malaria but an increased chance of having homozygous recessive offspring.

42 Beyond Mendel

43 Beyond Mendel Incomplete and Co-dominance l there are patterns of inheritance that do not follow the same patterns that Mendel observed. they still follow the same rules as laid out by Mendel s laws l Incomplete dominance occurs when neither of the two alleles for the same gene can completely conceal the presence of the other. example: Mirabilis jalapa (Four o clock Plant)

44 Beyond Mendel l Incomplete dominance example: Mirabilis jalapa (Four o clock Plant) a cross between a true breeding red-flowered plant and a true-breeding white-flower produces offspring with pink flowers. when representing incomplete dominance upper and lower case letters are not used. all upper case letters are used with subscripts to denote the alleles. R 1 R 1 red flower R 2 R 2 white flower R 1 R 2 pink flower

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46 Beyond Medel l Incomplete dominance two human examples of incomplete dominance are sickle cell anemia and familial hypercholesterolemia Sickle Cell Anemia Hb A Hb A normal red blood cells Hb s Hb s sickle shaped red blood cells Hb A Hb s have the sickle trait this is called heterozygous advantage because if you have one copy of the you don t have the disease and you are resistant to malaria.

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48 Beyond Mendel l Incomplete dominance Familial Hypercholesterolemia a genetic condition that prevents the tissues from removing low-density lipoproteins (bad cholesterol) from the blood. if you are homozygous for the trait you have six times the amount of cholesterol in your blood. most have a heart attack by the age 2 heterozygous individuals have about twice as much cholesterol in their blood and may have a heart attack by the age 35.

49 Beyond Mendel l Co-dominance occurs when both alleles are fully expressed. example: Blue Roan Horses a heterozygous animal where both the base colour and white are expressed. both black and white hairs grow on the body creating a blue appearance

50 Beyond Medel Chromosomal Theory Walter Sutton and Theodor Boveri (1902) l observed chromosomes came in pairs and segregated during meiosis. chromosomes formed new pairs when the egg and sperm united. this supported Mendel s observations on inheritance and his factors became alleles of a gene.

51 Beyond Medel l humans have 44 autosomal chromosomes and 2 sex chromosomes. humans have thousands of different traits. Sutton hypothesized that each chromosome carries multiple genes genes that are located on the same chromosome are said to be linked genes.

52 Beyond Medel l the Chromosomal theory of inheritance: chromosomes carry genes, the units of heredity paired chromosomes segregate during meiosis. Each sex cell or gamete has half the number of chromosomes found in the somatic cells. This explains why each gamete has one one of each of the paired alleles.

53 Morgan s Experiment l studied the principles of inheritance using Drosophila melanogaster, fruit flies l fruit flies a great animals to study because: they reproduce rapidly (in 10 to 15 days) offspring can mate shortly after leaving the egg females produce over 100 eggs they are small and easy to take care of. males can be easily distinguished from females. males have smaller-rounded abdomen, females have a pointed abdomen.

54 Beyond Medel

55 Beyond Medel l Morgan observed a white-eyed phenotype and after a number of test crosses figured out it only occurred in males. a sex-linked trait is one that is determined by genes located on the sex chromosomes l Morgan first crossed a white eyed male with a red eyed female (red eyed being dominant) all members of the F 1 generation had red eyes l Morgan then crossed to members of the F 1 generation he observed ¾ red eyes and ¼ whites eyes.

56 Beyond Medel l Morgan then crossed two members of the F 1 generation he observed ¾ red eyes and ¼ whites eyes in the F 2 generation he noticed all the females had red eyes and the white eyed phenotype only appeared in the males. l because the sex chromosomes in males are not homologous they contain different genes. l Morgan concluded that the Y chromosome does not carry the gene to determine eye colour. we now know the gene for eye colour in fruit flies is on the X chromosome.

57 Beyond Medel Punnett Squares for Sex Linked Inheritance F 1 Generation X R X R F 2 Generation X R X r X r Y X R X r X R Y X R X r X R Y X R Y X R X R X R Y X R X r X r Y 4/4 red eyed fruit flies 3/4 red eyed fruit flies (2 female and 1 male) ¼ white eyed, 1 male

58 Beyond Medel l l in humans it is estimated that the X chromosome carries between 100 and 200 genes the Y chromosome carries less than 100 genes disorders that require two recessive alleles, one on each X chromosome only need to be present once in males. this is why some sex linked disorders occur more frequently in males. examples: colour blindness, hemophilia, near-sightedness (myopia), night-blindness. recessive lethal X-linked disorders also occurs more frequently in males. example: infantile spinal muscular atrophy

59 Beyond Medel Dr. Murray Barr (Uni. of Western Ontario) l recognized dark spots in some somatic cells of female mammals the spot turned out to be sex chromatin. it comes about when one of the X chromosomes becomes inactivated randomly. this spot is now called a Barr Body this means not all female somatic cells are identical, some have two active X chromosomes and some have one. homozygous recessive X disorders can appear in some cells that are heterozygous but have lost the dominant allele.

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61 Beyond Medel Multiple Alleles l Trifolium repens (Clover) thus far there has been only three types of genotypes (homozygous recessive or dominant and heterozygous) but in Clover one gene is responsible for all the patterns on the leaves. in most organisms many genes have more than two alleles. a gene with more than two alleles is said to have multiple alleles.

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63 Beyond Medel l In humans a single gene determines a person s ABO blood type. this gene determines the type of antigen, if any, that is attached to the cell membrane or red blood cells. the gene is designated as I and has three common alleles: I A I B i the different combinations of the three alleles produce four phenotypes known as A, B, AB and O

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65 Beyond Mendel l the I A allele is responsible for the A antigen l the I B allele is responsible for the B antigen l the i allele causes there to be no antigen l the I A and I B heterozygous mix are co-dominant with each other.

66 Beyond Mendel Polygenic Inheritance l Mendel selected characteristics that were distinct so there would be no question of phenotypes. since then people have looked at continuous traits traits that gradually change from one extreme to another. examples: ears, length in corn, weight of beans continuous traits are usually controlled by more than one gene. traits that are controlled by many genes are called polygenetic traits. a group of genes that all contribute to the same trait is called a polygene

67 Beyond Mendel Polygenic Inheritance l example: corn length an ear of corn is controlled by two genes, A and B each dominant allele contributes to length, recessive alleles do not contribute AABB is the genotype with the largest length phenotype aabb is a genotype with the smallest length phenotype

68 Beyond Mendel Polygenic Inheritance l a true-breeding lines for longest and shortest ear lengths are crossed P generation AABB x aabb F 1 generation AaBb l with four genes you start to see a range of lengths. continuous phenotypic traits

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70 Gene Linkage and Crossover Back to Morgan and the fruit flies l first Morgan crossed wild-type body colour (AA) and straight wings (BB) with black body colour (aa) and curved wings (bb) AABB x aabb l the F 1 generation is heterozygous for both traits AaBb l Morgan then crossed the F 1 generation and expected the Mendelian ratio of 9:3:3:1 for a dihybrid cross. instead all the individuals with wild-type body colour had straight wings and all those with black body colour had curved wings.

71 Gene Linkage and Crossover Back to Morgan and the fruit flies l Morgan concluded that the two genes: did not undergo independent segregation for this to happen genes would have to be on the same chromosome the genes are linked

72 Gene Linkage and Crossover Back to Morgan and the fruit flies for this to happen genes would have to be on the same chromosome the genes are linked l because the genes are linked the two gametes form an individual that is heterozygous for both traits. l Morgan predicted that crossing the F 1 generation would produce an F 2 generation what would have a 3:1 phenotypic ratio (3 flies with wildtype body type and straight wings)

73 l Morgan found a number of linked genes Trait Dominant/ Recessive Location wingless (wg) recessive lethal chromosome 2 curly wings (Cy) dmoninant chromosome 2 purple eyes (pr) recessive nonlethal chromosome 2 stubble bristles (Sb) dominant chromosome 3 ebony body (e) recessive nonlethal chromosome 3 miniature wings (m) sex-linked recessive chromosome 4 cut wings (ct) sex-linked recessive chromosome 4 white eyes (w) sex-linked recessive chromosome 4 vermillion eyes (v) sex-linked recessive chromosome 4

74 Crossing Over Morgan observed that in a small number of dihybrid crosses the offspring had different combinations of traits than the parents Data: Phenotype Number Possible Genotypes wild-type body colour, straight wings black body colour, curved wings wild-type body colour, curved wings black body colour, straight wings 290 AABB 92 Aabb 9 AAbb or Aabb (recombinant types) 9 AaBB or aabb (recombinant types)

75 Crossing Over l the new combinations of alleles that Morgan observed came about by DNA crossing over during meiosis.

76 Crossing Over and Chromosome Mapping l there are groups of linked genes on a chromosome. these are called a linkage group l particular genes are always found on the same location (locus) on a chromosome. l Morgan showed that the frequency of crossovers between any two genes in a linkage group is always the same. the frequency of crossing over between any two genes can be stated as a percent.

77 crossover percentage = (number of recombinations/total number of offspring) x 100% using the previous table s data: crossover % = 18/400 x 100% crossover % = 4.5% l geneticists use this number to say the two alleles are 4.5 map units apart.

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79 probability = (genotype looking for)/(total number of possible genotypes) probability = (phenotype looking for)/(total number of possible phenotype)

80 Pedigree Practise

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84 What is the genotype of each individual in this pedigree?

85 What is the genotype of each individual in this pedigree?????????

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87 What is the genotype of each individual in this pedigree?

88 What is the genotype of each individual in this pedigree?

89 What is the blood type of individuals I-4 and I-6