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CH. 6: Patterns of Inheritance Major Concepts: Genes are discrete sequences of DNA on chromosomes; chromosomes consist of DNA and associated proteins. Genes are the units of inherited information. Genes code for several RNA types; mRNA is the template for proteins. Inheritance of genes occurs in regular patterns that can be predicted by the rules of probability. Genetic variation, from mutation and recombination, is essential for evolution. The products of genetic engineering give rise to ethical consideration of benefits and risks to human well-being and environmental integrity. Genes Determine Biological Potential Genes Nucleotides Genes (DNA) Chromosomes Heredity and Environment Genetics: branch of biology that deals with inherited variation. Heredity and Environment Nature vs. Nurture: Both heredity and environment influence an individual’s development. e.g. Siamese cats inherit genes for enzymes that produce dark pigment for fur. The enzymes function best at temps below normal body temp., thus dark markings are at extremities: ears, face, paws, tail. One can change coloration by keeping certain portions of cat’s body cool. Heredity and Environment The environment has a strong impact on gene expression. Heredity and Environment Studies of twins: Fraternal twins: develop from separate eggs each fertilized by separate sperm cells. Identical twins: develop from one zygote forming two complete embryos. Heredity and Environment If a trait shows up more often in identical twins than fraternal: characteristic is probably genetic. If a trait differs in identical twins: probably environmentally influenced. Heredity and Environment Blended inheritance: notion that mixing of parents’ genes resulted in an “averaging” of parental characteristics no longer seriously considered, as it would not allow for passing on of traits separately to future generations, which is observed. Genes Information is stored in genes in the sequence of nucleotide bases that make up DNA: a molecular code. The code directs the cell processes involved in development and function of cells and, thus, the entire organism. Genes provide instructions for the structure, function and development of a cell/organism. Genes Many genes code for the synthesis of specific proteins, e.g. an enzyme, muscle protein, pigment, regulatory proteins, etc.; other genes code for various forms of RNA Through processes of meiosis and fertilization, chromosomes are passed on from generation to generation. Genes Determine Biological Potential Heredity - passing of traits from parent to offspring Genes – basic units of genetic info Genetics - study of heredity Involves predictions referred to as probability - predicts the chances that a certain event will occur Geneticist – one who studies heredity and the actions of genes. Genes and Chromosomes Prokaryotic chromosomes: single circular DNA molecule with little protein; generally no introns. ~ 90% of DNA is translated. Prokaryotic Chromosomes Often have small circles of additional DNA: plasmids. Plasmids may move from one bacterial cell to another, thereby introducing genetic variation. Geneticists use plasmids to introduce modified genetic material into bacterial cells: genetic engineering, e.g. insulin production. Plasmids carry genes for antibiotic resistance. Genes and Chromosomes Eukaryotic chromosomes: consist of long molecules of DNA wrapped around proteins. Only part of the DNA codes for proteins. Some noncoding sections of DNA consist of sequences repeating thousands of times. Genes and Chromosomes Only ~ 1.5% of human DNA codes for proteins. (We don’t know importance of rest.) Some introns are involved with gene expression. Repetitive sequences may serve to stabilize DNA’s bond with associated proteins. Mutations can convert inactive DNA sequences into active genes, or inactivate functional genes may be a source of new alleles in natural selection. Genes and Chromosomes Homologous chromosomes carry same genes, though not necessarily the same alleles for those genes. Genes and Chromosomes Chromosomes may be distinguished by their banding pattern (pattern of dye that occurs when chromosome is stained (Fig. 8.9, p. 191). Ea. chromosome has a distinctive banding pattern. Genes and Chromosomes Karyotype: (Fig. 13.11, p. 349) a display of human chromosomes arranged as homologous pairs. Karyotypes Used in genetic studies of disease to search for hereditary causes. Members of each pair have a specific banding pattern when dyed, as the stains bind to specific regions of the chromosomes. Karyotypes White blood cells frequently used for such studies: They can be made to divide easily. Grow well in culture. Chemicals interrupt cell cycle at metaphase. Why? Cells are placed on microscope slide and treated with water to spread chromosomes apart. Stains cause the banding pattern to appear. Unique banding of each chromosome pair enables researchers to detect missing or extra chromosome parts and extra chromosomes themselves. Have also helped with mapping of genes on chromosomes. Genes and Chromosomes Karyotype: Allows one to count and identify chromosomes, and spot any unusual, missing or extra chromosomes fairly quickly. Genes and Chromosomes Human karyotype made up of 22 pairs of autosomes, chromosomes that are the same in ♂ & ♀, and one pair of sex chromosomes, chromosomes that are different in ♂ & ♀. ♀: XX; ♂: XY Probability Probability: an area of mathematics that predicts the chances that a certain event will occur. Using the rules of probability, one can predict the most probable outcome of randomly ordered events; the actual outcome, however, may not match the prediction, i.e. the prediction is simply that: a prediction, not a guarantee. Investigation 13A: Probability, pp. 748 – 49 (Lab write-up due ___) Gregor Mendel 1822 – 1884 Austrian monk Studied science & math at the University of Vienna Formulated the laws of heredity in the early 1860's Did a statistical study of traits in garden peas over an eight year period Mendel’s work led to the concept of the gene - Mendelian Genetics Why garden peas (Pisum sativum)? Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate several generations (truebreeding) Can be artificially cross-pollinated Garden pea flowers contain both male & female reproductive parts Self-pollination (pollinates itself) Cross-pollination (collect pollen from flowers of one pea plant & transfer to another) Mendel and the Idea of Alleles 4 ways experiments were unique: Looked at only one trait at a time Used large numbers Combined results of many identical experiments Analyzed results using rules of probability Thus, Mendel was able to see patterns of inheritance Mendel and the Idea of Alleles Mendel studied 22 simple traits of pea plants (e.g. seed color & shape, pod color & shape, etc.). Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring. He used his math principles of probability to interpret results. Mendel studied pea traits, each of which had a dominant & a recessive form. Inheritance of Alleles Trait: any characteristic that can be passed from parent to offspring Allele: one of two or more possible forms of a gene (e.g. dominant & recessive) Dominant: an allele that masks the presence of another allele of the same gene in a heterozygous organism, represented by capital letter, e.g. B Recessive: a trait (allele) whose expression is masked (hidden) in a heterozygous organism, represented by lower-case letter, e.g. b Genotype: genetic makeup of an organism; gene combination for a trait (e.g. RR, Rr, rr) Phenotype: observable appearance or trait determined by the genotype; the physical feature resulting from a genotype (e.g. tall, short) Inheritance of Alleles Homozygous: The condition (genotype) in which both alleles are the same form, e.g. RR, rr; can produce only one type of gamete; also called “pure.” Heterozygous: The condition in which two alternate forms (alleles) of a gene are contained within the organism, e.g. Rr; also called “hybrid.” Monohybrid cross: a cross (mating) involving a single trait. Dihybrid cross: a cross (mating) involving two traits. Punnett Square: graphic tool (grid) used to solve genetics problems. Inheritance of Alleles Inheritance of Alleles A Punnett Square: Inheritance of Alleles A Punnett Square: The dominant (shows up most often) gene or allele is represented with a capital letter, & the recessive gene with a lower case of that same letter (e.g. B, b) Mendel’s traits included: a. Seed shape --- Round (R) or Wrinkled (r) b. Seed Color ---- Yellow (Y) or Green (y) c. Pod Shape --- Smooth (S) or wrinkled (s) d. Pod Color --- Green (G) or Yellow (g) e. Seed Coat Color --- Gray (G) or White (g) f. Flower position --- Axial (A) or Terminal (a) g. Plant Height --- Tall (T) or Short (t) h. Flower color --- Purple (P) or white (p) Mendel’s Experiments (cont.) 1st: Mendel tested each strain of plant he used to ensure that it was true-breeding (homozygous), i.e. genetically true, producing offspring identical to themselves generation after generation. 2nd: He worked with strains that were true-breeding in all but one characteristic, e.g. tall vs. short plant form; green vs. yellow seeds, round vs. wrinkled seeds, etc. This allowed him to follow the pattern of inheritance of one trait at a time from generation to generation, e.g. round vs. wrinkled seeds: Mendel’s Experiments (cont.) Parental generation (P1): Crossed round seed-producing plants with wrinkled seed-producing plants. First filial generation (F1): All offspring of the above cross produced round seeds. Second filial generation (F2): ¾ produced round seeds; ¼ produced wrinkled seeds. Thus, wrinkled seeds seemed to disappear in one generation (F1), then reappear in the next (F2). Mendel called the round seed condition, “dominant,” and the wrinkled seed condition, “recessive.” He surmised that the recessive form of a trait could only be manifest when the individual inherited that trait from both parents, i.e. received two “doses” of the trait. This is known as Mendel’s principle of dominance. Mendel’s Experiments (cont.) Mendel’s Experiments (cont.) Mendel’s Experiments (cont.) Alleles are not dominant or recessive; only their effects on a trait are. At the molecular level, the genotype, both alleles are present. At the level of the organism, the phenotype, the effects of one allele (the dominant form) may mask those of the other (recessive form) allele. Mendel repeated this type of experiment for other traits (outlined in Fig. 6.10, p. 179). He calculated the ratio of dominant to recessive forms for each trait and it was always essentially the same: the dominant form appeared in approx. ¾ of the F2 plants, while the recessive form appeared in ¼ of the F2 plants. Thus, the ratio was always 3:1. (See p. 179) Inheritance of Alleles Mendel did not know about genes. He referred to dominant and recessive “factors” to describe the results of his experiments. He did not know where these “factors” were located in cells. Hypothesized that only one copy of a factor went into each sperm or ovum, i.e. if a parent were truebreeding for round seeds, for example, all its gametes would have the “round-seed factor.” Similarly for “wrinkled-seed factor.” Offspring of a round-seed by wrinkled seed cross would have one factor of each from each parent: the principle of segregation. Inheritance of Alleles Genotype: genetic makeup of an organism; made up of the alleles for any gene, e.g. RR, Rr, rr. Phenotype: the observable appearance or trait that is determined by the genotype, e.g. Three distinct genotypes: BB phenotype: purple flowers (dominant) Bb phenotype: purple flowers (dominant) bb phenotype: white flowers (recessive) Inheritance of Alleles Inheritance of Alleles Mendel also worked with plants that varied in two traits at a time, e.g. round vs. wrinkled seeds and yellow vs. green seed color. Dihybrid cross: a cross that results in offspring that are heterozygous for two (di) traits. See Fig. 13.14, p. 353; a Punnett square F2 Phenotypes occur in 9:3:3:1 ratio (9/16, 3/16, 3/16, 1/16). Each trait individually displays the 3:1 ratio of a monohybrid cross. The genes for the various traits separate independently from one another . . . Principle of independent assortment: alleles for one trait segregate independently of alleles for the other trait during gamete formation. Dihybrid Cross Dihybrid Cross Dihybrid Cross Patterns of Inheritance Testcross: a cross between an organism with an unknown genotype and an organism with the recessive phenotype The recessive phenotype individual must be homozygous recessive, thus one knows the genotype. The resulting Punnett Square results will allow you to determine the genotype of the “unknown” parent. Quick Lab: Using a Testcross (p. 185) Patterns of Inheritance Mendel worked with garden peas, but his results apply to inheritance in other organisms as well, e.g cystic fibrosis, which geneticists now know is a recessive trait. F: allele for no disease f: allele for cystic fibrosis (Read story from p. 183 in green Biology text) What are the genotypes of Bob, Mary and Lisa? What are the chances that Bob and Mary’s next child will have cystic fibrosis? 7.2 Multiple Alleles & Alleles Without Dominance Incomplete Dominance: A heterozygous condition in which both alleles at a gene locus are partially expressed, often producing an intermediate phenotype. (Fig. 8.7, p. 190) P1 ♂: Red; P2 ♀: White F1: All Pink F2: 25% Red; 50% Pink; 25% White 7.2 Multiple Alleles & Alleles Without Dominance Codominance: The situation in which a heterozygote shows the phenotypic effects of both alleles fully & equally, (e.g. blood group antigens). R = allele for red flowers W = allele for white flowers red x white red & white spotted RR x WW 100% RW 7.2 Multiple Alleles & Alleles Without Dominance Codominance: The situation in which a heterozygote shows the phenotypic effects of both alleles fully & equally, (e.g. mottled coat coloration in a cow). 7.2 Multiple Alleles & Alleles Without Dominance Multiple alleles: Genes that have more than two types of alleles; provide another example of codominance, e.g. three alleles for blood type in humans (Table 8.2, p. 190): Genotype IAIA or IAi IBIB or IBi I AI B ii Blood phenotype A B AB (codominance) O (Three different alleles exist, but an individual has only two of the alleles.) Blood Types 7.2 Multiple Alleles & Alleles Without Dominance 7.2 Multiple Alleles & Alleles Without Dominance Multifactorial Inheritance: Inheritance determined by the combined effects of genetic and environmental factors. 7.2 Multiple Alleles & Alleles Without Dominance Discontinuous (Discrete) Traits: traits like cystic fibrosis, that are either present or absent. Generally controlled by a single pair of alleles. Continuous Traits: traits that vary across a broad range, e.g. height, weight, intelligence, hair color, skin color, eye color in humans. 7.2 Multiple Alleles & Alleles Without Dominance Continuous variation is the result of multifactorial inheritance, the interaction of at least several genes with a large number of possible environmental variables, e.g. skin pigment, melanin, is result of four genes; exposure to sun also plays a role. 7.2 Multifactorial Inheritance Cleft lip and spina bifida also result from combination of genes and environmental conditions in mother’s womb. 7.2 Multiple Alleles & Alleles Without Dominance Polygenic inheritance (p. 206): Multiple genes influence phenotype, but there is no environmental influence. Incomplete dominance, codominance, multiple alleles, and multifactorial inheritance don’t yield the same ratios as Mendel’s simple crosses, but are still considered a form of Mendelian inheritance, i.e. they are the result of genes residing on chromosomes that are transmitted by meiosis during sexual reproduction. 7.2 Multiple Alleles & Alleles Without Dominance Epistasis (from Gk. “to stand upon”): The phenotypic expression of a gene at one locus alters that of a gene at a second locus, e.g. coat color in labrador retrievers. Gene 1: Coat color – Black (B) or Brown (b) Gene 2: Allows (E) or disallows (e) coat pigment to be deposited. EE or Ee: coat will be black (BB or Bb) or brown (bb) ee: coat will be yellow 7.2 Multiple Alleles & Alleles Without Dominance Epistasis: 7.2 Multiple Alleles & Alleles Without Dominance Pleiotropy: Genes that have multiple phenotypic effects Pleiotropic alleles are responsible for the multiple symptoms associated with certain hereditary diseases, e.g. cystic fibrosis. 7.2 Multiple Alleles & Alleles Without Dominance Some genetic information is passed on differently: the DNA of chloroplasts and mitochondria is passed in a different manner. Endosymbiont Theory: Tracing Human Evolution: Tracing Human Evolution: 7.2 Multiple Alleles & Alleles Without Dominance Bacteria do not undergo meiosis, but exhibit a great variety of non-Mendelian forms of inheritance. 7.3 Linked Genes There are many genes on each chromosome. Genes that are found close to one another on a chromosome are said to be linked; they are, therefore, frequently inherited together (Fig. 7.11, p. 211). As an example: the gene for ABO blood type is found on chromosome 9; so is an oncogene that may be partially responsible for certain types of cancer. Geneticists almost always find these genes together, i.e. little or no evidence of independent assortment. 7.3 Linked Genes Quiz Question: Why do geneticists find little or no evidence of independent assortment among the genes for blood type and a particular oncogene involved in certain types of cancer? 7.3 Linked Genes Linked genes do not always remain together, but the closer on the chromosome they are to one another, the more likely it is that they will remain together (less chance of the chromosome crossing over right between the genes in question). The farther apart two genes are from one another on a chromosome, the more likely a break will occur somewhere between them. 7.3 Linked Genes 7.3 Linked Genes If two genes are far enough apart on a chromosome, the principle of independent assortment may apply to their inheritance, i.e. the genes exhibit no linkage. 7.3 Linked Genes Genetic maps/Linkage maps: Geneticists can use the frequency with which two linked traits become separated to determine the relative distance between the two genes on the chromosome. From this information, they can construct genetic maps, diagrams of the locations of genes on chromosomes (See Fig. 7.10, p. 211). 7.3 Linked Genes 7.3 Linked Genes 7.1 X-Linked Traits Show a Modified Pattern of Inheritance Human karyotype made up of 22 pairs of autosomes, chromosomes that are the same in ♂ & ♀, and one pair of sex chromosomes, chromosomes that are different in ♂ & ♀. ♀: XX; ♂: XY 7.1 X-Linked Traits Show a Modified Pattern of Inheritance Other species: Some insects have an X-O system: ♀: XX; ♂: X (no Y chromosome exists). Birds, some fish, and some insects have a Z-W system: ♂: ZZ; ♀: ZW. Some plants have an XY system. Most plants and some animals have no sex chromosomes; sex is determined by a single pair of alleles. 7.1 X-Linked Traits Show a Modified Pattern of Inheritance During meiosis, chromosomes separate. Each ovum contains an X chromosome. Males produce two types of sperm though: some with X chromosome; some with Y. If ovum is fertilized by X-bearing sperm, zygote develops into ♀ If fertilized by Y-bearing sperm, ♂. Thus, in humans, the father determines sex of offspring. 7.1 X-Linked Traits Show a Modified Pattern of Inheritance Thomas Hunt Morgan: studied fruit flies (Drosophila melanogaster) at Columbia University. Noticed differences among certain flies, e.g. white eyes, instead of red; short wings, instead of long; yellow or black bodies, instead of gray. 7.1 X-Linked Traits Show a Modified Pattern of Inheritance Considered these differences mutations, variations of normal genetic material; alternative forms of a gene. Mutations are how new alleles come to be formed. Some mutations are beneficial (provide new material for evolution); some have no effect on organism at all. In other words, not all mutations are bad or deleterious. Mutations are the raw material for evolution. 7.1 X-Linked Traits Show a Modified Pattern of Inheritance Not all mutations are bad or deleterious. Mutations are the raw material for evolution. 7.1 X-Linked Traits Show a Modified Pattern of Inheritance Morgan et al. studied mutations by mating mutant flies with nonmutants. Most mutations were inherited according to Mendelian patterns, except . . . 7.1 X-Linked Traits Show a Modified Pattern of Inheritance . . . White eyes: Morgan crossed white-eyed ♂ with normal redeyed ♀’s (Fig. 7.3, p. 201): F1 generation: all red eyed (as expected, if white eye allele was recessive). F2 generation: only males displayed white eyes. 7.1 X-Linked Traits Show a Modified Pattern of Inheritance P1 Generation: XRXR x XrY 7.1 X-Linked Traits Show a Modified Pattern of Inheritance In another experiment, he crossed white-eyed ♀’s with red-eyed ♂’s: F1 generation: only ♀’s were red-eyed; all ♂’s had white eyes (because they inherited their single X chromosome from their mother). XR red eyes Xr white eyes Y chromosome does not contribute to eye color Key to understanding: ♂ = XY; ♀= XX 7.1 X-Linked Traits Show a Modified Pattern of Inheritance P1 Generation: XrXr x XRY 7.1 X-Linked Traits Show a Modified Pattern of Inheritance X-linked inheritance: An X-linked trait is a trait whose gene is only on the X chromosome. Over 300 X-linked traits have been identified in humans, including hemophilia, red-green colorblindedness, and Duchenne muscular dystrophy. ** X-linked traits occur more often in males than females. Autosomal traits occur in both sexes more or less equally. 7.1 X-Linked Traits Show a Modified Pattern of Inheritance 7.4 X-Linked Traits Show a Modified Pattern of Inheritance Color Blindness: Color Blindness: 7.4 X-Linked Traits Show a Modified Pattern of Inheritance Hmwk: 1. 2. Redraw figure (next slide) on paper. Complete the genotypes for all individuals. X-Linked Traits Hmwk: XBXB XBY XBXb XbY XbXb = male = female Blue = color blind = male X-Linked Traits = female XBXB XBXb Hmwk: XBY XBXB XBXb XbY XBXb XBY XBXb XBY XBXB XBY XbY XbY XBY XBY XBXb XBXB XBXb XBXB XBXb XBY XBY XBXB XBXb Blue = color blind XBY XBXb XBXB XBXb XBXB XBXb XBY XbY XBXb XBXb XbY XbY XBXb Investigation: Pedigree Analysis P. 218 in text Is supertasting an X-linked trait? Complete handouts for credit as lab activity See pedigree on next slide T1T1 = Supertaster = male T1T2 = Medium taster T2T2 = Nontaster = female T1T1 = Supertaster = male T1T2 = Medium taster T1T2 T2T2 T2T2 = Nontaster T1T2 T1T2 T2T2 T1T2 T1T2 T2T2 T1T2 T1T1 T1T2 T2T2 T1T2 T2T2 T1T2 T2T2 T1T2 T2T2 T1T2 T1T1 T1T2 T1T1 T1T2 T1T2 T1T2 T1T1 T1T2 T2T2 Jack T1T2 T1T1 T2T2 ? = female T1T2 T1T1 T1T2 T2T2 T1T1 T1T2 T2T2 Jill T1T2 T2T2 T1T2 T2T2 T1T2 T2T2 ? T1T2 T2T2 T1T2 T2T2 T1T2 T2T2 T1T2 T1T2 T1T2 T2T2 T2T2 T1T2 T1T1 Nondisjunction (p. 192) Abnormal numbers or types of chromosomes can result in certain developmental errors. Result of nondisjunction. Nondisjunction Down’s Syndrome (AKA Trisomy 21): A condition characterized by distinctive features of the eyes, mouth, hands, and sometimes internal organs, retarded mental development (though the degree of delayed development varies greatly). Down’s syndrome individuals have 47 chromosomes, instead of 46; extra chromosome 21, Trisomy: Having three copies of a given chromosome (Fig. 7.16, p. 217) Syndrome: a group of symptoms associated with a particular disease or condition. Nondisjunction Down’s Syndrome (Trisomy 21) Nondisjunction Turner syndrome (45, X karyotype): a condition resulting from having only one X chromosome and no Y chromosome. Usually short, underdeveloped and sterile ♀. XXX syndrome (47, XXX karyotype): ♀ with limited fertility; may have slight intellectual impairment. Nondisjunction Klinefelter syndrome (47, XXY karyotype): ♂; often tall and sexually underdeveloped. Nondisjunction These conditions occur when chromosome pairs do not separate in meiosis, an event called nondisjunction. Results in the formation of abnormal gametes, i.e. some sperm or ova get extra chromosomes; some get too few. When these gametes fuse with normal gametes abnormal development usually occurs. Nondisjunction Nondisjunction Evidence suggests that every cell must contain at least two of each type of chromosome for the embryo to develop. An exception is the X chromosome, 45. A missing X chromosome, or extra sex chromosomes (X or Y) usually permit a fetus to develop, though that development may be abnormal. Except for trisomy 21, an extra autosome usually results in the death of the fetus. Most spontaneously aborted fetuses have abnormal chromosome numbers. Nondisjunction 13.10 Nondisjunction Mary Lyon: British geneticist Proposed that early in development of a normal ♀, one X chromosome in each body cell is inactivated. Based proposal on finding that in each cell nuclei of ♀’s, but not ♂’s, a darkly-staining body appears: a Barr body. (See Fig. 7.4, p. 203) Nondisjunction ♀’s with three X chromosomes have two Barr bodies, suggesting that all but one X chromosome are rendered inactive. How many would someone with Turner syndrome have? What about XXX syndrome? Nondisjunction May be one reason trisomies of X chromosomes are not as disruptive as autosomal trisomies. Nondisjunction Some gene function on the condensed X chromosomes is apparently maintained, resulting in the abnormalities associated with extra X chromosomes. Nondisjunction Chromosomal Abnormalities Some abnormalities result from altered chromosome structure, e.g. Deletion: a piece missing Inversion: a section reversed Duplication: a piece attaches to a pairing partner Translocation: a piece attaches to another, unrelated chromosome. Chromosomal Abnormalities Some abnormalities result from altered chromosome structure, e.g. Deletion: a piece missing Chromosomal Abnormalities Inversion: a section reversed Chromosomal Abnormalities Duplication: a piece attaches to a pairing partner Chromosomal Abnormalities Translocation: a piece attaches to another, unrelated chromosome.