Genetics PDF

Topic 10: Genetics © 2017 Cengage Learning. All Rights Reserved. Genetic Variation in Rabbits © 2017 Cengage Learning. All Rights Reserved. Why It Matters … A genetic disorder called sickle-cell anemia develops when a person inherits two...

Topic 10: Genetics © 2017 Cengage Learning. All Rights Reserved. Genetic Variation in Rabbits © 2017 Cengage Learning. All Rights Reserved. Why It Matters … A genetic disorder called sickle-cell anemia develops when a person inherits two mutated copies of a gene (one from each parent) that codes for a subunit of hemoglobin When oxygen is low, the altered hemoglobin forms crystal-like structures that push red blood cells into a sickle shape The altered protein differs from the normal protein by just a single amino acid © 2017 Cengage Learning. All Rights Reserved. Sickle-Cell Anemia A. A normal red blood cell B. A sickled red blood cell Dr. Stanley Flegler/Visuals Unlimited Dr. Stanley Flegler/Visuals Unlimited © 2017 Cengage Learning. All Rights Reserved. Gregor Mendel Mendel discovered the fundamental rules that govern inheritance M. Hofer/National Library of Medicine © 2017 Cengage Learning. All Rights Reserved. The Beginnings of Genetics: Mendel’s Garden Peas Until about 1900, scientists believed in the blending theory of inheritance In the 1860s, Gregor Mendel studied patterns of inheritance by experimenting with garden peas Mendel studied specific heritable features (characters) that had alternative forms (character differences or traits) © 2017 Cengage Learning. All Rights Reserved. The Beginnings of Genetics (cont'd.) Mendel established that characters are passed to offspring in the form of discrete hereditary factors (genes) Mendel observed that many parental traits appear unchanged in offspring – others disappear in one generation to reappear unchanged in the next The inheritance patterns he observed are the result of the segregation of chromosomes to gametes in meiosis © 2017 Cengage Learning. All Rights Reserved. True-Breeding Garden Peas Mendel chose the garden pea (Pisum sativum) for his genetics experiments Normally, pea plants self-fertilize (self-pollinate) Mendel prevented self-fertilization by cutting off the anthers, and used pollen from a different plant to fertilize the flowers (cross-fertilization or cross-pollination) To begin his experiments, Mendel chose pea plants that were true-breeding (pure-breeding) © 2017 Cengage Learning. All Rights Reserved. Research Method: Crossing Peas Pea plant Ovary, containing ovules in which eggs—female gametes—are produced and fertilization occurs, leading to the development of seeds. Stigma, the upper end of the carpel where pollen lands and begins the Carpel—female fertilization structure; lower end process. contains the ovary with the ovules in which eggs (female Anthers, structures gametes) are Stamen—male where pollen—male produced. structure; ends gametes are produced. with an anther. © 2017 Cengage Learning. All Rights Reserved. Research Method: Crossing Peas Male parent Female parent F1 generation F1 generation seeds adult plant Seed Transfer pollen Anthers from male are parent onto removed. stigma of female parent. 1. Remove the anthers from one of the parents 2. The cross-fertilized plant produces seeds. (the white-flowered plant) to prevent self- Seeds may be scored for seed traits, such fertilization. Transfer pollen from the male as round vs. wrinkled shape. Seeds are parent (the purple-flowered plant) onto the grown into adult plants. Plants may be stigma of the white flower (the female scored for adult traits, such as purple vs. parent). This results in cross-fertilization, the white flower color. fertilization of one plant with pollen from another. © 2017 Cengage Learning. All Rights Reserved. Crosses with One Character Mendel crossed true-breeding plants that had purple flowers with true-breeding plants that had white flowers purple ♂ X white ♀ He also carried out a reciprocal cross in which the two parents were switched white ♂ X purple ♀ © 2017 Cengage Learning. All Rights Reserved. Generations True-breeding plants used in an initial cross are called the parental or P generation The first generation of offspring from a cross is called the filial or F1 generation Self-pollination of individuals from the F2 generation produces an F2 generation © 2017 Cengage Learning. All Rights Reserved. Crosses with One Character (cont'd.) Plants that grew from Mendel’s F1 seeds all had purple flowers, as if the trait for white flowers had disappeared In the F2 generation, the missing trait reappeared – both traits were present among the offspring About ¾ of the plants that grew from F2 seeds had purple flowers, but ¼ had white flowers (a ratio of 3:1) For each character Mendel tested, the ratio of the two traits in the F2 generation was close to 3:1 © 2017 Cengage Learning. All Rights Reserved. Mendel’s Crosses with Peas Characte Traits F F Rati r crossed 1 2 o Flower purple × All 705 224 3.15 : color white purple purple white 1 Seed round × All 5,474 1,850 2.96 : shape wrinkled round round wrinkled 1 Seed yellow × All 6,022 2,001 3.01 : color green yellow yellow green 1 Pod inflated × All 882 299 2.95 : shape constricted inflated inflated constricted 1 Pod green × All 428 152 2.82 : color yellow green green yellow 1 Flower axial (along stems) × All 651 207 3.14 positio terminal (at tips) axial axial terminal :1 n Stem tall × All 787 277 2.84 : length dwarf tall tall dwarf 1 © 2017 Cengage Learning. All Rights Reserved. Mendel’s Three Conclusions Adult plants carry a pair of factors (alleles of genes) that govern the inheritance of each trait If an individual’s pair of genes consists of different alleles, one allele is dominant over the other, which is recessive Pairs of alleles segregate as gametes are formed; half the gametes carry one allele, and the other half carry the other allele (Mendel’s Principle of Segregation) © 2017 Cengage Learning. All Rights Reserved. Key Genetics Terms Homozygote A true-breeding individual: both alleles of a gene are the same Produces only one type of gamete A homozygote is said to be homozygous for the particular allele of the gene Heterozygote An individual with two different alleles of a gene Produces two types of gametes – half have one allele, half have the other allele A heterozygote is said to be heterozygous for the pair of different alleles of a gene © 2017 Cengage Learning. All Rights Reserved. Key Genetics Terms (cont'd.) Monohybrid An F1 heterozygote produced from a cross that involves a single character Monohybrid cross A cross between two individuals that are each heterozygous for the same pair of alleles © 2017 Cengage Learning. All Rights Reserved. Genotype and Phenotype Genotype refers to the genetic constitution of an organism in terms of genes and alleles Phenotype refers to the organism’s appearance Example: Two alleles for flower color: P = purple (dominant), and p = white (recessive) Genotypes PP (homozygous dominant) and Pp (heterozygous) produce the purple phenotype Genotype pp (homozygous recessive) produces the white phenotype © 2017 Cengage Learning. All Rights Reserved. Experimental Research: Mendel's Principle of Segregation 1. P generation Purple Whit P is the dominant allele p is the recessive allele e for purple; the true- for white; the true breeding purple- breeding white-flowered × flowered parent has the parent has the pp PP combination of combination of alleles. alleles. The plant is PP p The plant is homozygous homozygous for p for the p allele. the P allele. 2. Haploid gametes The two alleles separate The two alleles separate during gamete during gamete P p formation: only gametes formation: only gametes with the P allele are with the p allele are produced in a PP plant. produced in a pp plant. © 2017 Cengage Learning. All Rights Reserved. Experimental Research: Mendel's Principle of Segregation (cont'd.) 3. F1 generation Gamete from parent with white flowers Fusion of the P gamete from the purple-flowered parent with the p p gamete from the white-flowered parent produces an F1 generation of all Pp plants, which have purple Gamete flowers because the P allele is from P dominant to the p allele. Because parent with they have two different alleles of purple P a gene, the plants are said to be flowers p heterozygous for that gene. The F1 heterozygote is called a monohybrid. © 2017 Cengage Learning. All Rights Reserved. Experimental Research: Mendel's Principle of Segregation (cont'd.) 4. F1 × F1 self Mendel now performed a monohybrid cross by × allowing P P F1 purple Pp plants to p p self and produce the F2 5. F2 generation. generation Gametes from Pp Fl plant P P P Gametes from P P P p The P and p gametes Pp F1 plant fused to produce the F2generation. p P P p P © 2017 Cengage Learning. All Rights Reserved. Probability Probability is the mathematical possibility that an outcome will occur if it is a matter of chance – as in the random fertilization of an egg by a sperm A certain outcome has a probability of 1, and an impossible outcome has a probability of 0 If two different outcomes are equally likely, we divide the probability of one outcome by the total number of possible outcomes (probability of a head when flipping a coin is ½) © 2017 Cengage Learning. All Rights Reserved. The Product Rule in Probability When two or more events are independent, we calculate the probability that they will occur in succession using the product rule (multiply their individual probabilities) Example: Flipping a coin two times Probability of getting heads on the first flip = ½ Probability of heads on the second flip = ½ Probability of getting two heads in a row = ½ X ½ =¼ © 2017 Cengage Learning. All Rights Reserved. Rules of Probability Second toss Probability is 1/2 Probability is 1/2 Probability is 1/2 1/2 × 1/2 = 1/4 1/2 × 1/2 = 1/4 First toss Probability is 1/2 1/2 × 1/2 = 1/4 1/2 × 1/2 = 1/4 © 2017 Cengage Learning. All Rights Reserved. The Sum Rule in Probability When there are two or more different ways of obtaining the same outcome, we determine the probability using the sum rule (add the individual probabilities) Example: Getting a head and a tail in two flips of a coin There are two ways this can happen: head/tail (probability ¼) and tail/head (probability ¼) Probability of 1 head and 1 tail in either order = ¼ + ¼=½ © 2017 Cengage Learning. All Rights Reserved. Probability in Mendel’s Crosses Probability of obtaining purple flowers in the cross Pp X Pp: Two ways to get purple flowers – genotypes PP and Pp Add the individual probabilities: ¼ PP + ½ Pp = ¾ The Punnett square method is also used to determine genotypes of offspring and their expected proportions The probability of obtaining gametes with each type of allele is written at the top for one parent, and on the side for the other parent © 2017 Cengage Learning. All Rights Reserved.  Gametes from F1 purple Pp plant Gametes from F1 purple Pp plant Stepped Art Testcrosses A testcross is a cross between an individual with the dominant phenotype and a homozygous recessive individual Geneticists use a testcross to determine whether an individual with a dominant trait is a heterozygote or a homozygote If half of the offspring have the dominant trait and half the recessive trait, then the tested individual is a heterozygote If all offspring have the dominant trait, the tested individual is a homozygote © 2017 Cengage Learning. All Rights Reserved.  Experimental Research: Genetic Crosses 1. F1 purple plant X true-breeding white plant Purple Whit Fl e (heterozygous) purple- flowered plant from a True-breeding cross of a true- X (homozygous) white- breeding purple- flowered plant P pp flowered plant and a p true 2. breeding Offspring white- Gamete from pp flowered plant plant 1 p 1/2 P The heterozygous Pp plant produces two types of Gametes from 1/2 Pp gametes: 1/2 are P and Pp plant 1/2 are p. The homozygous pp plant Produces one type of 1/2 p gamete: 1 p. Combination 1/2 pp of the gametes produces the offspring. © 2017 Cengage Learning. All Rights Reserved. Experimental Research: Genetic Crosses (cont'd.) 1. True-breeding purple plant X true-breeding white plant Purple White Homozygo True-breeding uspurple- X white-flowered plant flowered plant PP pp 2. Offspring Gamete from pp plant 1 p The homozygous PP plant produces one type of gamete: 1 P. The homozygous pp Gamete from 1 P plant produces one type of gamete: 1 p. PP plant Combination of the gametes produces 1 Pp the offspring. © 2017 Cengage Learning. All Rights Reserved. Crosses Involving Two Characters Mendel next experimented with crosses in which two characters (seed shape and seed color) were involved For seed shape, round is dominant to wrinkled: RR or Rr genotypes produce round seeds The rr genotype produces wrinkled seeds For seed color, yellow is dominant to green: YY or Yy genotypes produce yellow seeds The yy genotype produces green seeds © 2017 Cengage Learning. All Rights Reserved. Key Genetic Terms Dihybrid An F1 that is produced from a cross that involves two characters and is heterozygous for each of the pairs of alleles of the two genes involved Example: Aa Bb, where genes A and B control different traits (upper case = dominant, lower case = recessive) Dihybrid cross A cross between two individuals that are each heterozygous for the pairs of alleles of two genes © 2017 Cengage Learning. All Rights Reserved. Crosses Involving Two Characters (cont'd.) Mendel crossed a true-breeding plant with round, yellow seeds (RR YY) with a true-breeding plant with wrinkled, green seeds (rr yy) through to the F2 generation The F2 generation showed a phenotypic ratio of 9:3:3:1 The allele for seed shape that the gamete receives (R or r) has no influence on which allele for seed color it receives (Y or y) and vice versa – the two events are independent © 2017 Cengage Learning. All Rights Reserved. Experimental Research: Independent Assortment 1. P generation Round, Wrinkled, yellow green X RR YY rr yy 2. Haploid gametes RY ry 3. F1 generation Round and yellow Rr Yy © 2017 Cengage Learning. All Rights Reserved. Experimental Research: Independent Assortment (cont'd.) 4. F1 X F1 self Round, Round, yellow yellow X Rr Rr Yy Yy 5. F2 generation Gamet es ¼RY ¼RY ¼ ry ¼ ry ¼RY 1/16 RR YY 1/16 RR1/16 Rr 1/16 Rr Yy YY Yy ¼RY Gamete 1/16 RR 1/16 RR 1/16 Rr 1/16 Rr s Yy yy Yy yy (eggs) ¼ ry 1/16 Rr YY1/16 Rr 1/16 rr 1/16 rr Yy YY Yy ¼ ry 1/16 Rr Yy1/16 Rr yy1/16 rr Yy1/16 rr yy © 2017 Cengage Learning. All Rights Reserved. Mendel’s Principle of Independent Assortment Mendel showed that traits of different characters were distributed to offspring independently, not inherited together – a property known as independent assortment Mendel’s Principle of Independent Assortment The alleles of genes that govern two characters assort independently during formation of gametes © 2017 Cengage Learning. All Rights Reserved. Molecular Insights: Genes and Dwarfing Research Question: What is the function of Mendel’s Le gene in controlling stem length? Conclusion: The methods of molecular biology allowed contemporary researchers to study a gene first studied genetically in the mid-nineteenth century. The findings leave no doubt that the gene codes for an enzyme that catalyzes formation of a plant hormone responsible for causing plant stems to elongate. © 2017 Cengage Learning. All Rights Reserved. Foundation of the Field of Genetics Mendel’s findings in the 1860s anticipated in detail the patterns by which genes and chromosomes determine inheritance His work was overlooked until the early 1900s, when investigators performed similar experiments and reached the same conclusions Meiosis, which related Mendel’s “factors” to cell structures, was not discovered until the 1890s © 2017 Cengage Learning. All Rights Reserved. Sutton’s Chromosome Theory of Inheritance Walter Sutton recognized the similarities between Mendel’s genes and the behavior of chromosomes: Chromosomes occur in pairs, as do alleles of each gene Chromosomes of each pair are separated and delivered singly to gametes, as are alleles of a gene © 2017 Cengage Learning. All Rights Reserved. Sutton’s Chromosome Theory of Inheritance (cont'd.) Walter Sutton recognized the similarities between Mendel’s genes and the behavior of chromosomes: Separation of a chromosome pair in meiosis is independent of the separation of other pairs, as in independent assortment in Mendel’s dihybrid crosses In fertilization, one member of each chromosome pair is derived from the male parent, and one from the female parent – an exact parallel with the two alleles of a gene © 2017 Cengage Learning. All Rights Reserved.  Behavior of Meiosis in male or female Behavior of genes and alleles in meiosis chromosomes in meiosis diploid parent and correspondence to Mendel’s principles Chromosomes occur Diploid nucleus Alleles of genes occur in in pairs in diploid before replication pairs in diploid individuals individuals (R/r is a pair of alleles and Y/y is another) Alternative path 1 Alternative path 2 Chromosomes replicate before meiosis (follows either left or right path) Metaphase I of meiosis During chromosome First meiotic Principle of segregation: separation, the division Two alleles of a gene chromosomes segregate from each other of different during gamete formation pairs segregate independently Second meiotic Principle of independent division assortment: During the Two meiotic divisions segregation of alleles into separate chromosome gametes, alleles of pairs and deliver them Gametes different pairs assort singly to gametes independently Stepped Art ¼R Y ¼r y ¼R y ¼r Y The Chromosome Theory of Inheritance Sutton correctly concluded that genes and their alleles are carried on the chromosomes, known as the chromosome theory of inheritance The particular site on a chromosome at which a gene is located is called the locus (plural, loci) of the gene The locus is a particular DNA sequence that typically encodes a protein responsible for a phenotype © 2017 Cengage Learning. All Rights Reserved. A Locus Different alleles consist Homologous of differences in DNA chromosome pair (unreplicated) Allele a sequence of a gene, Allele A of gene of gene which may result in A Gene locus a functional differences in (the location of a gene on a chromosome) the protein encoded by the gene © 2017 Cengage Learning. All Rights Reserved. Later Modifications and Additions to Mendel’s Principles Further research revealed many variations on Mendel’s basic principles of dominant and recessive inheritance: Incomplete dominance Codominance Multiple alleles Epistasis Polygenic Inheritance Pleiotropy © 2017 Cengage Learning. All Rights Reserved. Incomplete Dominance When one allele of a gene is not completely dominant over another allele of the same gene, it shows incomplete dominance The phenotype of the heterozygote is intermediate between the phenotypes of the dominant and recessive heterozygotes In a monohybrid cross, the phenotypes of F 2 individuals are seen in a 1:2:1 ratio Example: Flower color in snapdragons © 2017 Cengage Learning. All Rights Reserved. Experimental Research: Incomplete Dominance 1. P generation iStockphoto.com iStockphoto.com The red- The white-flowered X flowered snapdragon is snapdragon is Homozygous Homozygous homozygous for red parent Red C C R R homozygous for White C WC white parent the C W allele. the W 2. F1 C R allele. generation Fusion of CR gametes from the red-flowered plant and CW gametes from the white-flowered plant produces CRCW iStockphoto.com heterozygotes in the F1. These plants have pink flowers, an intermediate phenotype between red and white. This phenotype is not that expected if one of the alleles shows complete dominance to the other allele. This phenotype is, F1 however, consistent with incomplete dominance. offspring Pink C C R W all pink © 2017 Cengage Learning. All Rights Reserved. Experimental Research: Incomplete Dominance (cont'd.) 3. Fl X Fl cross X F1 pink-flowered plants are crossed to produce the F2 generation. Pink R C C Pink R C C W W 4. F2 generation Gametes from one R W C C F1 pink- flowered plant R W C C R C R R R W Gametes from C C C C Each parent plant produces two types of another C RC W F1 gametes, C R and C W. Random fusion of pink-flowered the gametes from the two parents plant produces the F2 generation. W C R W W W C C C C © 2017 Cengage Learning. All Rights Reserved. Codominance Codominance occurs when the effects of two alleles of a gene are equally detectable in heterozygotes Example: Human MN blood group If the genotype is LMLM, the blood type is M If the genotype is LNLN, the blood type is N In heterozygotes – genotype LMLN – two glycoprotein types are present, producing the blood type MN © 2017 Cengage Learning. All Rights Reserved. Multiple Alleles Although an individual can have only two alleles for a gene, multiple alleles (more than two different alleles of a gene) may be present in the population as a whole Example: Gene B may have several altered alleles (b1, b2, b3, etc.), any two of which may be found in an individual Multiple alleles of a gene each contain differences at one or more points in their DNA sequences © 2017 Cengage Learning. All Rights Reserved. Multiple Alleles B allele 5' 3' 3' 5' b1 allele 5' 3' 5' 3' 3' b2 allele 5' 3' 5' b3 allele 5' 3' 3' 5' © 2017 Cengage Learning. All Rights Reserved. ABO Blood Group The human ABO blood group is an example of multiple alleles, dominance, and codominance Red blood cells from one blood type are agglutinated (clumped) by an antibodies in the serum of another type, sometimes causing fatal transfusion reactions Example: People with type A blood have antigen A on their red blood cells, and anti-B antibodies in their blood – if they receive a type B transfusion, the blood will clump © 2017 Cengage Learning. All Rights Reserved. Human Blood Types © 2017 Cengage Learning. All Rights Reserved. ABO Blood Group (cont'd.) The four blood types – A, B, AB, and O – are produced by different combinations of multiple (three) alleles of a single gene I designated IA, IB, and i IA and IB are codominant alleles that are each dominant to the recessive i allele © 2017 Cengage Learning. All Rights Reserved. Inheritance of Blood Types Possible alleles in gametes from father: A or B or i I I A AB A IA IAIA IAIB IAi or Possible AB B B alleles I B in gamete from mother: IAIB IBIB IBi or A B O i IAi IBi ii © 2017 Cengage Learning. All Rights Reserved. Epistasis In epistasis, two genes interact – alleles of a gene at one locus inhibit or mask the effects of alleles of a different gene at a different locus The result of epistasis is that some expected phenotypes do not appear among offspring Epistasis is an important factor in determining an individual’s susceptibility to common diseases such as insulin resistance © 2017 Cengage Learning. All Rights Reserved. Epistasis in Labrador Retrievers The dominant B allele produces black fur color in BB or Bb Labs – the recessive b allele produces brown fur in bb Labs The dominant allele E of a second gene permits pigment deposition – pigment deposition is blocked in homozygous recessive ee individuals Epistasis by the E gene eliminates some of the expected classes from crosses among Labs – BB ee, Bb ee, and bb ee genotypes all have yellow fur © 2017 Cengage Learning. All Rights Reserved. Epistasis in Labradors A. Black labrador B. Chocolate brown C. Yellow labrador labrador Erik Lam/Shutterstock.com c.byatt-norman/Shutterstock.com c.byatt-norman/Shutterstock.com © 2017 Cengage Learning. All Rights Reserved.  Black Yellow Homozygous parents: Black F1 puppies: F2 offspring from cross of Gametes from one Bb Ee F1 dog: two F1 Bb Ee dogs: Gametes from another Bb Ee F1 dog: Stepped Art F2 phenotypic ratio is 9 black : 3 chocolate : 4 yellow Polygenic Inheritance A continuous distribution of phenotypes (such as height) typically results from polygenic inheritance, in which several to many different genes contribute to the same character These characters are known as quantitative traits – individual genes that contribute to a quantitative trait are known as quantitative trait loci or QTLs When numbers of individuals in a series of defined classes are plotted as a graph, polygenic inheritance produces a bell-shaped curve © 2017 Cengage Learning. All Rights Reserved. Continuous Variation A. Students at Brigham Young University, arranged according to height B. Actual distribution of individuals in the C. Idealized bell-shaped curve for a population photo according to height that displays continuous variation in a trait (Line of bell- in each height category in each height category shaped curve Number of individuals Number of individuals indicates continuous variation in population.) 1 4 8 1016 1616 15 141313 11 9 8 8 5 1 2 Shortest Range of heights Tallest Shortest Range of heights Tallest If the sample in the photo included more individuals, the distribution would more closely approach this ideal. © 2017 Cengage Learning. All Rights Reserved. Polygenic Inheritance and the Environment Polygenic inheritance is often modified by the environment Example: Height in humans is not the result of genetics alone Poor nutrition during infancy and childhood limits growth Good nutrition promotes growth © 2017 Cengage Learning. All Rights Reserved. Pleiotropy In pleiotropy, a single gene affects more than one character of an organism Example: Sickle-cell anemia is caused by a recessive allele of a single gene that alters hemoglobin – wide-ranging pleiotropic effects damage many tissues and organs in the body and affect many body functions © 2017 Cengage Learning. All Rights Reserved. Pleiotropy in Sickle-Cell Anemia Homozygous recessive individual Abnormal hemoglobin Sickling of red blood cells Rapid destruction of Clumping of cells and sickle cells leads to anemia interference with blood circulation leads to local failures in blood supply Impaired mental function Pneumonia Heart failure Heart failure Kidney failure Weakness and Abdominal fatigue pain Paralysis