Extensions of Mendelian Inheritance PDF

Extensions of Mendelian Inheritance © McGraw Hill ©Dimitriosp/123RF 1  Introduction Mendelian inheritance describes inheritance patterns that obey two laws Law of segregation Law of independent assortment Simple Mendelian inheritance involves...

Extensions of Mendelian Inheritance © McGraw Hill ©Dimitriosp/123RF 1  Introduction Mendelian inheritance describes inheritance patterns that obey two laws Law of segregation Law of independent assortment Simple Mendelian inheritance involves A single gene with two different alleles Alleles display a simple dominant/recessive relationship © McGraw Hill 2  Introduction In this chapter we will examine traits that deviate from a simple dominant/recessive relationship The inheritance patterns of these traits still obey Mendelian laws, but they are more complex and interesting than Mendel had realized © McGraw Hill 3  Simple Mendelian Inheritance Pattern (the heterozygote phenotype is the same with the homozygous dominant but different from the homozygous recessive) There are many ways in which two alleles of a single gene may govern the outcome of a trait Several different patterns of Mendelian inheritance are described in the next table These patterns are examined with two goals in mind Predict the outcome of crosses Understand the relationship between the molecular expression of a gene and the trait itself © McGraw Hill 4  Mendelian Inheritance Patterns Involving Single Genes Type Description Inheritance: This term is commonly applied to the inheritance of alleles that obey Mendel’s laws and follow a strict dominant/recessive relationship. In this chapter, we will see that Simple some genes occur as three or more alleles, making the relationship more complex. Mendelian Molecular: 50% of the protein, produced by a single copy of the dominant (functional) allele in the heterozygote, is sufficient to produce the dominant trait. Inheritance: In the case of dominant traits, this pattern occurs when a dominant phenotype is not expressed even though an individual carries a dominant allele. An example is an individual who carries the polydactyly allele but has a normal number of fingers and toes. Incomplete penetrance Molecular: Even though a dominant allele is present, the protein encoded by the gene may not exert its effects. This can be due to environmental influences or due to other genes that may encode proteins that counteract the effects of the protein encoded by the dominant allele. Inheritance: This pattern occurs when the heterozygote has a phenotype that is intermediate between either corresponding homozygote. For example, a cross between homozygous red-flowered and homozygous white-flowered parents produces heterozygous Incomplete offspring with pink flowers. dominance Molecular: 50% of the protein, produced by a single copy of the functional allele in the heterozygote, is not sufficient to produce the same trait as in a homozygote making 100% of that protein. © McGraw Hill 5  Mendelian Inheritance Patterns Involving Single Genes Type Description Inheritance: This pattern occurs when the heterozygote has a trait that confers a greater level of reproductive success than either homozygote. Overdominance Molecular: Three common ways that heterozygotes may gain benefits: (1) Their cells may have increased resistance to infection by microorganisms; (2) they may produce more forms of protein dimers with enhanced function; or (3) they may produce proteins that function under a wider range of conditions. Inheritance: This pattern occurs when the heterozygote expresses both alleles simultaneously without forming an intermediate phenotype. For example, with regard to human blood types, an individual carrying the A and B alleles will have an AB blood type. Codominance Molecular: The codominant alleles encode proteins that function slightly differently from each other, and the function of each protein in the heterozygote affects the phenotype uniquely. Inheritance: This pattern involves the inheritance of genes that are located on the X chromosome. In mammals and fruit flies, males have one copy of X-linked genes, whereas females have two copies. X-linked Molecular: If a pair of X-linked alleles shows a simple dominant/recessive relationship, 50% of the protein, produced by a single copy of the dominant allele in a heterozygous female, is sufficient to produce the dominant trait. Males have only one copy of X-linked genes and therefore express the copy they carry. © McGraw Hill 6  Mendelian Inheritance Patterns Involving Single Genes Type Description Inheritance: This pattern refers to the effect of sex on the phenotype of the individual. Some alleles are recessive in one sex and dominant in the Sex-influenced opposite sex. (autosomal inheritance) inheritance Molecular: Sex hormones may regulate the molecular expression of genes. This regulation can influence the phenotypic effects of alleles. Inheritance: In this pattern, a trait occurs in only one of the two sexes. An example is breast development in mammals. (autosomal inheritances) Sex-limited Molecular: Sex hormones may regulate the molecular expression of inheritance genes. This regulation can influence the phenotypic effects of alleles. In this pattern of inheritance, sex hormones that are primarily produced in only one sex are essential for an individual to display a particular phenotype. Inheritance: A lethal allele is one that has the potential of causing the death of an organism. Lethal alleles Molecular: Lethal alleles are most commonly loss-of-function alleles that encode proteins that are necessary for survival. In some cases, such an allele may be due to a mutation in a nonessential gene that changes a protein so that it functions with abnormal and detrimental consequences. © McGraw Hill 7  Dominant and Recessive Alleles Prevalent alleles in a population are termed wild-type alleles These typically encode proteins that Function normally Are made in the proper amounts Genetic Polymorphism can produce more than one wild-type in large populations © McGraw Hill 8  Dominant and Recessive Alleles Alleles that have been altered by mutation are termed mutant alleles They are often defective in their ability to express a functional protein These tend to be rare in natural populations They are typically, but not always, inherited in a recessive fashion © McGraw Hill 9  Dominant and Recessive Alleles In a simple dominant/recessive relationship, the recessive allele does not affect the phenotype of the heterozygote There are two possible explanations to explain the wild-type phenotype of a heterozygote 50% of the normal protein is enough to accomplish the protein’s cellular function The heterozygote may actually produce more than 50% of the functional protein The normal gene is “up-regulated” to compensate for the lack of function of the defective allele © McGraw Hill 10  A Comparison of Protein Levels Among Genotypes Phenotype: Simple dominant/recessive relationship PP-Purple Pp-Purple pp-White © McGraw Hill 11  A Comparison of Protein Levels Among Genotypes Dominant (functional) allele: P (purple) Recessive (defective) allele: p (white) Genotype: Amount of functional protein P PP: 100% Pp: 50% pp: 0% © McGraw Hill 12  Genetic Diseases Are Usually Caused by Mutant Alleles In many human genetic diseases, the recessive allele contains a mutation This prevents the allele from producing a fully functional protein © McGraw Hill 13  Examples of Recessive Human Diseases Protein That Is Produced by Disease Description the Normal Gene* Phenylketonuria Phenylalanine hydroxylase Inability to metabolize phenylalanine. The disease can be prevented by following a phenylalanine-free diet. If the diet is not followed early in life, the result can be severe mental impairment and physical degeneration. Albinism Tyrosinase Lack of pigmentation in the skin, eyes, and hair. Tay-Sachs disease Hexosaminidase A Defect in lipid metabolism. Leads to paralysis, blindness, and early death. Sandhoff disease Hexosaminidase B Defect in lipid metabolism. Muscle weakness in infancy, early blindness, and progressive mental and motor deterioration. Cystic fibrosis Chloride transporter Inability to regulate ion balance across epithelial cells. Leads to production of thick mucus and results in chronic lung infections, poor weight gain, and organ malfunctions Lesch-Nyhan Hypoxanthine-guanine Inability to metabolize purines, which are bases syndrome phosphoribosyl transferase found in DNA and RNA. Leads to self-mutilation behavior, poor motor skills, and usually mental impairment and kidney failure. © McGraw Hill 14  Dominant Mutants Dominant Mutants are much less common than recessive Three explanations for most dominant mutations Gain-of-function Protein encoded by the mutant gene is changed so it gains a new or abnormal function Dominant-negative Protein encoded by the mutant gene acts antagonistically to the normal protein Haploinsufficiency mutant is loss-of-function heterozygote does not make enough product to give the wild type phenotype © McGraw Hill 15  Incomplete Penetrance In some instances, a dominant allele does not influence the outcome of a trait in a heterozygote individual Example: Polydactyly Autosomal dominant trait Affected individuals have additional fingers and/or toes A single copy of the polydactyly allele is usually sufficient to cause this condition In some cases, however, individuals carry the dominant allele but do not exhibit the trait © McGraw Hill 16  Incomplete Penetrance © McGraw Hill 17  Expressivity Expressivity is the degree to which a trait is expressed In the case of polydactyly, the number of digits can vary A person with several extra digits has high expressivity of this trait A person with a single extra digit has low expressivity Expressivity is related to the intensity of a given phenotype © McGraw Hill 18  Expressivity and Incomplete Penetrance The molecular explanation of expressivity and incomplete penetrance may not always be understood In most cases, the range of phenotypes is thought to be due to influences of the Environment Other ‘modifier’ genes © McGraw Hill 19  Environmental Effects on Gene Expression Environmental conditions may have a great impact on the phenotype of the individual Some animals like the arctic fox change coat color Grayish brown in summer, white in winter This is an example of a temperature-sensitive allele © McGraw Hill 20  Environmental Effects on Gene Expression Geneticists often examine a range of conditions when studying the effect of environment on phenotype This allows them to see the norm of reaction of the environmental influence on eye facet number © McGraw Hill 21  Norm of Reaction © McGraw Hill (right): ©Tomatito/Shutterstock 22  Incomplete Dominance, Overdominance, and Codominance (Heterozygote phenotype different from both types of homozygotes) Incomplete Dominance In incomplete dominance the heterozygote exhibits a phenotype that is intermediate between the corresponding homozygotes Example: Flower color in the four o’clock plant Two alleles CR = wild-type allele for red flower color CW = allele for white flower color © McGraw Hill 23  Incomplete Dominance In F1: 50% of the CR protein is not sufficient to produce the red phenotype In F2: 1:2:1 phenotypic ratio NOT the 3:1 ratio observed in simple Mendelian inheritance © McGraw Hill 24  Incomplete Dominance Whether a trait is dominant or incompletely dominant may depend on how closely the trait is examined Example: characteristic of pea shape Mendel visually concluded that RR and Rr genotypes produced round peas rr genotypes produced wrinkled peas However, a microscopic examination of round peas reveals that not all round peas are the same © McGraw Hill 25  Phenotype Comparison A comparison of phenotype at the macroscopic and microscopic levels Dominant (functional) allele: R (round) Recessive (defective) allele: r (wrinkled) Genotype: Amount of functional (starch-producing) protein RR: 100% Rr: 50% Rr: 0% © McGraw Hill 26  Phenotype Comparison Phenotype: With unaided eye (simple dominant/recessive relationship) Round Round Wrinkled © McGraw Hill 27  Phenotype Comparison Phenotype: With microscope (incomplete dominance) Heterozygotes look round, but they only have half the amount of starch found in homozygous dominants Round Round Wrinkled © McGraw Hill 28  Overdominance Overdominance is the phenomenon in which a heterozygote has greater reproductive success compared to both of the corresponding homozygotes It is also called heterozygote advantage Example: Sickle-cell disease Autosomal recessive disorder Affected individuals produce abnormal form of hemoglobin Two alleles HbA  Encodes the normal hemoglobin, hemoglobin A HbS  Encodes the abnormal hemoglobin, hemoglobin S © McGraw Hill 29  Overdominance HbSHbS individuals have red blood cells that deform into a sickle shape under conditions of low oxygen tension This has two major ramifications Sickling phenomenon greatly shortens the life span of the red blood cells Anemia results Odd-shaped cells clump Partial or complete blocks in capillary circulation Thus, affected individuals tend to have a shorter life span © McGraw Hill 30  Overdominance HbAHbS individuals have an “advantage” over HbSHbS, because they do not suffer from sickle cell disease HbAHbA, because they are more resistant to malaria © McGraw Hill 31  The Hbs Allele Is Found at a Fairly High Frequency in Parts of Africa Where Malaria Is Found Malaria is caused by a protozoan, Plasmodium This parasite undergoes its life cycle in two main parts One inside the Anopheles mosquito The other inside red blood cells Red blood cells of heterozygotes, are likely to rupture when infected by Plasmodium This prevents the propagation of the parasite © McGraw Hill 32  Inheritance of Sickle Cell Disease (a) Normal red blood cell (b) Sickled red blood cell © McGraw Hill (a): © Mary Martin/Science Source; (b): © Science Source 33  Inheritance of Sickle Cell Disease © McGraw Hill 34  Explanations for Overdominance At the molecular level, overdominance is due to two alleles that produce slightly different proteins These two protein variants produce a favorable phenotype in the heterozygote There are three possible explanations for overdominance at the molecular/cellular level Disease resistance Homodimer formation Variation in functional activity © McGraw Hill 35  1. Disease Resistance A microorganism will infect a cell if certain cellular proteins function optimally Heterozygotes have one altered copy of the gene Have altered protein function Not enough to cause serious side effects, but is enough to prevent infections Examples include Tay-Sachs disease and resistance to tuberculosis PKU and resistance to fungal toxins © McGraw Hill 36  2. Homodimer Formation Some proteins function as homodimers Composed of two subunits, encoded by the same type of gene, but the alleles of that gene can be different A1A2 heterozygotes Make A1A1 and A2A2 homodimers AND A1A2 homodimers For some proteins, the A1A2 homodimer may have better functional activity Gives the heterozygote superior characteristics © McGraw Hill 37  2. Homodimer Formation The homozygotes that are A1A1 or A2A2 will make homodimers that are A1A1 and A2A2, respectively. The A1A2 heterozygote can make A1A1 and A2A2 and can also make A1A2 homodimers, which may have better functional activity. © McGraw Hill 38  3. Variation in Functional Activity A gene, E, encodes a metabolic enzyme Allele E1 encodes an enzyme that functions better at lower temperatures Allele E2 encodes an enzyme that functions better at higher temperatures E1E2 heterozygotes produce both enzymes An advantage is that the combination of both enzymes function over a wider temperature range than either E1E1 or E2E2 alone © McGraw Hill 39  Multiple Alleles Many genes have multiple alleles Multiple alleles are commonly found within natural populations Typically three or more different alleles However, for genes present in a single copy/haploid genome, a maximum of two alleles are found in any particular diploid individual Example: the ABO blood group © McGraw Hill 40  Multiple Alleles Blood type is determined by the type of antigen present on the surface of red blood cells Antigens are substances that are recognized by antibodies produced by the immune system There are three different alleles that determine which antigen(s) are present on the surface of red blood cells Allele IA, produces antigen A Allele IB, produces antigen B Allele i, does not produce either antigen © McGraw Hill 41  Codominance: ABO Blood Types Allele i is recessive to both IA and IB Alleles IA and IB are codominant They are both expressed in a heterozygous individual © McGraw Hill 42  Codominance: ABO Blood Types The carbohydrate tree on the surface of RBCs is composed of three sugars A fourth can be added by the enzyme glycosyl transferase The i allele encodes a defective enzyme The carbohydrate tree is unchanged IA encodes a form of the enzyme that can add the sugar N-acetylgalactosamine to the carbohydrate tree IB encodes a form of the enzyme that can add the sugar galactose to the carbohydrate tree © McGraw Hill 43  Formation of A and B Antigens by Glycosyl Transferase © McGraw Hill 44  Codominance: ABO Blood Types The A and B antigens are different enough to be recognized by different antibodies For safe blood transfusions to occur, the donor’s blood must be an appropriate match with the recipient’s blood Example: If a type O individual received blood from a type A, type B or type AB blood Antibodies in the recipient blood will react with antigens in the donated blood cells This causes the donated blood to agglutinate A life-threatening situation may result because of clogging of blood vessels © McGraw Hill 45  4.5 Genes on Sex Chromosomes Many species have males and females that differ in their sex chromosome composition Certain traits are governed by genes on the sex chromosomes A pedigree for an X-linked disease shows that it is mostly males that are affected with their mothers as carriers © McGraw Hill 46  Pedigree for Duchenne Muscular Dystrophy © McGraw Hill 47  Punnett Squares of X-linked Traits Punnett squares of X-linked traits show different results depending on which sex is affected Male can transmit his one X or a Y chromosome An affected male and unaffected female have no affected offspring, but females are carriers An affected female and unaffected male have all male offspring affected and females all carriers © McGraw Hill 48  Examples of X-linked Dystrophy Inheritance Patterns © McGraw Hill 49  Sex Chromosomes and Traits Sex-linked genes are those found on one of the two types of sex chromosomes, but not both X-linked Hemizygous in males Only one copy Males are more likely to be affected Y-linked Relatively few genes in humans Referred to as holandric genes Transmitted only from father to son © McGraw Hill 50  Sex Chromosomes and Traits Pseudoautosomal inheritance refers to the very few genes found on both X and Y chromosomes Found in homologous regions needed for chromosome pairing © McGraw Hill 51  Sex-influenced and Sex-limited Inheritance Sex-influenced Inheritance Traits where an allele is dominant in one sex but recessive in the opposite sex Sex influence is a phenomenon of heterozygotes Sex-influenced does not mean sex-linked Sex-influenced traits are autosomal © McGraw Hill 52  Sex-influenced Traits Scur formation in cattle is characterized by small hornlike growths on the frontal bone. © McGraw Hill Courtesy of Sheila M. Schmutz, Ph.D. 53  Sex-influenced Inheritance Example: Scurs (hornlike growth) in cattle Caused by an autosomal gene Allele Sc is dominant in males, but recessive in females Genotype Phenotype in Males Phenotype in Females ScSc Scurs Scurs Scsc Scurs No scurs (hornless) scsc No scurs No scurs © McGraw Hill 54  Sex-limited Inheritance Traits that occur in only one of the two sexes Genes are controlled by sex hormones or the sexual development pathway For example: in humans Ovary development is limited to females Testes growth is limited to males For example: in birds Males have more ornate plumage © McGraw Hill 55  Example of a Sex-limited Trait Feather plumage in chickens (a) Hen (b) Rooster © McGraw Hill (a): ©Pixtal/agefotostock; (b): ©Image Source/PunchStock 56  Lethal Alleles A lethal allele is one that has the potential to cause the death of an organism These alleles are typically the result of mutations in essential genes They are usually inherited in a recessive manner Essential genes are those that are absolutely required for survival The absence of their protein product leads to a lethal phenotype It is estimated that about one-third of all genes are essential for survival Nonessential genes are those not absolutely required for survival © McGraw Hill 57  Lethal Alleles A lethal allele may produce ratios that seemingly deviate from Mendelian ratios Example: Manx cat Carries a dominant mutation that affects the spine This mutation shortens the tail This allele is lethal as a homozygote © McGraw Hill 58  Manx Cat © McGraw Hill 59  Example of Manx Inheritance Patterns © McGraw Hill 60  Lethal Alleles Time when lethal effect is exerted can vary Many lethal alleles prevent cell division These will kill an organism at an early age Some lethal alleles exert their effect later in life Huntington disease Characterized by progressive degeneration of the nervous system, dementia and early death The age of onset of the disease is usually between 30 and 50 © McGraw Hill 61  Conditional Lethal Alleles Conditional lethal alleles may kill an organism only when certain environmental conditions prevail Temperature-sensitive (ts) lethals A developing Drosophila larva may be killed at 30° Celsius But it will survive if grown at 22° Celsius (permissive temperature) Typically caused by mutations that alter structure of the protein at the nonpermissive temperature © McGraw Hill 62  Semilethal Alleles Semilethal alleles kill some individuals in a population, but not all of them Environmental factors and other genes may help prevent the detrimental effects of semilethal genes © McGraw Hill 63  Pleiotropy Most genes actually have multiple effects Multiple effects of a single gene on the phenotype of an organism is called pleiotropy Pleiotropy can be caused because The gene product can affect cell function in more than one way The gene may be expressed in different cell types The gene may be expressed at different stages of development © McGraw Hill 64  Pleiotropic Effects Example: Cystic fibrosis Functional (wild-type) allele encodes the cystic fibrosis transmembrane conductance regulator (CFTR) Regulates ionic balance by transporting Cl- ions Mutant does not transport Chloride effectively In lungs, this causes very thick mucus On the skin, causes salty sweat Poor weight gain due to blockages in tubes that carry digestive enzymes Defect in CFTR can have multiple effects © McGraw Hill 65  Embryonic Development Determines White Spotting Coat Color in Dogs © McGraw Hill 66  Embryonic Development Determines White Spotting Coat Color in Dogs In the white-spotted dogs, portions of an animal’s fur lack pigmentation, including the legs, belly, neck, and the tip of the tail. The spotting (S) gene exists in multiple alleles that affect the amount of pigmentation of an animal’s fur: S+: full pigmentation (that is, no white spotting) sI: Irish spotting sw: extreme-white spotting © McGraw Hill 67  Gene Interactions Gene interactions occur when two or more different genes influence the outcome of a single trait Essentially all traits are affected by contributions of many genes Morphological traits such as height, weight and pigmentation are affected by many different genes in combination with environmental factors © McGraw Hill 68  Types of Mendelian Inheritance Patterns Involving Two Genes Type Description Epistasis An inheritance pattern in which the alleles of one gene mask the phenotypic effects of the alleles of a different gene. Complementation A phenomenon in which two parents that express the same or similar recessive phenotypes produce offspring with a wild-type phenotype. Gene modifier A phenomenon in which an allele of one gene modifies the effect phenotypic outcome of the alleles of a different gene. Gene redundancy A pattern in which the loss of function in a single gene has no phenotypic effect, but the loss of function of two genes has an effect. Functionality of only one of the two genes is necessary for a normal phenotype; the genes are functionally redundant. © McGraw Hill 69  Gene Interactions A Gene interaction can exhibit epistasis and complementation Example: Flower color inheritance in the sweet pea Lathyrus odoratus normally has purple flowers Bateson and Punnett obtained several true-breeding varieties with white flowers Next they crossed different strains of white flowered plants © McGraw Hill 70  Cross Between Two White Pea Varieties © McGraw Hill 71  Cross Between Two White Pea Varieties The F2 generation contained purple and white flowers in a ratio of 9 purple: 7 white Bateson and Punnett reasoned that flower color is determined by two different genes C (one purple-color-producing) allele is dominant to c (white) P (another purple-color-producing) allele is dominant to p (white) cc or pp masks P or C alleles, producing white color A plant that is homozygous for either recessive white allele would develop a white flower regardless of whether or not the other gene contains a purple-producing allele © McGraw Hill 72  Cross Between Two White Pea Varieties Epistasis: Homozygosity for the recessive allele of either gene results in a white phenotype, thereby masking the purple (wild-type) phenotype. Both gene products encoded by the wild-type alleles (C and P) are needed for a purple phenotype. © McGraw Hill 73  Epistasis The term epistasis describes the situation in which a gene can mask the phenotypic effects of another gene Epistatic interactions often arise because two (or more) different proteins participate in a common cellular function Example pathway: colorless Enzyme C colorless purple   Enzyme P precusor intermediate pigment © McGraw Hill 74  Epistasis Recessive alleles do not produce a functional enzyme If an individual is homozygous for either recessive allele It will not make any functional enzyme C or enzyme P Therefore, the flowers remain white © McGraw Hill 75  Epistasis Note: Mendel’s laws of segregation and independent assortment still hold! Geneticists consider epistasis relative to a particular phenotype (cc genotype is epistatic to a purple phenotype/pp genotype is also epistatic to a purple phenotype) If possible, the wild-type phenotype is the reference In the previous example, the cc and pp genotypes exhibit recessive epistasis © McGraw Hill 76  Cross Between Two White Pea Varieties Complementation: In the F1 generation, each recessive allele (c and p) is complemented by a wild-type allele (C and P). This phenomenon indicates that the recessive alleles are in different genes. © McGraw Hill 77  Complementation explanation © McGraw Hill 78  Complementation colorless Enzyme C colorless purple   Enzyme P precusor Cc intermediate Pp pigment © McGraw Hill 79  Gene Modifier Effect A gene interaction can result in a gene modifier effect Example: Inheritance of coat color in rodents A true-breeding black rat crossed to true-breeding albino results in agouti coat color Agouti have black at the tips of each hair that changes to brown (a mixture of yellow and black) near the root If two F1 agouti animals are crossed, they produce offspring in the following ratios 9 agouti 3 black 4 albino © McGraw Hill 80  Coat Color Inheritance in Rodents © McGraw Hill 81  Coat Color Inheritance in Rodents Inheritance of coat color in rodents Is regulated by two genes A for agouti C for colored The A gene regulates the shift from black at the tips to brown near the roots; it allows the synthesis of yellow pigment near the roots, which mixes with black to make brown aa results in black throughout the entire hair The C gene allows any pigmentation to occur cc results in albino © McGraw Hill 82  Coat Color Inheritance in Rodents At least one copy of each dominant allele results in agouti color. An animal with one dominant C but recessive aa will be black. The four cc animals are albino, even if the dominant A is present. c is epistatic to A. aa can also be considered epistatic to C where aa modifies agouti to black © McGraw Hill 83  Gene Redundancy Due to gene redundancy, loss-of-function alleles may have no effect on phenotype Geneticists have developed techniques to directly generate loss-of-function alleles This is called a gene knockout Allows scientists to understand the affects of the gene on structure or function of the organism Many knockouts have no obvious effect on phenotype © McGraw Hill 84  A Molecular Explanation of Gene Redundancy Gene duplication A species may have two or more copies of similar genes These copies are not identical due to the accumulation of random changes during evolution, called paralogs If one gene is missing, a paralog may be able to carry out the missing function Proteins that are involved in a common cellular function When one of the proteins is missing, the function of another protein may be increased to compensate for the missing protein and thereby overcome the defect © McGraw Hill 85  A Molecular Explanation for Gene Redundancy © McGraw Hill 86  Gene Redundancy in Seed Capsule Shape Shepherd’s purse seed capsule shape investigated by George Shull Triangular capsule plant crossed with Ovate capsule plant F1 was all triangular F1 self fertilized to make F2 F2 Ratio: 15 triangular to 1 Ovate At least one copy of T or V present is triangular Only recessive for both genes ttvv is ovate © McGraw Hill 87  Inheritance of Capsule Shape © McGraw Hill 88