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Gregor Mendel on Vimeo http://vimeo.com/8854352 Pea Plant Characteristics Why Pea Plants? • Easy, cheap, and quick to grow • 7 easily distinguishable contrasting traits • Normally self-pollinate due to tight enclosure of the parts of the flower • Can be cross-pollinated by cutting out the stamens from one flower and using them to pollinate another flower with a contrasting trait Fig. 14-2a TECHNIQUE 1 2 Parental generation (P) Stamens Carpel 3 4 Mendel’s Crosses • Mendel crossed plants that had bred true for a certain trait for multiple generations with plants that bred true for the contrasting trait for multiple generations. • He called these the P generation. • The offspring from these crosses were called the F1 generation. • The offspring from crosses between the F1 plants were the called F2 generation. Fig. 14-2b RESULTS First filial generation offspring (F1) 5 • Mendel observed: • the F1 generation always had a single phenotype • the phenotype of the other parent remained hidden. Where does the recessive trait go during the F1 generation? He hypothesized that it is still present in all the offspring, but is not expressed. Mendel’s Laws • The Law of Dominance: • When two plants are crossed that are pure for contrasting traits, then one of the traits, the dominant one, will be expressed in the offspring. The other trait, the recessive one, will not be expressed. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Without knowing about meiosis and genes and how gametes are formed, Mendel determined: • The Law of Segregation: • When an individual produces gametes, the two “factors” responsible for a trait separate so that a gamete will receive only one factor or the other. Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Mendel asked: How will the recessive trait ever be expressed if it is dominated by the other trait? • It can only be expressed when there is no dominant trait present. • He called this genotype homozygous recessive. When writing genotypes, a capital letter represents the dominant trait and a lower-case letter represents the recessive trait. Tall – T Short - t The same letter is normally used. Yellow seed – Y Green seed - y Gamete Formation • Due to meiosis, only one chromosome from each homologous pair is in each gamete. • Thus, there will be one allele for each trait in each gamete. • Mendel tracked one or two alleles at a time. Pure Tall Pea Plant X Pure Short Pea Plant TT T X T tt t t Hybrid Tall Pea Plant X Hybrid Tall Pea Plant Tt T X t Tt T t Punnett Squares • A Punnett square makes it easy to track the alleles which are passed on to offspring by the fertilization of gametes. Using a Punnett square: Male gametes A A Female gametes a a Possible Genotypes A a A AA Aa a Aa aa Tracking Traits • genotype • the alleles of a given gene locus • phenotype • the physical expression of a gene What are the possible genotypes for the offspring of each of the following crosses? 1) AA x AA 2) Aa x aa 3) AA x aa 4) aa x Aa 5) AA x Aa 6) Aa x Aa 1) AA x AA? 2) Aa x aa? 3) AA x aa? 4) aa x Aa? 5) AA x Aa? 6) Aa x Aa? 100% AA 50% Aa; 50% aa 100% Aa 50% Aa, 50% aa 50% Aa, 50% AA 25% AA, 50% Aa, 25% aa With only two alleles: • there are a limited number of possible combinations for any one gene • these examples represent the simplest circumstances Sometimes there are genes with three or more alleles • Ex. blood types Sometimes one allele does not have complete dominance over the other. Probability If I flip a coin one time, what is the probability that it will land on heads? 1 out of 2 chances, or 50% Since I got heads (or tails) the first time, what is the probability that I will get heads on my next flip? It is still 50%. If I keep flipping the coin and getting heads, does that affect the probability of the outcome of my next flip? No, the chances of getting heads or tails is the same on each flip. What is the probability that I will get heads on two coin tosses in a row? You are now asking for the probability of flipping heads and flipping heads again. When you have two separate probabilities and both must occur, the two separate probabilities must be multiplied to find the overall probability—so the probability of heads and heads occurring is 1/2 x 1/2 = 1/4 • The probability of two events combined occurring will be much lower than one event alone • the chance of two coin tosses both being of a particular outcome is less likely than one coin toss of a particular outcome. Rule of Multiplication • Multiply the probabilities of a combination of possible outcomes. • Multiply whenever there is a statement of one probability “and” another probability. What is the probability that a couple would have three boys? The separate probabilities are 1/2 for each boy, so the combined probability would be 1/2 x 1/2 x 1/2 = 1/8 What is the probability that parents with the genotypes Aa and Aa would have a child with the genotype AA? • The probability of the father giving his child a gamete with the “A” allele is ½ • The probability of the mother giving her child a gamete with the “A” allele is ½ 1/2 x 1/2 = 1/4 If the probability of getting dealt an ace from a deck of cards is 4 out of 52 (4/52), what is the probability of getting dealt an ace or a king? The probability of getting an ace is 4/52 and the probability of getting a king is 4/52; to learn what chance there is of getting dealt either card, you must add the probabilities together to get the total probability: 4/52 + 4/52 = 8/52 What is the probability of getting any face card? Calculate this two ways: • first, count up the total number of face cards (12/52) • verify this, using the rule of addition: (kings) 4/52 + (queens) 4/52 + (jacks) 4/52 = 12/52 Rule of Addition Adding the probabilities of a combination of independent outcomes is called the rule of addition. Use this whenever there is a statement of one probability “or” another probability. What is the probability of my being dealt an ace or a face card from one deck of cards? The probability of getting an ace is 4/52 and the probability of getting a face card is 12/52, so the probability of either outcome occurring is 4/52 + 12/52 = 16/52 If a child with a dominant gene A is affected by a particular disease, what is the probability of a couple having a child with this disease if they have the genotypes of Aa (father) and aa (mother)? The probability of the father giving the child an “A” allele is 1/2 and the probability of the mother giving the child an “A” allele is 0, so the total probability is 1/2 + 0 = ½ Verify this using a Punnett square. The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-7 TECHNIQUE Dominant phenotype, Recessive phenotype, unknown genotype: known genotype: PP or Pp? pp Predictions If PP Sperm p p P Pp Eggs If Pp Sperm p p or P Pp Eggs P Pp Pp pp pp p Pp Pp RESULTS or All offspring purple 1/2 offspring purple and 1/2 offspring white Fig. 14-7a TECHNIQUE Dominant phenotype, Recessive phenotype, known genotype: unknown genotype: pp PP or Pp? Predictions If PP Sperm p p P P Pp Eggs If Pp Sperm p p or Pp P Eggs Pp Pp pp pp p Pp Pp Fig. 14-7b RESULTS or All offspring purple 1/2 offspring purple and offspring white 1/2 The Law of Independent Assortment • Mendel derived the law of segregation by following a single trait • The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one trait • A cross between such heterozygotes is called a monohybrid cross Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Mendel identified his third law of inheritance by following two characters at the same time • Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters • A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-8 EXPERIMENT YYRR P Generation yyrr Gametes YR F1 Generation yr YyRr Hypothesis of dependent assortment Predictions Hypothesis of independent assortment Sperm or Predicted offspring of F2 generation 1/ 4 Sperm 1/ YR 1/ 2 2 yr 1/ 4 1/ 2 YR 1/ 4 Yr 1/ 4 yR 1/ 4 yr YR YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YYRR Eggs 1/ 2 YyRr 1/ 4 Yr Eggs yr YyRr 3/ 4 yyrr 1/ 4 yR 1/ 4 Phenotypic ratio 3:1 1/ 4 yr 9/ 16 3/ 16 3/ 16 1/ 16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 Fig. 14-8a EXPERIMENT YYRR P Generation yyrr Gametes YR F1 Generation yr YyRr Hypothesis of independent assortment Hypothesis of dependent assortment Predictions Sperm or Predicted offspring of F2 generation 1/ 4 Sperm 1/ 2 YR 1/ 2 1/ 4 Yr yR 1/ 4 yr YR YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YYRR Eggs 1/ 2 1/ 4 yr 1/ 4 1/ 2 YR YyRr 1/ 4 Yr Eggs yr yyrr YyRr 3/ 4 1/ 4 yR 1/ 4 Phenotypic ratio 3:1 1/ 4 yr 9/ 16 3/ 16 3/ 16 1/ 16 Phenotypic ratio 9:3:3:1 Fig. 14-8b RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 • Using a dihybrid cross, Mendel developed the • Law of independent assortment • Each pair of alleles segregates independently of each other pair of alleles during gamete formation Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings We now know • Mendel’s Law of Independent Assortment • Applies only to genes on different, nonhomologous chromosomes • The traits he studied were, in fact, on nonhomologous chromosomes • Genes located near each other on the same chromosome tend to be inherited together Solving Complex Genetics Problems with the Rules of Probability • The multiplication and addition rules can predict the outcome of crosses involving multiple characters – A dihybrid cross is equivalent to two or more independent monohybrid crosses occurring simultaneously – In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Inheritance patterns are often more complex than predicted by simple Mendelian genetics • Many heritable characters are not determined by only one gene with two alleles. • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Extending Mendelian Genetics for a Single Gene • Inheritance of characters by a single gene may deviate from Mendelian patterns: – When alleles are not completely dominant or recessive – When a gene has more than two alleles – When a gene produces multiple phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Degrees of Dominance • In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties • In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-10-1 P Generation Red CRCR Gametes White CWCW CR CW Fig. 14-10-2 P Generation Red CRCR Gametes White CWCW CR CW Pink CRCW F1 Generation Gametes 1/2 CR 1/ 2 CW Fig. 14-10-3 P Generation Red CRCR White CWCW CR Gametes Incomplete dominance in snapdragon color CW Pink CRCW F1 Generation Gametes 1/2 CR 1/ CW 2 Sperm 1/ 2 CR 1/ 2 CW F2 Generation 1/ 2 CR Eggs 1/ 2 CRCR CRCW CRCW CWCW CW The Relation Between Dominance and Phenotype • A dominant allele does not subdue a recessive allele; alleles don’t interact • Alleles are simply variations in a gene’s nucleotide sequence • For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain – At the organismal level, the allele is recessive – At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant – At the molecular level, the alleles are codominant Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Frequency of Dominant Alleles • Dominant alleles are not necessarily more common in populations than recessive alleles • For example, one baby out of 400 in the United States is born with extra fingers or toes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage • In this example, the recessive allele is far more prevalent than the population’s dominant allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Multiple Alleles • Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-11 Allele IA IB Carbohydrate A B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype Red blood cell appearance Phenotype (blood group) IAIA or IA i A IBIB or IB i B IAIB AB ii O (b) Blood group genotypes and phenotypes Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Extending Mendelian Genetics for Two or More Genes • Some traits may be determined by two or more genes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus – For example, in mice and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-12 BbCc EpistasisOne gene affects the expression of another gene BbCc Sperm 1/ 4 BC 1/ 4 bC 1/ 4 Bc 1/ 4 bc Eggs 1/ 1/ 1/ 1/ 4 BC BBCC BbCC BBCc BbCc BbCC bbCC BbCc bbCc BBCc BbCc BBcc Bbcc BbCc bbCc Bbcc bbcc 4 bC 4 Bc 4 bc 9 : 3 : 4 Polygenic Inheritance • Quantitative characters vary in the population along a continuum • usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-13 Polygenic Inheritance AaBbCc AaBbCc Sperm 1/ Eggs 1/ 8 1/ 8 1/ 8 1/ 8 1/ 1/ 8 1/ 1/ 8 8 1/ 8 1/ 64 15/ 8 1/ 1/ 8 8 8 1/ 8 1/ 8 1/ 8 8 1/ Phenotypes: 64 Number of dark-skin alleles: 0 6/ 64 1 15/ 64 2 20/ 3 64 4 6/ 64 5 1/ 64 6 Height in Human Beings Frequency Distributions in Polygenic Inheritance Nature and Nurture: The Environmental Impact on Phenotype • Phenotype for a character depends on environment as well as genotype – The norm of reaction is the phenotypic range of a genotype influenced by the environment – For example, hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-14 Himalayan Rabbit When an ice pack was kept on a shaved portion of the rabbit’s back, the new fur grew back black instead of white. Temperature affected the expression of the gene for fur color. Many human traits follow Mendelian patterns of inheritance • Humans are not good subjects for genetic research – Generation time is too long – Parents produce relatively few offspring – Breeding experiments are unacceptable • However, basic Mendelian genetics endures as the foundation of human genetics Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Pedigree Analysis • A pedigree is a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-15a Key Male Female Affected male Affected female Mating Offspring, in birth order (first-born on left) Fig. 14-15b 1st generation (grandparents) 2nd generation (parents, aunts, and uncles) Ww ww ww Ww ww ww Ww Ww Ww ww 3rd generation (two sisters) WW or Ww Widow’s peak ww No widow’s peak (a) Is a widow’s peak a dominant or recessive trait? Fig. 14-15c 1st generation (grandparents) Ff 2nd generation (parents, aunts, and uncles) FF or Ff ff Ff ff ff Ff Ff Ff ff ff FF or Ff 3rd generation (two sisters) Attached earlobe Free earlobe (b) Is an attached earlobe a dominant or recessive trait? • Pedigrees can also be used to make predictions about future offspring • We can use the multiplication and addition rules to predict the probability of specific phenotypes Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Recessively Inherited Disorders • Many genetic disorders are inherited in a recessive manner Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Behavior of Recessive Alleles • Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal (i.e., pigmented) – Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-16 Parents Normal Aa Normal Aa Sperm A a A AA Normal Aa Normal (carrier) a Aa Normal (carrier) aa Albino Eggs • If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low • Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele • Most societies and cultures have laws or taboos against marriages between close relatives Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Cystic Fibrosis • Cystic fibrosis – the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent – results in defective or absent chloride transport channels in plasma membranes – Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Sickle-Cell Disease • Sickle-cell disease affects one out of 400 AfricanAmericans – caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells – Symptoms include physical weakness, pain, organ damage, and even paralysis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Dominantly Inherited Disorders • Some human disorders are caused by dominant alleles – Dominant alleles that cause a lethal disease are rare and arise by mutation – Achondroplasia is a form of dwarfism caused by a rare dominant allele Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-17 Parents Dwarf Dd Normal dd Sperm D d d Dd Dwarf dd d Dd Dwarf Eggs Normal dd Normal Huntington’s Disease • Huntington’s disease is a degenerative disease of the nervous system • The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Multifactorial Disorders • Many diseases, such as heart disease and cancer, have both genetic and environmental components • Little is understood about the genetic contribution to most multifactorial diseases Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Genetic Testing and Counseling • Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Tests for Identifying Carriers • For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fetal Testing • In amniocentesis, the liquid that bathes the fetus is removed and tested • In chorionic villus sampling (CVS), a sample of the placenta is removed and tested • Other techniques, such as ultrasound and fetoscopy, allow fetal health to be assessed visually in utero Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-18a Amniotic fluid withdrawn Centrifugation Fetus Placenta Uterus Cervix Fluid Fetal cells BioSeveral chemical hours tests Several weeks Several weeks Karyotyping (a) Amniocentesis Fig. 14-18b Fetus Placenta Biochemical tests Karyotyping Suction tube inserted through cervix Chorionic villi Several hours Fetal cells Several hours (b) Chorionic villus sampling (CVS) Newborn Screening • Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States – Phenylketonuria – inability to metabolize the amino acid phenylalanine Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 14-UN2 Degree of dominance Description Complete dominance of one allele Heterozygous phenotype same as that of homozygous dominant Incomplete dominance of either allele Heterozygous phenotype intermediate between the two homozygous phenotypes Example PP Pp C RC R Codominance Multiple alleles Pleiotropy Heterozygotes: Both phenotypes expressed In the whole population, some genes have more than two alleles One gene is able to affect multiple phenotypic characters C RC W C W C W IAIB ABO blood group alleles IA , IB , i Sickle-cell disease Fig. 14-UN3 Relationship among genes Epistasis Example Description One gene affects the expression of another BbCc BbCc BC bC Bc bc BC bC Bc bc 9 Polygenic inheritance A single phenotypic character is affected by two or more genes AaBbCc :3 :4 AaBbCc