Overview: Drawing from the Deck of Genes

Transcription

1 Chater 4 Mendel and the Gene Idea Overview: Drawing from the Deck of Genes What genetic rinciles account f the assing of traits from arents to offsring? The blending hyothesis is the idea that genetic material from the two arents blends together (like blue and yellow aint blend to make green) The articulate hyothesis is the idea that arents ass on discrete heritable units (genes) Mendel documented a articulate mechanism through his exeriments with garden eas 4.: Mendel and the two laws of inheritance Mendel discovered the basic rinciles of heredity by breeding garden eas in carefully lanned exeriments Mendel s Exerimental, Quantitative Aroach Advantages of ea lants f genetic study: There are many varieties with distinct heritable features, characters (such as flower col); character variants (such as urle white flowers) are called traits Mating of lants can be controlled Each ea lant has serm-roducing gans (stamens) and egg-roducing gans (carels) Cross-ollination (fertilization between different lants) can be achieved by dusting one lant with ollen from another

2 TECHNIQUE TECHNIQUE arental generation () RESULTS First filial generation offsring (F ) 2 Stamens Carel arental generation () 2 Stamens Carel 3 4 RESULTS First filial generation offsring (F ) 5 The Law of Segregation Mendel chose to track only those characters that varied in an either- manner He also used varieties that were true-breeding (lants that roduce offsring of the same variety when they selfollinate) In a tyical exeriment, Mendel mated two contrasting, true-breeding varieties, a rocess called hybridization The true-breeding arents are the generation The hybrid offsring of the generation are called the F generation When F individuals self-ollinate, the F 2 generation is roduced When Mendel crossed contrasting, true-breeding white and urle flowered ea lants, all of the F hybrids were urle When Mendel crossed the F hybrids, many of the F 2 lants had urle flowers, but some had white Mendel discovered a ratio of about three to one, urle to white flowers, in the F 2 generation 2

3 EXERIMENT EXERIMENT EXERIMENT (true-breeding arents) urle flowers White flowers (true-breeding arents) urle flowers White flowers (true-breeding arents) urle flowers White flowers F Generation (hybrids) All lants had urle flowers F Generation (hybrids) All lants had urle flowers F 2 Generation 705 urle-flowered lants 224 white-flowered lants Mendel s Model Mendel reasoned that only the urle flower fact was affecting flower col in the F hybrids Mendel called the urle flower col a dominant trait and the white flower col a recessive trait Mendel observed the same attern of inheritance in six other ea lant characters, each reresented by two traits What Mendel called a heritable fact is what we now call a gene Mendel develoed a hyothesis to exlain the 3: inheritance attern he observed in F 2 offsring Four related concets make u this model These concets can be related to what we now know about genes and chromosomes 3

4 The first concet is that alternative versions of genes account f variations in inherited characters F examle, the gene f flower col in ea lants exists in two versions, one f urle flowers and the other f white flowers These alternative versions of a gene are now called alleles Each gene resides at a secific locus on a secific chromosome Locus f flower-col gene Allele f urle flowers Allele f white flowers Homologous air of chromosomes The second concet is that f each character an ganism inherits two alleles, one from each arent Mendel made this deduction without knowing about the role of chromosomes The two alleles at a locus on a chromosome may be identical, as in the true-breeding lants of Mendel s generation Alternatively, the two alleles at a locus may differ, as in the F hybrids The third concet is that if the two alleles at a locus differ, then one (the dominant allele) determines the ganism s aearance, and the other (the recessive allele) has no noticeable effect on aearance In the flower-col examle, the F lants had urle flowers because the allele f that trait is dominant The fourth concet, now known as the law of segregation, states that the two alleles f a heritable character searate (segregate) during gamete fmation and end u in different gametes Thus, an egg a serm gets only one of the two alleles that are resent in the somatic cells of an ganism This segregation of alleles cresonds to the distribution of homologous chromosomes to different gametes in meiosis Mendel s segregation model accounts f the 3: ratio he observed in the F 2 generation of his numerous crosses The ossible combinations of serm and egg can be shown using a unnett square, a diagram f redicting the results of a genetic cross between individuals of known genetic makeu A caital letter reresents a dominant allele, and a lowercase letter reresents a recessive allele 4

5 Fig Aearance: urle flowers White flowers Genetic makeu: Aearance: urle flowers White flowers Genetic makeu: Aearance: urle flowers White flowers Genetic makeu: Gametes: Gametes: Gametes: F Generation F Generation Aearance: Genetic makeu: urle flowers Aearance: Genetic makeu: urle flowers Gametes: / 2 / 2 Gametes: / 2 / 2 F 2 Generation 3 Useful Genetic Vocabulary henotye Genotye An ganism with two identical alleles f a character is said to be homozygous f the gene controlling that character An ganism that has two different alleles f a gene is said to be heterozygous f the gene controlling that character Unlike homozygotes, heterozygotes are not true-breeding Because of the different effects of dominant and recessive alleles, an ganism s traits do not always reveal its genetic comosition Therefe, we distinguish between an ganism s henotye, hysical aearance, and its genotye, genetic makeu In the examle of flower col in ea lants, and lants have the same henotye (urle) but different genotyes urle 3 urle urle White (homozygous) (heterozygous) (heterozygous) (homozygous) 2 Ratio 3: Ratio :2: 5

6 The Testcross How can we tell the genotye of an individual with the dominant henotye? Such an individual must have one dominant allele, but the individual could be either homozygous dominant heterozygous The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual If any offsring dislay the recessive henotye, the mystery arent must be heterozygous TECHNIQUE Dominant henotye, unknown genotye:? redictions Recessive henotye, known genotye: If If RESULTS All offsring urle /2 offsring urle and /2 offsring white TECHNIQUE Dominant henotye, Recessive henotye, unknown genotye: known genotye:? redictions If If The Law of Indeendent Asstment RESULTS All offsring urle /2 offsring urle and /2 offsring white Mendel derived the law of segregation by following a single character The F offsring roduced in this cross were monohybrids, individuals that are heterozygous f one character A cross between such heterozygotes is called a monohybrid cross Mendel identified his second law of inheritance by following two characters at the same time Crossing two true-breeding arents differing in two characters roduces dihybrids in the F generation, heterozygous f both characters A dihybrid cross, a cross between F dihybrids, can determine whether two characters are transmitted to offsring as a ackage indeendently 6

7 EXERIMENT EXERIMENT YYRR yyrr YYRR yyrr Gametes YR yr Gametes YR yr F Generation YyRr Hyothesis of Hyothesis of deendent indeendent redictions asstment asstment redicted YR /4 /4 Yr /4 yr /4 yr offsring of F 2 generation YR /2 /2 yr /4 YR YYRR YYRr YyRR YyRr /2 YR YYRR YyRr /4 Yr YYRr YYrr YyRr Yyrr yr /2 YyRr yyrr /4 yr YyRR YyRr yyrr yyrr 3 /4 /4 /4 yr henotyic ratio 3: YyRr Yyrr yyrr yyrr F Generation redictions redicted offsring of F 2 generation /2 YR /2 yr Hyothesis of deendent asstment YR /2 /2 yr /4 YYRR YyRr /4 YyRr yyrr /4 3 /4 /4 YyRr Hyothesis of indeendent asstment /4 YR Yr yr /4 /4 /4 yr YR YYRR YYRr YyRR YyRr Yr YYRr YYrr YyRr Yyrr yr YyRR YyRr yyrr yyrr RESULTS henotyic ratio aroximately 9:3:3: RESULTS 9 /6 3 /6 3 /6 /6 henotyic ratio 9:3:3: henotyic ratio 3: /4 yr YyRr Yyrr yyrr yyrr 9 /6 /6 /6 / henotyic ratio aroximately 9:3:3: henotyic ratio 9:3:3: Using a dihybrid cross, Mendel develoed the law of indeendent asstment The law of indeendent asstment states that each air of alleles segregates indeendently of each other air of alleles during gamete fmation Strictly seaking, this law alies only to genes on different, nonhomologous chromosomes Genes located near each other on the same chromosome tend to be inherited together 4.2: The laws of robability Mendel s laws of segregation and indeendent asstment reflect the rules of robability When tossing a coin, the outcome of one toss has no imact on the outcome of the next toss In the same way, the alleles of one gene segregate into gametes indeendently of another gene s alleles The Multilication and Addition Rules Alied to Monohybrid Crosses The multilication rule states that the robability that two me indeendent events will occur together is the roduct of their individual robabilities robability in an F monohybrid cross can be determined using the multilication rule Segregation in a heterozygous lant is like fliing a coin: Each gamete has a 2 chance of carrying the dominant allele and a chance of carrying the recessive allele 2 7

8 Rr Segregation of alleles into eggs / 2 / 2 R r / 2 R r R R R / 2 R / 4 / 4 r Rr Segregation of alleles into serm r r r The rule of addition states that the robability that any one of two me exclusive events will occur is calculated by adding together their individual robabilities The rule of addition can be used to figure out the robability that an F 2 lant from a monohybrid cross will be heterozygous rather than homozygous Solving Comlex Genetics roblems with the Rules of robability We can aly the multilication and addition rules to redict the outcome of crosses involving multile characters A dihybrid other multi-character cross is equivalent to two me indeendent monohybrid crosses occurring simultaneously In calculating the chances f various genotyes, each character is considered searately, and then the individual robabilities are multilied together / 4 / 4 Concet 4.3: Inheritance atterns are often me comlex than redicted by simle Mendelian genetics The relationshi between genotye and henotye is rarely as simle as in the ea lant characters Mendel studied Many heritable characters are not determined by only one gene with two alleles However, the basic rinciles of segregation and indeendent asstment aly even to me comlex atterns of inheritance Gene Inheritance of characters by a single gene may deviate from simle Mendelian atterns in the following situations: When alleles are not comletely dominant recessive When a gene has me than two alleles When a gene roduces multile henotyes Coyright 2008 earson Education Inc., ublishing as earson Benjamin Cummings Coyright 2008 earson Education Inc., ublishing as earson Benjamin Cummings 8

9 Degrees of Dominance Comlete dominance occurs when henotyes of the heterozygote and dominant homozygote are identical In incomlete dominance, the henotye of F hybrids is somewhere between the henotyes of the two arental varieties In codominance, two dominant alleles affect the henotye in searate, distinguishable ways Red C R C R Gametes C R C W White C W C W F Generation Red C R C R Gametes C R Gametes / C R 2 / 2 C W C W ink C R C W White C W C W Coyright 2008 earson Education Inc., ublishing as earson Benjamin Cummings F Generation F 2 Generation Red C R C R Gametes C R Gametes / C R 2 / 2 / 2 C R C W C W / C R 2 / 2 C R C R C W C R C W ink C R C W White C W C W The Relation Between Dominance and henotye A dominant allele does not subdue a recessive allele; alleles don t interact, the dominant allele masks the effect of the recessive allele. Alleles are simly variations in a gene s nucleotide sequence Frequency of Dominant Alleles Dominant alleles are not necessarily me common in oulations than recessive alleles F examle, one baby out of 400 in the United States is bn with extra fingers toes This is called olydactyl, it is a genetic disder caused by dominant allele. / 2 C W C R C W C W C W 9

10 Multile Alleles Most genes exist in oulations in me than two allelic fms F examle, the four henotyes of the ABO blood grou in humans are determined by three alleles f the enzyme (I) that attaches A B carbohydrates to red blood cells: I A, I B, and i. The enzyme encoded by the I A allele adds the A carbohydrate, whereas the enzyme encoded by the I B allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither Allele I A I B Carbohydrate A i none (a) The three alleles f the ABO blood grous and their associated carbohydrates Genotye I A I A I A i I B I B I B i I A I B ii B Red blood cell aearance henotye (blood grou) A B AB (b) Blood grou genotyes and henotyes O leiotroy leiotroy occurs when one gene influences multile henotyic traits. Consequently, a mutation in a leiotroic gene may have an effect on some all traits simultaneously. Most genes have multile henotyic effects, a roerty called leiotroy F examle, leiotroic alleles are resonsible f the multile symtoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease Extending Mendelian Genetics f Two Me Genes Some traits may be determined by two me genes Eistasis In eistasis, a gene at one locus alters the henotyic exression of a gene at a second locus F examle, in mice and many other mammals, coat col deends on two genes One gene determines the igment col (with alleles B f black and b f brown) The other gene (with alleles C f col and c f no col) determines whether the igment will be deosited in the hair / 4 BC / 4 bc / 4 Bc / 4 bc BbCc BbCc / 4 BC / 4 bc / 4 Bc / 4 bc BBCC BbCC BBCc BbCc BbCC bbcc BbCc bbcc BBCc BbCc BBcc Bbcc BbCc bbcc Bbcc bbcc 9 : 3 : 4 0

11 olygenic Inheritance Quantitative characters are those that vary in the oulation along a continuum Quantitative variation usually indicates olygenic inheritance, an additive effect of two me genes on a single henotye Skin col in humans is an examle of olygenic inheritance / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 AaBbCc / 8 / 8 / 8 / 8 / 8 / 8 / 8 / 8 henotyes: Number of / 64 6 / 5 64 / / 5 64 / 6 64 / 64 / 64 dark-skin alleles: AaBbCc Nature and Nurture: The Environmental Imact on henotye Another dearture from Mendelian genetics arises when the henotye f a character deends on environment as well as genotye The nm of reaction is the henotyic range of a genotye influenced by the environment F examle, hydrangea flowers of the same genotye range from blue-violet to ink, deending on soil acidity Nms of reaction are generally broadest f olygenic characters Such characters are called multifactial because genetic and environmental facts collectively influence henotye Integrating a Mendelian View of Heredity and Variation An ganism s henotye includes its hysical aearance, internal anatomy, hysiology, and behavi An ganism s henotye reflects its overall genotye and unique environmental histy

12 4.4: Many human traits follow Mendelian atterns of inheritance Humans are not good subjects f genetic research Generation time is too long arents roduce relatively few offsring Breeding exeriments are unaccetable However, basic Mendelian genetics endures as the foundation of human genetics edigree Analysis A edigree is a family tree that describes the interrelationshis of arents and children across generations Inheritance atterns of articular traits can be traced and described using edigrees Key Male Female Affected male Affected female Mating Offsring, in birth der (first-bn on left) st generation (grandarents) 2nd generation (arents, aunts, and uncles) 3rd generation (two sisters) Ww ww Ww ww ww Ww WW Ww ww Ww ww Ww ww st generation (grandarents) 2nd generation (arents, aunts, and uncles) 3rd generation (two sisters) Ff Ff FF Ff ff ff Ff Ff ff ff ff FF Ff Ff edigrees can also be used to make redictions about future offsring We can use the multilication and addition rules to redict the robability of secific henotyes Widow s eak (a) Is a widow s eak a dominant recessive trait? No widow s eak Attached earlobe Free earlobe (b) Is an attached earlobe a dominant recessive trait? 2

13 Recessively Inherited Genetic Disders Many genetic disders are caused by recessive alleles. Recessively inherited disders show u only in individuals homozygous f the allele Carriers are heterozygous individuals who carry the recessive allele but are henotyically nmal (i.e., igmented) Albinism is a recessive condition characterized by a lack of igmentation in skin and hair A a Nmal Aa A AA Nmal Aa Nmal (carrier) arents Nmal Aa a Aa Nmal (carrier) aa Albino 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 taboos against marriages between close relatives Cystic Fibrosis Sickle-Cell Disease Dominantly Inherited Disders Cystic fibrosis is the most common lethal genetic disease in the United States,striking one out of every 2,500 eole of Euroean descent The cystic fibrosis allele results in defective absent chlide transt channels in lasma membranes Symtoms include mucus buildu in some internal gans and abnmal abstion of nutrients in the small intestine Sickle-cell disease affects one out of 400 African- Americans The disease is caused by the substitution of a single amino acid in the hemoglobin rotein in red blood cells Symtoms include hysical weakness, ain, gan damage, and even aralysis Some human disders are caused by dominant alleles Dominant alleles that cause a lethal disease are rare and arise by mutation Achondrolasia is a fm of dwarfism caused by a rare dominant allele Huntingtons disease is also caused by a rare dominant allele 3

14 Fig. 4-7 Huntington s Disease Multifactial Diseases d arents Dwarf Dd Nmal dd D d Dd dd Dwarf Nmal Huntington s disease is a degenerative disease of the nervous system The disease has no obvious henotyic effects until the individual is about 35 to 40 years of age, at which oint the individual has already likely reroduced and assed on the dominant allele. Many diseases, such as heart disease and cancer, have both genetic and environmental comonents Little is understood about the genetic contribution to most multifactial diseases d Dd Dwarf dd Nmal Genetic Testing and Counseling Fetal Testing Amniotic fluid withdrawn Genetic counsels can rovide infmation to rosective arents concerned about a family histy f a secific disease. Using family histies, genetic counsels hel coules determine the odds that their children will have genetic disders F a growing number of diseases, tests are available that identify carriers and hel define the odds me accurately In amniocentesis, a samle of the liquid that bathes the fetus, amniotic fluid, is collected and tested In chionic villus samling (CVS), a samle of the lacenta is removed and tested Other techniques, such as ultrasound and fetoscoy, allow fetal health to be assessed visually in utero Fetus lacenta Uterus Cervix Fluid Fetal cells Centrifugation Several hours Several weeks Biochemical tests lacenta Chionic villi Several hours Fetus Fetal cells Suction tube inserted through cervix (a) Amniocentesis Several weeks Karyotying Several hours (b) Chionic villus samling (CVS) 4

15 Newbn Screening Some genetic disders can be detected at birth by simle blood tests that are now routinely erfmed in most hositals in the United States NJ screens all newbns f the following gentic disders: Male syru Urine Diseas henylketourea (KU) Sickle cell and other blood anemias Cystic Fibrosis Galactosemia Congential Adrenal Hyerlasia Congenital Hyothyroidism Degree of dominance Comlete dominance of one allele Incomlete dominance of either allele Codominance Multile alleles leiotroy Descrition Heterozygous henotye same as that of homozygous dominant Heterozygous henotye intermediate between the two homozygous henotyes Heterozygotes: Both henotyes exressed In the whole oulation, some genes have me than two alleles One gene is able to affect multile henotyic characters C R C R I A I B I A, I B, i Examle C R C W C W C W ABO blood grou alleles Sickle-cell disease Fig. 4-UN3 Relationshi among genes Eistasis olygenic inheritance Descrition One gene affects the exression of another A single henotyic character is affected by two me genes BbCc Examle BbCc BC bc Bc bc BC bc Bc bc AaBbCc 9 : 3 : 4 AaBbCc You should now be able to: Gege Sandra Tom Sam Daniel Alan Tina Christoher Arlene Wilma Ann Michael Carla. Define the following terms: true breeding, hybridization, monohybrid cross, generation, F generation, F 2 generation 2. Distinguish between the following airs of terms: dominant and recessive; heterozygous and homozygous; genotye and henotye 3. Use a unnett square to redict the results of a cross and to state the henotyic and genotyic ratios of the F 2 generation 4. Exlain how henotyic exression in the heterozygote differs with comlete dominance, incomlete dominance, and codominance 5. Exlain why dominant gene disders are much rarer than recessive gene disders 6. Exlain how carrier recognition, fetal testing, and newbn screening can be used in genetic screening and counseling 5