Bio 1107: Biology - Study Guide
Bio 1107: Biology - Study Guide Bio 1107
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This 60 page Study Guide was uploaded by Mo-Mo Miller on Wednesday October 15, 2014. The Study Guide belongs to Bio 1107 at University of Connecticut taught by Katherine Shaw in Fall. Since its upload, it has received 492 views. For similar materials see Biology in Science at University of Connecticut.
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Date Created: 10/15/14
Basic Genetics Inheritance Genotypes Phenotypes GeneGene and GeneEnvironment Interactions Biology 1107 Lecture 14 1 39 f U11trEt 5itjJ sot tm ut Greener Ii1tti r d CHI I1ptL1E Lecture 14 Overview 250 Q Overview Inheritance hypotheses 3 Q Overview terminology 47 Q Mende s genetic experiments 819 Q Pea plant crosses 811 Q Results 1214 Q Mende s model 1519 Q Law of segregation 19 Q Punnett squares 2030 Q Monohybrid crosses and multiplication and addition rules 2123 Q Genotypic amp phenotypic ratios 24 Q Test crosses 2526 Q Dihybrid crosses and the law of independent assortment 2730 Q Inheritance patterns more complex than Mendelian genetics 3145 Q Different degrees of dominance 3233 Q Multiple alleles 3435 Q Pleiotropy 36 Q Gene Gene interactions 3739 Q Epistasis 38 Q Polygenic inheritance 39 Q Gene Environment interactions 40 Q Multifactorial disorders 42 360 Overview Drawing from the Deck of Genes What genetic principles account for the passing of traits from parents to offspring inheritance Two alternative hypotheses proposed in the 18005 1 blending hypothesis U quot 0 The idea that the genetic material from the two parents blends together blue amp yellow paint blend to make green Predicts that over many generations a freely mating population will give rise to a uniform population of individuals 2 quotparticuate hypothesis A 5 The idea that parents pass on discrete heritable units genes Unlike blending hypothesis can explain the reappearance oftraits after several generations ie traits skipping generations Genes can be shuffled and passed along and traits will not be diluted 460 Overview Terminology Genes are discrete heritable units that consist of specific nucleotide bases The genetic makeup or set of alleles of an organism is called the genotype The observable traits of an organism which are determined by the genotype is called the phenotype Overview Terminology Describing phenotypes Character A character is a heritable Howe feature that varies among W individuals HOW eg pea plant flower color pomquot A trait is a variant of that character Seed eg purple and white flowers Color 33 Dominant Trait Purple Axial Yellow Round X X X X 560 Recessive Trait White V l K 4 Terminal Green Wrinkled 660 Overview Terminology Describing genotypes Alternate versions of the same gene are called alleles Each allele codes a different trait for the same character eg white or purple flower color Each genotype for a character is controlled by two alleles in a diploid organism one allele from mother and one allele from father The same gene is always found in the same location on homologous chromosomes regardless of whether the alleles differ this location is called the gene39s locus plural loci Allele for purple flowers Pair of Locus for flowercolor gene gt homologous chromosomes Allele for white flowers Overview Terminology Each of the two alleles for a character in a diploid organism can be either a Dominant allele determines the organisms appearance denoted as a capital letter eg P trait purple coor 760 Recessive allele only expressed when a dominant allele is not present denoted as a ower case letter eg p trait white color If an organism has a pair of identical alleles for a character the organism is homozygous for the gene controlling the character eg PP or pp Individuals can be homozygous dominant PP or homozygous recessive pp If an organism has two different alleles for a gene the organism is heterozygous for that gene Pp Since heterozygotes carry a dominant and recessive allele only the trait of the dominant allele will be expressed in the phenotype Phenotype Purple Purple Purple White Genotype PP homozygous Pp heterozygous Pp heterozygous PP homozygous 860 Mende s Experiments Quantitative Approach Gregor Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments Mendel developed a theory of inheritance several decades before chromosomes were first observed under a microscope Advantages of using pea plants for genetic study distinct heritable features or characters flower color pea color etc Short generation time and production of a large number of offspring Mating can be controlled Each flower has sperm producing organs stamens and egg producing organ carpel Cross poination fertilization between different plants involves dusting flowers of one plant with pollen from another TECHNIQUE Parental generation P RESULTS First filial generation E offspring quot F1 950 Mende s Experiments Quantitative Approach Mendel chose to track only those characters that occurred in two distinct alternative forms ie purple OR white flower yellow OR green peas He also used varieties that were truebreeding plants that produce offspring of the same variety when they sef poinate Homozygous individuals eg with genotypes PP or pp are truebreeding individuals Gametes from PP X individual heterozygotes not Ferti ZatiOn true breeding Why are produce all PP individuals 1060 Why Are Heterozygotes Not True Breeding Heterozygous individuals Pp can produce either P or p gametes Crosses between different combinations of gametes will not always result in offspring of the same variety as the parent due to random fertilization Potential X X X Gametes from I I I Fertilization could produce PP Pp or pp offspring 1160 Mende s Experiments Crosses EXPERIMENT Mendel mated two contrasting true P Generation or x 0 P breeding varnetles PP truebreeding quot yP C 39 39 39 P I Wh39 arid PP 39 d39V39g a395r 3 parents flgwpefs flowters process ca e hybridization truebreeding parents are the P generation hybrid offspring of the P generation are called the F1 generation When F1 individuals selfpollinate or cross F1 Generation hybrids All plants had purple flowers Sef or crosspollinationl F2 Generation pollinate with other F1 705 I 224 hjt purp e w I e hybndsgthe F2 flowered flowered generation IS produced plants plants I 1260 Mendel s Results Mendel crossed EXPERIMENT contrasting truebreeding white and purple P Generation truebreeding flowered pea plants Parents H l White owers flowers all of the F1 hybrids were purple F1 Generation S Ivlendel Crossed the F1 hybrids All plants had purple flowers hybrids many of the F2 plants had purple flowers but some had white Self or crosspollinationl F2 Generation Mendel discovered a ratio N of about three to one 705 purpe 224 white fl d fl d purple to white flowers In quotY fj i the E generation I 1360 Mendel s Results Experiments supported the particulate hypothesis of inheritance the heritabe factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation This heritabe factor described by Mendel is what we now call a gene Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids Mendel called the purple flower color the dominant trait Mendel reasoned that because the white color reappeared in the F2 generation it was somehow hidden or masked in the F1 generation so he called the white flower color the recessive trait 15so Mende s Model Mendel developed a hypothesis to explain the 31 inheritance pattern he observed in F2 offspring Four related concepts make up this model These concepts can be related to what we now know about genes and chromosomes 15so Mende s Model 1 Alternate forms of genes exist Alternative versions of genes ie alleles account for variations in inherited characters The gene for flower color in pea plants exists in two versions one for purple flowers P and the other for white flowers p Each gene resides at a specific location locus on a specific chromosome Allele for purple flowers Pair of Locus for flower color gene gt homologous chromosomes Allele for white flowers 17so Mende s Model 2 Identical vs Nonidentical Alleles For each character an organism inherits two alleles one from each parent The two alleles at a particular locus may be identical homozygous as in the truebreeding plants of ende s parent generation PP x pp Alternatively the two alleles at a locus may differ heterozygous as in the F1 hybrids Pp Iendel made this prediction without knowing about the role or even existence of chromosomes 13so Mende s Model 3 Some alleles can mask others If the two alleles at a locus differ then one the dominant allele determines the organism39s appearance and the other the recessive allele has no noticeable effect on appearance In the flowercolor example the F1 hybrid plants had purple flowers because the allele for that trait is dominant P vs p 19so Mende s Model 4 Alleles for a particular gene are NOT inherited together The two alleles for a heritable character separate segregate during gamete formation and end up in different gametes Thus an egg or a sperm gets only one of the two alleles that are present in the organism Segregation of alleles corresponds to the random distribution of homologous chromosomes to different gametes in meiosis specifically Anaphase I of meiosis I This fourth part of the model is now known as the law of segregation Individual Meiosis 50 50 or Haploid gametes 2060 Modelling Mende s Results Punnett Squares Mende s segregation model accounts for the 3391 ratio he quot quot quot p 39 Ag 3 39quotquot X observed in the F2 generation i of numerous Crosses Etaia ca eup Purple ggwers WhItepfowers Gametes P p The possible combinations of Y sperm and egg can be shown F Geea O using a Punnett square a 0 diagram for predicting the Appearance Purple flowers results of a enetic cross Genetic makeUP Pp between in ividuals of Gametes V21 21 known genetic makeup Sperm from F1 Pp plant Dia ram An egg with a P 392G 39a P pr alle e has an equal chance of s i being fertilized by a s erm P with either a P or p a lele The E99 quot39 l t same is true for an egg with a P paquot p allele thus there are four equally likely combinations of egg and sperm 2160 The Multiplication and Addition Rules Applied to Monohybrid Crosses Multiplication rule Rh X R Segregation of Segregation of the probablhty W two or allelesInto eggs alleles Into sperm more independent events will occur together is the product of their individual probabilities Segregation for the offspring of two heterozygous individuals is like flipping a coin Each gamete has a 2 chance of carrying the dominant allele and a 2 chance of carrying the recessive allele Each offspring then has a 2 x 2 1 chance of being RR or rr 2260 The Multiplication and Addition Rules Applied to Monohybrid Crosses Addition rule the probability that any two Rr gtlt Rr mutually exclusive events allger39 ttltIgtogfs aIelg eiSr1tgtquotrIcerfm occur is calculated by adding their P y individuals probabilities The probability of two heterozygous parents producing heterozygous offspring is determined by the addition of the two mutually exclusive events of producing heterozygous offspring An R egg x r sperm and a r egg and an R sperm will each produce an Rr offspring The total probability of producing Rr offspring is A A 2 2360 The Multiplication and Addition Rules Applied to Monohybrid Crosses Probabilities apply equally to each offspring Every offspring produced has the same chances of obtaining the phenotypes from the parental crosses in the same ratios This means that if a parental cross produces an offspring with blue eyes ii there is still a 25 chance that the next offspring will have blue eyes Each offspring genotype is independent of the others Gene Eye Pigment Alleles I pigment brown i no pigment blue Cross Ii x Ii Genotype amp Phenotype Ratios Because of the different effects of dominant and recessive alleles an organism39s traits do not always correspond to the genetic composition Heterozygotes and homozygous dominants will have the same phenotype but different genotypes PP and Pp plants both have a purple phenotype but different genotypes We can describe the results of crosses in terms of ratios Phenotypic Ratio Genotypic Ratio 3lt f 1lt Phenotype Purple Purple Purple White Ratio 31 Dominant Phenotype Recessive Phenotype 2460 Genotype PP homozygous Pp heterozygous Pp heterozygous PP homozygous Ratio 121 Homozygous Dominant Heterozygotes Homozygous Recessive gt1 gt2 J gt1 2550 The Testcross How can we determine the genotype of an individual with the dominant phenotype Such an individual could be either homozygous dominant PP or heterozygous Pp The answer is to carry out a testcross breeding the mystery individual with a homozygous recessive pp individual If any offspring display the recessive phenotype the mystery parent must be heterozygous 2550 The Testcross If any offspring display the recessive phenotype the mystery parent must be heterozygous TECHNIQUE ix Dominant phenotype Recessive phenotype unknown genotype known genotype PP or Pp pp Predictions If purpleflowered or If purpleflowered parent is PP parent is Pp P 39quot gr Cg riffs Pp Eggs Pp N P tp J 3 PP PP sees or fe 5 All offspring purple 12 offspring purple and 2 offspring white p Eggs r39 P 2 2760 The Law of Independent Assortment Mendel derived the law of segregation by following a single character eg flower color only The F1 offspring produced in this cross were monohybrids individuals that are heterozygous for one character eg Pp A cross between such heterozygotes is called a monohybrid cross 2860 The Law of Independent Assortment Mendel identified his second law of inheritance the Law of Independent Assortment by following two characters at the same time eg seed pea color seed pea shape Crossing two truebreeding parents differing in two characters produces dihybrids in the F1 generation heterozygous for both characters A dihybrid cross a cross between F1 dihybrids determine whether two characters are transmitted to offspring as a package or independently 2960 The Law of Independent Assortment Pea color character Traits alleles Y yellow y green Pea shape character Traits alleles R round r wrinkled EXPERIMENT P Generation YYRR 0 9 39 quot l Gametes x Q 0 PW Hypothesis of Hypothesis of dependent assortment independent assortment Sperm Of Sperm or 1 1 14 14 F1 Generation Predictions F2 generation 12 12 4 0 0 0 0 Y YRR Y YRr Y yRR Y yRr YYRR YyRr 4 O YyRr Yyrr E99812 a a Eggs YYRr YYrr G C 0 0 0 0 YyRR YyRr yyRR yyRr 000i 916 w 316 g 316 6 16 9 Phenotypic ratio 9331 RESULTS 3150 1039 1o1 32f Phenotypic ratio approximately 9331 3060 The Law of Independent Assortment Using a dihybrid cross in pea plants Mendel developed the law of independent assortment The law of independent assortment states that each pair of alleles Y and y segregates independently of every other pair of alleles R and r during gamete formation resulting in a phenotypic ratio that is NOT 31 Strictly speaking this law applies only to genes on different nonhomologous chromosomes or those far apart on the same chromosome genes located near each other on the same chromosome tend to be inherited together 3160 Extending Mendelian Genetics for a Single Gene Inheritance patterns are often more complex than predicted by simple Mendelian genetics Mende s finding apply to a very specific set of genetic conditions discrete characters with only two alleles although the basic principles of segregation and independent assortment are more widely applicable Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations When alleles are not completely dominant or recessive When a gene has more than two alleles When a gene produces multiple phenotypes 3260 Degrees of Dominance Complete dominance phenotypes of the heterozygote and dominant homozygote are identical Incomplete dominance phenotype of F1 hybrids is somewhere in between the phenotypes of the two parental varieties shown in diagram Codominance two dominant alleles affect the phenotype in separate distinguishable ways P Generation R d CRECR X 0J White CW Gametes F1 Generation v H 39 239 v A Gametes 12 2 Sperm F2 Generation 12 12 E99312 CRCR CREW 2 Cigw Cwgw Incomplete Dominance Example 33so Dominance 7 Frequency In Population Dominant alleles are not necessarily more common in populations than recessive alleles Dominance only describes a type of interaction between two different alleles For example one baby out of 400 in the United States is born with extra fingers or toes The allele that results in this unusual trait polydactyly 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 in the population than the dominant allele R 34so Multiple Alleles Most genes exist in populations in more than two allelic forms The ABO blood groups 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 the cell membrane of red blood cells IA 3 and i These carbohydrates antigens are used for cellular recognition and are important to immune system functioning 35so Multiple Alleles The enzyme encoded by the IA allele adds the A carbohydrate whereas the enzyme encoded by the 3 allele adds the B carbohydrate the enzyme encoded by the i allele adds neither o S resu H3 in fou r a quotl39139ebtre rltles for the ABO blood groups and their blood types A B AB O Allele A B i The AB blood e 5 an Carbohydrate A A B Q One exa m p Ie of b Blood group genotypes and phenotypes codo m I na nce 0 A and B Genotype IAIA or1A39 1313 or1B39 A13 5 bloord phenotypes ca n ave two Red blood cell e n 0 es appearance homozygous or Ph t eno e heterozygous blood group A B AB 0 35so Pleiotropy Ilost genes have multiple phenotypic effects a property called pleiotropy For example pleiotropic alleles are responsible for the multiple symptoms of certain hereditable diseases such as cystic fibrosis and sicklecell disease In pea plants the gene that determines flower color also affects the color of the pea coating white or gray Based on our knowledge of the complex molecular and cellular interactions that are responsible for an organism39s development and physiology it is not surprising that a single gene can affect multiple characteristics in an organism 3760 Extending Mendelian Genetics for Two or More Genes Some characters may be determined by two or more genes Epistasis Polygenic Inheritance Epistasis In epistasis a gene at one locus alters the phenotypic expression of a gene at a secondlocus For example in Labrador retrlevers 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 E for pigmentation deposition and e for no pigmentation deposition determines whether the pigment will be deposited in the hair A genotype with ee will result in yellow labs regardless of the genotype at the blackbrown locus Eggs 4 4 4 4 L I BbEe X BbEem Y 3860 Sperm 14 14 14 14 33EEamp BbEEamp BBE39 BbEeL BbEEL bbEEm bbEem quot BBEe PeW BbEe EW by Bbee 0FW BbEe 0W bbEe Bbee W bbee V V W gV 9L 3 L 9 139 3 4 I 31 n 39so Polygenic Inheritance 0 Quantitative characters are 999 x W those that vary in the AaBCc Aagbcc population along a continuum S Y perm quot g ma hag 148 148 148 188 148 48 148 48 8000 888 9 888 888 888 0 0 Quantitative variation usually 1800 8 898 889 w 82 7 indicates polygenic 1 o o O00 O00 O00 O00 O00 O00 O00 3 Qtquot inheritance an additive effect 8 000 3900 0390 00 00 of two or more genes on a Eggs 8000 888 9 888 88 as single phenotype 13o g 0 Q 0 Skin color in humans is an 139 i99g j example of polygenic 18 as 0H J inheritance At least three separately Phenotypes 164 inherited genes affect the Number of darkness of Skin darkskin alleles 0 1 2 3 4 5 6 4060 Nature and Nurture The Environmental Impact on Phenotype Sometimes the phenotype for a character depends on environment as well as genotype For example hydrangea flowers with the same genotype will produce a range of colors from blue violet to pink depending on soil acidity 4160 Nature and Nurture The Environmental Impact on Phenotype Growing evidence suggests that a genotype is generally not associated with a rigidly defined phenotype but rather with a range of phenotypic possibilities due to environmental influences this range of possibilities is called the norm of reaction eg human height is affected mainly by genes but ability to obtain maximal genetically determined height can be 1 2 influenced by nutritional health during Envlmnment development as well as other factors Norms of reaction are generally broadest for polygenic characters such characters are called multifactorial because genetic and environmental factors collectively influence phenotype Trait Value 42so Multifactorial Disorders p 279 Some disease are simple Mendelian disorders eg cystic fibrosis Huntington39s disease but many diseases have both genetic and environmental components heart disease diabetes alcoholism mental illnesses schizophrenia bipolar disorder and cancer In many cases the hereditable component is poygenic Example many genes affect cardiovascular health making some people more prone to heart attacks and strokes However no matter the genotype lifestyle has a tremendous effect on cardiovascular health eg exercise healthy diet Little is understood about the genetic contribution to most multifactorial diseases so most effort is directed at promoting healthy lifestyles Summary of Inheritance Patterns Relationship among alleles of a single gene Complete dominance of one allele Incomplete dominance of either allele Codominance Multiple alleles Pleiotropy Description Heterozygous phenotype same as that of homo zygous dominant Heterozygous phenotype intermediate between the two homozygous phenotypes Both phenotypes expressed in heterozygotes In the whole population some genes have more than two alleles One gene is able to affect multiple phenotypic characters Example 19 172 Q a pb CRCR CRCW CWCW IAIB ABO blood group alleles IA 131 Sicklecell disease 9 2011 Pearson Education Inc 4360 4460 Summary of Inheritance Patterns For Two or More Genes Relationship among two or more genes Description Example Epistasis The phenotypic BbEe L x L BbEe expression of one gene affects that Y of another L L L L Polygenic inheritance A single phenotypic character is affected A BbCquot A BbC by two or more genes oooooooooooooooooooooooo 3 8 uaaaaaaa maaaeasmg amp i Gene Allele Genotype Phenotype Homozygous Heterozygous Dominant Recessive Genotypic ratio Phenotypic ratio Terms to Know Punnett Square Monohybrid Cross Dihybrid Cross 4560 Incomplete Dominance Codominance Pleiotropy Epistasis Polygenic Norm of reaction Be able to recognize examples of these terms 4660 Review Key Concepts II Gregor Mendel laid the foundational work for the theory of inheritance supporting the particulate hypothesis of inheritance II Genotype is the genetic makeup of an organism while phenotype is the outward expression of those traits II Mende s model of inheritance for discrete characters involves four main concepts 1 2 3 Alternative forms of genes exist alleles Diploid organisms can carry identical or nonidentical pairs of alleles for a particular characteristic homozygous vs heterozygous The expression of some alleles recessive can be masked in the presence of other alleles dominant Alleles for a particular gene are not inherited together Law of segregation 4760 Review Key Concepts II Punnett squares can be used to determine the outcome of particular crosses II When the genotype of a dominant phenotype is unknown it can be determined by performing a testcross II In dihybrid crosses characters are transmitted independently to offspring the law of independent assortment II This law only applies to genes on different nonhomologous chromosomes or genes that are very far apart on the same chromosome 4860 Review Key Concepts Dominance does not refer to the frequency of a trait in a population in some cases recessive traits can be very common There are different degrees of dominance complete dominance incomplete dominance and codominance Mende s ideas can be extended to multiple allele systems Most genes have multiple phenotypic effects pleiotropy Some phenotypes may be determined by two or more genes epistasis and polygenic inheritance The environment can have a large impact on phenotype such that phenotype for any give genotype is an interaction between genes and environment many human diseases fall into this complex category multifactorial disorders 4960 Punnett Square Practice One cat carries heterozygous ong haired traits Ss and its mate carries homozygous short haired traits ss Use a Punnett square to determine the probability of one of their offspring having long hair 5060 Punnett Square Practice One flower is heterozygous red Rr and it is crossed with a homozygous white rr plant Use a Punnett square to determine the probability of one of their offspring having a red color Give the genotypic and phenotypic ratios for this cross 5160 Punnett Square Practice In a certain species of plant the color purple P is dominant to the color white p According to the Punnett Square what is the probability of an offspring being white Describe the genotypes and phenotypes of the parents Parent 1 Pp PP Pp Parent 2 Pp 5250 Punnett Square Practice Testcross In dogs hereditary deafness can be caused by a recessive gene d A kennel owner has a male dog that she wants to use for breeding purposes if possible The dog can hear so the owner knows his genotype is either DD or Dd If the dog39s genotype is Dd the owner does not wish to use him for breeding so that the deafness gene will not be passed on This can be tested by breeding the dog to a deaf female dd 1 Draw the Punnett squares to illustrate these two possible crosses In each case what percentagehow many of the offspring would be expected to be hearing deaf 2 How could you tell the genotype of this male dog 3 Also using Punnett squares show how it could be possible for two hearing dogs could produce deaf offspring 5360 Punnett Square Practice KEY One cat carries heterozygous ong haired traits Ss and its mate carries homozygous short haired traits ss Use a Punnett square to determine the probability of one of their offspring having long hair S long hair allele S short hair allele Cross Ss x ss S 5 S5 ss Ss ss Results 50 probability 24 that each offspring will have long hair heterozygous Ss 5460 Punnett Square Practice KEY One flower is heterozygous red Rr and it is crossed with a homozygous white rr plant Use a Punnett square to determine the probability of one of their offspring having a red color Give the genotypic and phenotypic ratios for this cross R red allele r white allele Cross Rr x rr Rr IT Rr IT Results 50 probability 24 that each offspring will have red color heterozygous Rr G Ratio 0 RR 2 Rr 2 rr P Ratio 2 red 2 white 5560 Punnett Square Practice KEY In a certain species of plant the color purple P is dominant to the color white p According to the Punnett Square what is the probability of an offspring being white Describe the genotypes and phenotypes of the parents P purple allele P p white allele Parent 1 Pp P Cross Pp x Pp Parent 2 Pp Results 25 probability 14 that each offspring pp will be white homozygous pp The parents of the cross are both heterozygotes with purple phenotypes 5660 Punnett Square Practice Testcross KEY In dogs there is an hereditary deafness caused by a recessive gene d A kennel owner has a male dog that she wants to use for breeding purposes if possible The dog can hear so the owner knows his genotype is either DD or Dd If the dog39s genotype is Dd the owner does not wish to use him for breeding so that the deafness gene will not be passed on This can be tested by breeding the dog to a deaf female dd 1 Draw the Punnett squares to illustrate these two possible crosses In each case what percentagehow many of the offspring would be expected to be hearing deaf 2 How could you tell the genotype of this male dog 3 Also using Punnett squares show how it could be possible for two hearing dogs could produce deaf offspring See following slides for KEY 5760 Punnett Square Practice Testcross KEY In dogs hereditary deafness can be caused by a recessive gene d A kennel owner has a male dog that she wants to use for breeding purposes if possible The dog can hear so the owner knows his genotype is either DD or Dd If the dog39s genotype is Dd the owner does not wish to use him for breeding so that the deafness gene will not be passed on This can be tested by breeding the dog to a deaf female dd 1 Draw the Punnett squares to illustrate these two possible crosses In each case what percentage how many of the offspring would be expected to be hearing deaf D normal hearing allele d deafness allele Cross x dd Must try both crosses to get offspring probabilities DDxdd Dd x dd d D D D d Dd Dd d Dd dd Dd Dd d Dd dd If the male dog is DD then the cross will produce only heterozygous dogs that can hear cross DD x dd If the male dog is Dd then the cross will produce hearing dogs 50 of the time and deaf dogs 50 of the time 5860 Punnett Square Practice Testcross KEY In dogs hereditary deafness can be caused by a recessive gene d A kennel owner has a male dog that she wants to use for breeding purposes if possible The dog can hear so the owner knows his genotype is either DD or Dd If the dog39s genotype is Dd the owner does not wish to use him for breeding so that the deafness gene will not be passed on This can be tested by breeding the dog to a deaf female dd 2 How could you tell the genotype of this male dog The only way you could determine the genotype of this dog short of an expensive genetics screening is to do a testcross with the homozygous recessive deaf female dd As it mentions in the lecture slides the way to determine if an individual expressing a dominant phenotype is heterozygous is if any of the offspring produced express the homozygous recessive phenotype dd 5950 Punnett Square Practice Testcross KEY In dogs there is an hereditary deafness caused by a recessive gene d A kennel owner has a male dog that she wants to use for breeding purposes if possible The dog can hear so the owner knows his genotype is either DD or Dd If the dog39s genotype is Dd the owner does not wish to use him for breeding so that the deafness gene will not be passed on This can be tested by breeding the dog to a deaf female dd 3 Also using Punnett squares show how it could be possible for two hearing dogs could produce deaf offspring You can find the answer to part 3 two different ways FIRST WAY Attempt all the crosses D D D D D d between two dogs that D DD DD D DD DD D DD Dd have hearing K phenotypes 9 DD DD d Dd Dd d Dd da DD x DD xx DD X Dd The only way two hearing dogs could produce a deaf offspring Dd X Dd dd is if both parental dogs were heterozygous Then there And C0mP3quote the results would be a 25 chance 14 that each offspring would be deaf 6060 Punnett Square Practice Testcross KEY In dogs hereditary deafness can be caused by a recessive gene d A kennel owner has a male dog that she wants to use for breeding purposes if possible The dog can hear so the owner knows his genotype is either DD or Dd If the dog39s genotype is Dd the owner does not wish to use him for breeding so that the deafness gene will not be passed on This can be tested by breeding the dog to a deaf female dd 3 Also using Punnett squares show how it could be possible for two hearing dogs could produce deaf offspring You can find the answer to part 3 two different ways SECOND WAY You can work backwards and save time making multiple punnett squares by asking yourself how it is possible to produce the offspring of interest deaf offspring dd D D D D D d 9 DD DD 9 DD DD 9 DD Dd 9 DD DD 1 Dd Dd d Dd dh 2 The only way hearing parental genotypes would produce any offspring with genotype dd is if both parents carried a d allele Thus only a Dd x Dd cross could produced deaf offspring
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