Attributes of Living Systems (GT
Attributes of Living Systems (GT LIFE 102
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This 7 page Class Notes was uploaded by Christop Jacobs on Monday September 21, 2015. The Class Notes belongs to LIFE 102 at Colorado State University taught by Stuart Field in Fall. Since its upload, it has received 13 views. For similar materials see /class/210126/life-102-colorado-state-university in Life Science at Colorado State University.
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Date Created: 09/21/15
o Blending Hypothesis 0 The idea that genetic material contributed by the two parents mixes in a manner analogous to the way blue and yellow paints blend to make green I If it were correct blending of F1 hybrids from a cross between purpleflowered and whiteflowered pea plants would have pale purple flowers 0 False giving rise to uniform populations of individuals 0 False t fails to explain other phenomena of inheritance such as traits reappearing after skipping a generation 0 Gregor Mendel O Developed a mechanism for inheritance I He was not aware of chromosomes He discovered the basic principles of heredity by breeding garden peas in carefully planned experiments He began in 1857 breeding peas I Peas are available in varieties example purple flowers and white flowers 0 Character a heritable feature that varies among individuals such as flower color 0 Trait Each variant for a character such as purple of white for flowers He removed the immature stamens pollenproducing organ from a purple plant before it produced pollen and then dusted pollen from a white plant onto the altered carpel egg bearing organ of the purple plant I Each resulting zygote then developed into a plant embryo encased in a seed Peal o This is called crosspollination fertilization between different plants He used truebreeding plants Varieties which over many generations of self pollination produce the same variety of the parent plant He crosspollinated two contrasting truebreeding pea varieties I This mating or crossing of two truebreeding varieties is called 0 Hybridization o The truebreeding parents are referred to as the P generation 0 The hybrid offspring are the F1 generation I If these F1 hybrids self pollinate they produce an F2 generation 0 He usually followed PF1 and F2 0 He came up with his two laws by utilizing up to the second generation 0 The Law of Segregation Extremely Important O In the case of pod color the Mendel Pea Experiment showed that a cross between a green pod plant and a yellow pod plant produced only green pod plants for the F1 generation It appeared that the yellow pod characteristic had disappeared I However the F2 generation threw up a surprising result the yellow pod variant appeared in a quarter of this generation I Clearly something strange was going on and in an inspired piece of thinking Mendel came up with his lLaw of Segregation 1 There are alternative forms of genes the units determining heritable characteristics This is now known as an allele 2 An organism inherits one allele from each parent The F1 generation inherited one green and one yellow pod allele from the parental generation 3 A sperm or egg carries only one allele for each characteristic which pair upon fertilization 4 When the alleles are different one is fully expressed and the other is masked now known as dominant and recessive genes o In the above case the green pod plant is a dominant trait and o The yellow pod plant is a recessive trait 0 31 is the ratio for the law of segregation I See page 265 table 141 o Mendel39s Model 0 The model explained the 31 inheritance pattern among the F2 offspring 0 Key Four related concepts make up Mendel s Model the fourth of which is the law of segregation I 1 Alternative versions of genes account for variations in inherited characters 0 Gene for flower color in pea plants for example two colors purple flowers and white flowers I These alternative versions are called alleles Key Allele for purple lowers Homologous owercolor r a gene chromosomes Allele or while owers I The DNA at the locus varies in its nucleotide sequence and hence its information content I 2 For each character an organism inherits two alleles one from each parent 0 Each somatic cell in a diploid organism has two sets of chromosomes I One inherited from each parent I Thus a genetic locus is actually represented twice in a diploid cell one on each homolog of a specific pair of chromosomes picture above 0 These two would be identical in the truebreeding plants of P generation 0 And different in F1 hybrids 3 If the two alleles at a locus differ then one the dominant allele determines the organism s appearance the other the recessive allele has no noticeable effect on the organism s appearance 4 Law of Segregation the two alleles for a heritable character segregate separate during gamete formation and end up in different gametes 0 So an egg or a sperm gets only one of the two alleles that are present in the somatic cells of the organism making the gamete KEY I In terms of chromosomes this segregation corresponds to the distribution of the two members of a homologous pair of chromosomes to different gametes in meiosis I If the organism is truebreeding has identical alleles for a particular character then that allele is present in all gametes o If different alleles are present then 50 ofthe gametes receive the dominant allele and 50 receive the recessive allele Punnett Square diagrammatic device for predicting the allele composition of offspring from a cross between individuals of known genetic makeup o In the Mendelian Model A capital letter symbolizes a dominant allele and a lowercase letter for a recessive allele Useful Genetic Vocabulary Homozygous an organism that has a pair of identical alleles for a character Heterozygous an organism that has two different alleles for a gene Phenotype an organisms appearance or observable traits Genotype an organisms genetic makeup Pp PP etc The Testcross Breeding an organism of unknown genotype with a recessive homozygote 0 It can then reveal the genotype of the unknown organism o The Law of Independent Assortment Key 0 The law of segregation was derived from experiments in which a single character was followed ie flower color 0 All the F1 progeny produced in the truebreeding crosses I Monohybrids They were heterozygous for one character 0 A cross between such heterozygotes is referred to as a monohybrid cross 0 Law of Independent Assortment 0 He identified this law by following two characters at the same time such as seed color and seed shape 0 Crossing two truebreeding varieties that differ in both characters I Cross between YYRR and yyrr o Dihybrids Individuals heterozygous for two characters Yer I Each pair of alleles segregates independently of each other pair of alleles during gamete formation 0 This seems key I This law only applies to genes allele pairs located on different chromosomes chromosomes that are not homologous I Genes located near each other on the same chromosome tend to be inherited together 0 The laws of probability govern Mendelian inheritance o The probability scales from 0 to 1 0 Event Certain to occur has a probability of 1 0 Event Not certain to occur has a probability of 0 0 Independent Events 0 Each coin toss is independent of every other coin toss o The alleles of one gene segregate into gametes independently of another gene s allele the law of independent assortment 0 Two basic rules of probability 0 The Multiplication and Addition Rules Key 0 Applied to Monohybrid Crosses I How do we determine the probability that two or more independent events will occur together in some specific combination 0 Example What is the chance that two coins tossed simultaneously will both land heads up I The multiplication rule Key states that to determine this probability we multiply the probability of one event by the probability of the other event I Both coins landing up V2 x 72 M o This can also be applied to F1 monohybrid cross 0 Each egg produced has a 72 chance of carrying the dominant allele and a V2 chance of carrying the recessive The addition rule key The probability that any one of two or more mutually exclusive events will occur is calculated by adding their individual probabilities Review page 270 Figure 149 this can get confusing and will guaranteed be a test question 0 Solving Complex Genetics Problems with the Rules of Probability o The rules of probability can also be applied to predict the outcome of crosses involving multiple characters 0 By knowing the probability of the offspring genotypes Yy Rr W Yy etc I We derive this information from a Dihybrids punnett square 0 We can multiple one genotype Yy by another genotype Rr and conclude the probability of having that genotype I Page 270 will walk you through the steps You need to do this 0 The addition rule also comes into play when we get into even more complex problems I Page 270 once again 0 Inheritance patterns are often more complex than predicted by simple Mendelian genetics 0 Mendel chose pea plant characters that are determined by one gene for which there are only two alleles one completely dominant and the other completely recessive o The relationship between genotype and phenotype is rarely this simple 0 Inheritance of characters determined by a single gene deviates from simple Mendelian patterns when 0 Alleles are not completely dominant or recessive 0 When a particular gene has more than two alleles 0 When a single gene produces multiple phenotypes 0 Degrees of Dominance 0 Complete Dominance one allele is shows up completely over the other 0 Incomplete Dominance neither allele is dominant or recessive there s a mixture 0 Table 1410 clears up why this is not the blending hypothesis 0 Codominance two alleles both affect the phenotype in separate distinguishable ways 0 It s different from incomplete dominance because both the phenotypes are distinguishable ie spotted colored cows The Relationship Between Dominance and Phenotype o Alleles are simply variations in a gene s nucleotide sequence 0 When a dominant allele coexists with a recessive allele in a heterozygote they do not actually interact at all I It is in the pathway from the genotype to phenotype that dominance and recessiveness come into play Frequency of Dominant Alleles 0 Dominant alleles are not more common than recessive allele Multiple Alleles 0 Only two alleles exist for the pea characters that Mendel Studied 0 But most genes exist in more than two allelic forms I Blood types A B AB and 0 are examples 0 The type refers to the type of carbohydrate o 0 means no carbohydrate Pleiotropy o A property that indicates that most genes have multiple phenotypic effects 0 In the garden pea the gene that determines flower color also affects the color of the coating on the outer surface of the seed Extending Mendelian Genetics for Two of More Genes 0 Two situations in which two or more genes are involved in determining a particular phenotype o Epistasis a gene at one locus alters the phenotypic expression of a gene at a second locus I Example In mice black coat color is dominant to brown Let s designate B and b as the two alleles for this character For a mouse to have brown fur its genotype must be bb But there is more to the story A second gene determines whether or not pigment will be deposited in the hair The dominant allele symbolized by C for color results in the deposition of either black or brown pigment depending on the genotype at the first locus But it the mouse is homozygous recessive for the second locus cc then the coat is white albino regardless of the genotype at the blackbrown locus In this case the gene for pigment deposition Cc is said to be epistatic to the gene that codes for black or brown pigment Bb Polygenic Inheritance 0 Mendel studied characters that could be classified on an eitheror basis such as purple versus white flower color 0 But for many characters such as human skin color and height an eitheror classification is impossible because the characters vary in population along a continuum I These are called quantitative characters 0 Quantitative variation usually includes polygenic inheritance I An additive effect of two or more genes on a single phenotypic character punnett square ex On pg 274 Fig 1413 I The opposite of Pleiotropy where a single gene affects several phenotypic characters Nature and Nurture The Environmental Impact on Phenotype 0 Another departure from simple Mendelian genetics arises when the phenotype for a character depends on the environment as well as the genotype 0 Genotype generally is not associated with a rigidly defined but rather with a range of39 I39 I quot quot39 39 due to Iinfluences I Norm of Reaction phenotypic range for a genotype 0 Environment contributes to the quantitative nature of characters as we have seen in the continuous variation of skin color in humans 0 Such characters are referred to as multifactorial I Meaning that many factors both genetic and environmental collectively influence phenotype Remember phenotype can refer not only to specific characters such as flower color and blood group but also to an organism in its entirety all aspects of its physical appearance internal anatomy physiology and behavior Genotype can refer to an organism s entire genetic makeup not just its alleles for a single genetic locus 0 Many human trait follow Mendelian patterns of inheritance o Pedigrees I A family tree that describes the traits of parents and children across generations I Note If some trait is a dominant allelea say W then for someone to lack that trait they must be homozygous ww w w w 14 Probability WW If 3 WW and Ww Windo have a child what is the prabiltytha the child will have a widows peak This is the equivalent to a Mende 39an F1 mo ohyhrid cross WWx Ww So the probabilty is 34 11 4 WW 12 WW 0 Recessively Inherited Disorders 0 The Behavior of Recessive Alleles I A recessiver inherited disorder shows up only in homozygous individuals aa KEY o This individual inherits one recessive allele from each parent Heterozygote s Aa phenotypically may be normal with regard to disorder but they may transmit the recessive aee to their offspring thus being called 0 Carriers Key Consanguineous quotsame blood matings o Increases chance of diseasecausing recessive allele 0 Indicated in pedigrees by double lines
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