Chapter 13: Genetics
Chapter 13: Genetics biol 3361
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Date Created: 10/23/15
Chapter 13 Genetics Evolution 0 The fossil record during Darwin s time showed evidence for ancient life but no one could explain how this evolution occurred 0 Lamark was the rst to try to explain evolution 0 Hypothesized inheritance of acquired characteristics from giraffes acquiring long necks by stretching up to eat leaves off of tall trees and then passing it to their offspring 0 False in his assumption of acquiring desirable traits by work like how a body builder acquires muscles with exercise 0 Darwin came up with natural selection that those who had desirable traits would survive to reproduce and pass though desirable traits to their offspring 0 With giraffes natural selection leads to a species of giraffes that have variation in neck lengths but the average neck length of the population is longer over time due to natural selection 0 The selective value of a longer neck for giraffes is actually because male giraffes compete for females with their necks and the longest necks win and get the females and reproduce more causing that trait to carry on 0 Natural selection occurs when 3 conditions are met 0 Individuals within a population vary in their characteristics 0 Trait can be passed on to offspring o n Certain traits cause a better chance of reproduction and survival Sickle Cell Disease 0 Example of natural selection 0 Children with sickle cell disease have inherited a mutant hemoglobin gene from both parents 0 The children who inherit the sickle cell gene are protected from malaria infections 0 So in areas where malaria is endemic the bad gene has selective value and a large number of people have the sickle cell gene Genech 0 During Darwin s time not much was known about genetics People thought that all traits in children were a blend of traits from their parents But this inheritance of blended traits doesn t work with natural selection because a population would blending traits would eventually give rise to uniform individuals Mendel discovered that traits behaved as if they are particulate unmixable Mendel found that he could explain results of his pea breeding experiments if he assumed that 0 Genes he didn t know what they were at the time came in pairs alleles 0 That one allele can be dominant over another 0 That each gamete could contain only one of the two alleles of its parent genes Which he called segregation of alleles This is now known to be because of meiosis 0 And that when he considered more than one gene each gene assorted independently to the gametes independent assortment Also because of meiosis With Mendel s discovery of the gene the physical basis for Darwin s theory of evolution by natural selection could begin to be understood Mendel studied various phenotypic observable traits in garden peas First experiment included crossing a round seeded father and a wrinkle seeded mother which resulted in a round seed progeny F1 generation which conclusively debunked the blending hypothesis o If the blending hypothesis were true you would expect to see a seed that was not round like the father or wrinkled like the mother but somewhere in between Second experiment to answer if the results were due to a maternal or paternal in uence Switched and crossed a wrinkled father with a rounded mother seed All progeny were again round seeded This concluded that the round seed trait is dominant to the wrinkled seed trait By Mendel s principle of segregation the gametes produced by each plant contain just one copy of the gene which can then combine randomly with each other during fertilization Mendel s are what we call Both alleles of the gene on the two homologous chromosomes are the same 0 In the cross of a homozygous dominant with a homozygous recessive all F1 progeny are heterozygous with the dominant phenotype Draw out the Punnett square of AA x aa 0 When this F1 generation is crossed with itself heterozygous x hetero it results in a 31 ratio of phenotype which is consistent with Mendel s principle of segregation Draw out the Punnett square of AA x Aa The Principle of Segregation can be represented by a Mendel then experimented with seeds differing in two traits 0 Each parent homozygous for different colors and different seed shapes 0 All offspring heterozygous for both with dominant phenotypes for both Selfpollination of these F1 plants all hetero for both traits led to a 9dominant for both traits 3 dominant A recessive B o 3 recessive A dominant B and o 1 recessive for both traits 0 Draw out the Punnett square for 2 traits AaBb x AaBb The 9331 was predicted by and con rms Mendel s principle of independent assortment Mendel then did a of a seed heterozygous for shape and color with a homozygous recessive on both traits parent and observed equal proportions of phenotypes in the offspring o A test cross is a cross of any phenotype with a homozygous recessive Mendel s ideas were not worked out until the early 19005 by Sutton and Boveri who worked out what was happening in meiosis Sutton and Boveri realized that is chromosomes contained the genetic material then their behavior in meiosis would exactly explain Mendel s observations Extending the chromosome theory Thomas Hunt Morgan early 19005 chose the fruit y as his study object 0 Fruit y has no preexisting phenotypic variants like the plant has round and wrinkled seeds It took Morgan a year of breeding ies and examining thousands before nding a mutant y with white eyes 0 Used notation w for the dominant allele and w for the recessive allele 0 Note that this is a different method of notation 0 Compare previous notation W dominant w recessive w dominant w recessive Ran basically same experiments as Mendel and found similar results except that the females were always redeyed and the only whiteeyed mutants were males 0 Crossed red eyed females with white eyed females and was able to breed whiteeyed females 0 Did a like Mendel and shows that the white eyed trait does depend on the sex of the donor only boys get it if the mother has it o This was explained by the discovery made by Nettie Stevens at about the same time that sex is governed by X and Y chromosomes XLinked Recessive Traits X amp Y are called the rest are called o In Diosophilia the fruit ies the gene for red or white eye color is on the 0 Females can either have no mutated X chromosomes have the disease 2 mutated X5 or be a carrier one mutated X and one normal X 0 Females can be XX unaffectednormal XX carrier or XX affected 0 Boys cannot be carriers they either have it mutated X or don t normal X o Males can be XY unaffectednormal or XY affected 0 A father with the trait will transmit the mutant allele to all daughters takes their only X but no sons only takes their Y Sex Chromosomes X chromosome contains 1400 genes and mutant X alleles are linked to 59 diseases 0 Y chromosomes contains only a few genes and no human diseases are known to be Ylinked o The only known Ylinked trait is hairy ear rims Hemophilia Recessive Xlinked mutation The wild type allele codes for an enzyme used in blood clotting boys with the defective allele have blood that fails to properly clot 0 Famous example intermarrying royal families of Europe RedGreen colorblindness o Xlinked recessive trait 0 One in 12 men have red or green color blindness while only one in 250 women do 0 The gene for synthesizing redsensitive opsin and greensensitive opsin are adjacent to each other 0 Some individuals can have 2 or 3 genes for greensensitive opsin Green genes can be lost by errors in crossing over Congenital Generalized Hypertrichosis Rare Xlinked dominant trait Hair covers face and upper body NonDisjunction of the Sex Chromosomes The sex chromosome equivalent to Down s syndrome where there are 3 copies of chromosome 21 o XX nondisjunction 0 XXX female normal slightly lower IQ slightly taller frequency unknown because phenotype is so normal 0 X0 female short stature webbed neck immature sex organs that do not undergo changes during puberty lownormal intelligence 0 XXY male low fertility many female body characteristics lowered IQ 1500 male births o OY nonviable o YY nondisjunction o XYY fertile normal appearance 20x more frequent in males in penal and mental institutions but most XYY males do not develop patterns of antisocial behaviors Barr bodies 0 One might expect an XO female to be same as XX female since one of the X in normal XX is silenced but not all genes are silenced in Barr bodies the silenced Xs Many of the genes that escape inactivation are in the regions of the X chromosome that contains genes also present on the Y chromosome unlike the majority of the X chromosome which are not also present on the Y chromosome These regions are called as individuals of either sex will receive two copies of every gene in these regions like a regular autosome unlike the majority of genes on the sex chromosomes in which they receive only one copy 0 Since individuals of either sex will receive two copies of every gene in a pseudoautosomal region no dosage compensation is needed for females so it is postulated that these regions of DNA have evolved mechanisms to escape Xinactivation o Shortly after fertilization all the cells in a female embryo randomly pick one of their two Xchromosomes and quotshut it offquot by converting it to a highly compacted form that is the Barr body In an XXX female there are 2 Barr bodies and one functional X chromosome The entire chromosome is not inactivated if they were XO s and XXY s would be phenotypically normal Females are mosaics because of X inactivation and because it occurs early in embryological development 0 The black and orange patches of Calico cats are an example of this X chromosome mosaicism in females female calico cats have black and orange alleles of an Xlinked gene 0 Xlinked genetic diseases can be mosaic is females meaning that they can be redgreen color blind in one eye Morgan s discovery of Xlinked inheritance was important evidence in support of the hypothesis that chromosomes are the carriers of the genes Mendel discovered The Morgan group Morgan and his students realized that chromosomes might indeed be a linear array of Mendel s genes The group then realized that if genes are located one after another along the length of a chromosomes then two genes close to each other on a chromosome would be quotlinkedquot unable to independently assort to the gametes in the way that traits governed by genes on separate chromosomes would The rst pair of linked genes that they discovered was on the X chromosome They were 1 the gene whose alleles determined the traits of red or white eyes and 2 a second gene that governs body color either wild type gray dominant of yellow recessive To examine the linkage between eye color and body color they bred a homozygous whiteeyed gray bodies female wywy and redeyed yellow bodied male wyY o w red w white 0 y redy yellow The F1 red eyed gray bodied females wywy heterozygous for both traits were then mated with their sibling whiteeyed gray bodied males wyY Rationale if the genes are located one after another along the length of a chromosome then only two different kinds of gametes could be formed by the F1 females a game with wy Xchromosome and a game with wy Xchromosome which would should that the traits could not switch location If the ww alleles were on different homologous pairs of chromosomes than the yy the mother could form 4 different kinds of gametes wywywywy because they re not connected so they can form any combination Furthermore if they re linked then only 4 different kinds of ies would be found Only two types of males would be found Half would be wyY and the other half would be wyY o This was not what was observed rather a small fraction 14 of the male progeny were unexpected novel phenotypes wyY and wyY 0 These novel phenotypes were due to the unexpected genotypes wy and wy X chromosomes 0 Morgan realized that the recombinant genotypes on the X chromosomes were being generated when crossing over occurred during prophase of Meiosis After Morgan s discovery that genes are linearly arrayed on chromosomes and that crossing over leads to recombinant chromosomes an undergrad student in his lab Sturtevant then realized that the frequency of crossing over would depend on the physical distance between genetic loci on a chromosome 0 The recombination frequency the map units now called centimorgans that the genes are apart l Genes that are closer together are less likely to recombine than genes very far apart Experimental crosses of F1 Progeny after Mendel s work in 1900 often did not result in the 9331 ratio Investigation of these results led to further study of different aspects of inheritance that can alter this ratio Heterozygous has a different phenotype because the dominant allele is not completely dominant Ex RR ower is red rr ower is white but Rr is pink having only one dominant copy of the gene results in only half as much pigment as it takes to make a red ower multiple alleles Multiple alleles at play rather than just the dominant allele and recessive allele Ex ABO blood types 3 rather than 2 alleles o The A B and 0 blood groups are de ned by the type of oligosaccharide present on the surface proteins and lipids of the blood cells in our bodies Being blood group 0 means that all your blood cells have the Oantigen on their surface and you make antibodies against the A antigen and Bantigen O O O Being blood group A means that all your blood cells have the Aantigen on their surface and you make antibodies against the Bantigen Being blood group B means that all your blood cells have the Bantigen on their surface and you make antibodies against the Aantigen ln blood group A and B other nonblood cells still express the 0 antigen If you are blood group AB your cells have the Aantigen and Bantigen and you do not make antibodies against the A B or 0 antigen The lA allele codes for an enzyme that attaches a sugar to the end of the Oantigen structure to create the Aantigen structure while the IB structure codes for a different enzyme that attaches a different sugar to the end of the Oantigen structure to create the Bantigen structure The i allele is a defective gene that doesn t code for any enzymes thus if a person is homozygous ii the Oantigen structure remains unmodi ed A heterozygous IAIB individual will make both enzymes and thus both antigens The gene that codes for the different enzymes IAIB or no enzyme at all i is located on the long arm of chromosome 9 No recipient has antibodies against type 0 cells universal donor AB is the universal acceptor Blood Type Summary Phenotype Genotype Antibodies Can accept blood antigens types Present Blood Group 0 ii None 0 Blood Group A IAIA or W A O A Blood Group B IBIB or lBi B O B Blood group AB IAIB A and B O A B AB Environmental Effect on Gene Expression Phenylketonuria PKU is a disease found in individuals who are homozygous for a defective allele for the PKU gene on chromosome 12 o The nondefective PKU gene encodes an enzyme Phenylalanine Hydroxylase PAH that converts phenylalanine to tyrosine If both alleles are defective phenylalanine and a related breakdown product phenylpyruvic acid accumulate in their bodies o The accumulation of these molecules interferes with developments of the nervous system and causes mental retardation 0 Genetic screening permits the identi cation of PKU babies at birth and placing the babies on a lowphenylalanine diet permits completely normal development 0 Some mutant genes encode proteins that are heatsensitive o The ch allele in Siamese cats encodes a heatsensitive enzyme involved in the synthesis of melanin a dark brown pigment o The ch version of the enzyme is inactive at temperatures at 33 C 0 The hair cells in the skin of the main body and head are above 33 C the enzyme is inactive and the hair is White or blonde 0 At the extremities the skin temp is cooler so the enzyme is active melanin is produced and the hair is dark brown 0 Less commonly some genes are coldsensitive O The arctic fox has a mutant gene encoding a cold sensitive protein that governs hair pigment production In the Winter it s too cold for the enzyme to be active so the fox is White In the summer it isn t too cold for the enzyme so the protein is active and hair color is brown Interactions with other genes 0 Cross of brown bell pepper with yellow red pepper creates a red bell peppeh 0 Cross of the F1 red pepper with itself let to production of 9 red 3 yellow 3 brown and 1 green 0 Bell pepper color is determined by two genes One gene R encodes an enzyme required for production of red pigment Another gene Y governs the presenceabsence of green in the form of controlling chlorophyll synthesis o R red r yellow 0 Y absence of green chorophyll y presence of green chlorophyll Furthermore orange peppers are produced when red pigment is made in reduced amounts A third gene that also produces an enzyme that functions in the synthesis of red pigment can also come into play o If this third gene makes a defective enzyme production of red pigment is reduced even when the R gene is wild type dominant 0 So two different plants can have the same genotype in terms of gene 1 and 2 both RRYY but one will make red peppers and one will make orange peppers This is called epistasis Quantitative traits Mendel worked with discrete traits yes or no qualitiesgreen or yellow but many traits aren t discrete For example height weight and skin color fall on a scale of continuous These type of traits are called quantitative traits Ex wheat kernel color Homozygous white crossed with homozygous red results in a blended F1 The F2 generation from a selfpollinated F1 generation exhibits in a normal distribution of colors that range from white to red NilssonEhle proposed a model to explain the quotnormal distributionquot range of colors produced by the F2 generation in wheat kernels In his model three genes contribute to gene color Each gene has two allelic forms 0 This model is correct and produces 64 different genotypes in a cross in a ratio a normal distribution Quantitative traits are produced by the independent actions of many genes in which each locus contributes a small amount to the value of the phenotype
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