BioNotesChapter14MendelandtheGeneIdea.pdf BIOL 1020 - 001
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Chapter 14 Mendel and the Gene Idea Drawing from the Deck of Genes What principles account for the passing of traits from parents to offspring The blending hypothesis is the idea that genetic material from the two parents blends together like blue and yellow paint blend to make green The quotparticulatequot hypothesis is the idea that parents pass on discrete heritable units genes Mendel documented a particulate mechanism through his experiments with garden peas Concept 141 Mendel used the scientific approach to identify two laws of inheritance Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments Mendel s Experimental Quantitative Approach Mendel s approach allowed him to deduce principles that had remained elusive to others A heritable feature that varies among individuals such as ower color is called a character Each variant for a character such as purple or white color for owers is called a trait Peas were available to Mendel in many different varieties Other advantages of using peas 0 Short generation time 0 Large number of offspring o Mating could be controlled plants could be allowed to selfpollinate or could be crosspollinated Mendel chose to track only those characters that occurred in two distinct alternative forms He also used varieties that were truebreeding plants that produce offspring of the same variety when they selfpollinate In a typical experiment Mendel mated two contrasting truebreeding varieties a process called hybridization The truebreeding parents are the P generation The hybrid offspring of the P generation are called the F1 generation When F1 individuals selfpollinate or crosspollinate with other F1 hybrids the F2 generation is produced The Law of Segregation When Mendel crossed contrasting truebreeding white and purple owered pea plants all the F1 hybrids were purple When Mendel crossed the F1 hybrids many of the F2 plants had purple owers but some had white Mendel discovered a ratio of about three to one purple to white owers in the F2 generation Mendel reasoned that only the purple ower factor was affecting ower color in the F1 hybrids Mendel called the purple ower color a dominant trait and the white ower colors a recessive trait The factor for white owers was not diluted or destroyed because it reappeared in the F2 generation Mendel observed the same pattern of inheritance in 6 other pea plant characters each represented by 2 traits What Mendel called a heritable factorquot is what we now call a gene Mendel 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 First alternative versions of genes alleles account for variations in inherited characters For example the gene for ower color in pea plants exists in 2 versions one for purple owers and the other for white owers Each gene resides at a specific locus on a specific chromosome Second for each character an organism inherits two alleles one from each parent Mendel made this deduction without knowing about chromosomes The 2 alleles at a particular locus may be identical as in the trueObreeding plants of Mendel s P generation Alternatively the two alleles at a locus may differ as in the F1 hybrids Third if the two alleles at a locus differ then one the dominant allele determines the organism s appearance and the other the recessive allele has no noticeable effect on appearance In the owercolor example the F1 plants had purple owers because the allele for that trait is dominant Dominant alleles may mask the presence of the recessive allele Fourth the law of segregation 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 This segregation of alleles corresponds to the distribution of homologues chromosomes to different gametes in meiosis Mendel s Law of Segregation Alleles pairs separate during gamete formation and reunite randomly at fertilization The model accounts for the 31 ratio observed in the F2 generation of Mendel s crosses Possible combinations of sperm and egg can be shown using a Punnett square A capital letter represents a dominant allele and a lowercase letter represents a recessive allele Useful Genetic Vocabulary An organism with two identical alleles for a character is homozygous for the gene controlling that character An organism that has 2 different alleles for a gene is heterozygous for the gene controlling that character Unlike homozygotes heterozygotes are not true breeding Because of the different effects of dominant and recessive alleles an organism s traits do not always reveal its genetic composition Therefore we distinguish between an organism s phenotype or physical appearance and its genotype or genetic makeup In the example of ower color in pea plants PP and Pp plants have the same phenotype purple but different genotypes The Law of Independent Assortment Mendel derived the law of segregation by following a single character The F1 offspring produced in this cross were monohybrids heterozygous for one character A cross between such heterozygotes is called a monohybrid cross Mendel identified his second law of inheritance by following two characteristics at the same time Crossing two true breeding parents differencing in two characters produces dihybrid in the F1 generation heterozygous for both characters A dihybrid cross a cross between F1 dihybrid can determine whether two characters are transmitted to offspring as a package or independently Using a dihybrid cross Mendel developed the law of independent assortment It states that each pair of alleles segregates independently of each other pair of alleles during gamete formation 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 Concept 143 Inheritance patterns are often more complex than predicted by simple Mendelian genetics The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied 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 Extending Mendelian Genetics for a Single Gene Inheritance of characteristics by a single gene may deviate from simple Mendelian patterns in the following situations 0 When alleles are not completely dominant or recessive I Incomplete dominance I Codominance 0 When a gene has more than 2 alleles I Multiple alleles 0 When a gene produces multiple phenotypes I Pleiotrophy Degrees of Dominance Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical 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 The Relation Between Dominance and Phenotype A dominant allele does not subdue a recessive allele alleles don t interact that way 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 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 US is born with extra fingers or ties Polydactyl many fingers The allele for this unusual trait is dominant to the allele for the more common trait of 5 digits per appendage In this example the recessive allele is far more prevalent than the population s dominant allele Multiple Alleles Most genes exist in populations in more than 2 allelic forms For example the 4 phenotypes of the ABO blood group in humans are determined by 3 alleles for the enzyme 1 that attaches A or B carbohydrates to red blood cells IA 13 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 Pleiotrophy Most genes have multiple phenotypic effects a property called Pleiotrophy For example pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases such as cystic fibrosis and sicklecell disease HOWEVER All of the symptoms are traceable to a SINGLE alleles Many phenotypic effects of a single genotype PLEIOTROPY Extending Mendelian Genetics for 2 or more Genes 2 or more genes may determine some traits Epistasis In epistasis a gene at one locus alters the phenotypic expression of a gene at a second locus For example in Lab retrievers and many other mammals coat color depends on 2 genes One gene determines the pigment color with alleles B for black and b for brown The other gene with alleles E for color and e for no color determines whether the pigment will be deposited in the hair Polygenic Inheritance Quantitative characters are those that vary in the population along a continuum Quantitative variation usually indicates polygenic inheritance an additive effect of 2 or more genes on a single phenotype For example skin color Nature and Nurture The Environmental Impact on Phenotype Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype The phenotypic range is broadest for polygenic characters Traits that depend on multiple genes combined with environmental in uences are called multifactorial 0 Example Hydrangeas A Mendelian View of Heredity and Variation An organism s phenotype includes its physical appearance internal anatomy physiology and behavior An organism s phenotype re ects its overall genotype and unique environmental history Concept 144 Many human traits follow Mendelian patterns of inheritance Humans are not good subjects for genetic research 0 Generation time is too long 0 Parents produce relatively few offspring o Breeding experiments are unacceptable However basic Mendelian genetics endures as the foundation of human genetics 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 and described using pedigrees 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 Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These range from relatively mild to life threatening The Behavior of Recessive Alleles Recessively inherited disorders show up only in individuals homozygous for the allele Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal most individuals with recessive disorders are born to carrier parents Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair If a recessive allele that causes a disease is rare then the chance of two carriers meeting and mating is low Consanguineous mating s ie mating s between close relatives increased the chance of mating between two carriers of the same rare allele Most societies and cultures have laws and taboos against marriages between close relatives SickleCell Disease A genetic disorder with Evolutionary Implications Sicklecell disease affects 1 out of 400 African Americans The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells In homozygous individuals all hemoglobin is abnormal sicklecell Symptoms include physical weakness pain organ damage and even paralysis Heterozygotes said to have sicklecell trait are usually healthy but may suffer some symptoms About one in 10 African Americans has sicklecell trait an unusually high frequency Heterozygotes are less susceptible to the malaria parasite so there is an advantage to being heterozygous in regions where malaria is common 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 Huntington s Disease A LateOnset Lethal Disease Huntington s disease is a degenerative disease of the nervous system The disease has no obvious phenotypic effects until the individual is about 3540 years old Once the deterioration of the nervous system begins the conditions is irreversible and fatal Marfan Syndrome Anotehr dominantlyinherited disease Affects connective tissue formation Affected individuals typically are tall and lanky Isaiah Austin Multifactorial Disorders Many diseases such as heart disease diabetes alcoholism mental illness and cancer have both genetic and environmental components No matter what our genotype our lifestyle ahs a tremendous effect on phenotype Genetic Testing and Counseling Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease Counseling Based on Mendelian Genetics and Probability Rules Using family histories genetic counselors help couples determine the odds that their children will have genetic disorders It is important to remember that each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings Tests for Identifying Carriers For a growing number of diseases tests are available that identify carriers and help define the odds more accurately The tests enable people to make more informed decisions about having children However they raise other issues such as whether affected individuals fully understand their genetic test results Fetal Testing In amniocentesis the liquid that bathes the fetus is removed and tested In chronic 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 Newborn Screening Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the US One common test is for phenylketonuria PKU a recessively inherited disorder that occurs in one of every 1000015000 births in the US Study Guide for Chp 14 Gregor Mendel and garden peas What are the laws of segregation and independent assortment How did Mendel deduce these laws What is a Punnett square How can Punnett squares be used to make predictions about progeny from a cross NonMendelian inheritance patterns due to o Codominance o Incomplete dominance OOOOO Multiple alleles Pleiotrophy Epistasis Polygenic inheritance Multifactorial conditions
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