Mendel and the Gene Idea Chapter 14 1. Historical view 2. Work of Mendel A. Principle of Segregation B. Independent Assortment C. Probability 3. Extending Mendel A. Intermidiate inheritance B. Multiple Alles C. Pleiotrophy D. Polygenic inheritance 4. Mendelian Inheritance in Humans A. Human Pedigree B. Recessive Disorders C. Dominant Disorders D. Genetic testing The History Of Genetics Pythagoras 500 B.C., a Greek philosopher, stated that human life began with male and female fluids The History Of Genetics Aristotle furthered this idea and suggested that these fluids, or ―semens,‖ were actually purified blood—therefore, blood must be part of heredity. Historical View Hippocrates and Aristotle proposed the idea of what they called pangenes, which they thought were tiny pieces of body parts. They thought that pangenes came together to make up the homunculus, a tiny pre-formed human that people thought grew into a baby Antonie van Leeuwenhoek In the 1600s, the development of the microscope brought the discovery of eggs and sperm thought he saw the homunculus curled up in a sperm cell. His followers believed that the homunculus was in the sperm, the father ―planted his seed,‖ and the mother just incubated and nourished the homunculus so it grew into a baby One Bizarre Theory •The theory of Homunculus -17th century •sex cells contained a complete miniature adult, perfect in form •This statement was popular way into the 18th Small century individual Regnier de Graaf He and his followers thought that they saw the homunculus in the egg, and the presence of semen just somehow stimulated its growth In the 1800s, a very novel, ―radical‖ idea arose: both parents contribute to the new baby, but people (even Darwin, as he proposed his theory) still believed that these contributions were in the form of pangenes Genetic Principles Drawing from the Deck of Genes? What accounts for the transmission of traits from parents to offspring? ―Blending‖ Hypothesis Possible explanation of heredity is a ―blending‖ hypothesis The idea that genetic material contributed by two parents mixes in a manner analogous to the way blue and yellow paints blend to make green ―Particulate‖ Hypothesis An alternative the gene idea Parents pass on discrete heritable units, genes Modern Genetics Traces the beginnings to Gregor Mendel, an Austrian monk, who grew peas in a monastery garden. Was unique among biologists of his time because he quantifiable data, and actually counted the results of his crosses. Published his findings in 1865 people didn’t know about mitosis and meiosis, so his conclusions seemed unbelievable His work was ignored until it was rediscovered in 1900 by a couple of botanists who were doing research on something else. Gregor Mendel Figure 14.1 Documented a particulate mechanism of inheritance through his experiments with garden peas He discovered the basic principles of heredity breeding garden peas in carefully planned experiments Chose to track Only those characters that varied in an ―either-or‖ manner Made sure that He started his experiments with varieties that were ―true-breeding‖ Genetic Vocabulary Character: a heritable feature, such as flower color Trait: a variant of a character, such as purple or white flowers The true-breeding parents Are called the P generation The hybrid offspring of the P generation Are called the F1 generation When F1 individuals self-pollinate The F2 generation is produced Phenotype Physical appearance Genotype Genetic makeup Dominant trait that is easily observed Recessive trait that is often masked Homozygous2 alleles for a trait are identical TT or tt Heterozygous – 2 alleles for a trait are not identical Tt Allele Alternative forms of a gene which determines a trait. Alleles cont. Uppercase (Capital) letters for dominant traits Lowercase letters for recessive traits Ex: tall = T short = t, expressed in pairs TT, Tt, tt Why peas? Rapid reproduction rate Presence of distinctive traits Closed structure of flowers (each pea plant has male (stamens) and female (carpal) sexual organs)allows self- fertilization Seven Traits Studied by Mendel The Law of Segregation When Mendel crossed contrasting, true- breeding white and purple flowered pea plants All of the offspring were purple When Mendel crossed the F1 plants Many of the plants had purple flowers, but some had white flowers Mendel’s Experiment Crossed pure purple and a pure white flower (P generation) =F1 generation All F1 plants (purple) are crossed by self pollination = F2 generation yields ¾ purple and ¼ yellow The Testcross In pea plants with purple flower The genotype is not immediately obvious Allows us to determine the genotype of an organism with the dominant phenotype, but unknown genotype The Testcross Individual with dominant phenotype not possible to predict the genotype run a test cross with individual with recessive phenotype to determine the allele Dominant Phenotype purple flower (genotype PP or Pp) Recessive Phenotype white flower ( genotype pp) Test Cross The Law of Independent Assortment Mendel derived the law of segregation By following a single trait The F1 offspring produced in this cross Were monohybrids, heterozygous for one character How are two characters transmitted from parents to offspring? As a package? Independently? Dihybrid Cross Mendel identified his second law of inheritance By following two characters at the same time Crossing two, true-breeding parents differing in two characters Produces dihybrids in the F1 generation, heterozygous for both characters The Y and R alleles and y and r alleles stay together F1 offspring would produce yellow, round seeds. The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross. If the two pairs of alleles segregate independently of each other Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts. These combinations produce four distinct phenotypes in a 9:3:3:1 ratio Using the information from a dihybrid cross, Mendel developed the law of independent assortment Each pair of alleles segregates independently during gamete formation Punnett Square Diagram which shows the possible outcome of a cross Probability Fractions or ratios that will predict that an event will occur Rr Rr Segregation of Segregation of alleles into eggs alleles into sperm Sperm 1⁄ R 1⁄ r 2 2 R R 1⁄ R R r 2 1⁄ 1⁄ 4 4 Eggs r r 1⁄ r R r 2 1⁄ 1⁄ 4 4 Figure 14.9 The Multiplication and Addition Rules Applied to Monohybrid Crosses The multiplication rule States that the probability that two or more independent events will occur together is the product of their individual probabilities The rule of addition States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities Extending Mendel The inheritance of characters by a single gene May deviate from simple Mendelian patterns The Spectrum of Dominance Complete dominance Occurs when the phenotypes of the heterozygote and dominant homozygote are identical Incomplete Dominance 100% Dominant/0% Codominant recessive 100% Dominant/100% recessive Incomplete Dominance The phenotype of F1 hybrids is somewhere between the P Generation phenotypes of the two parental Red White CW C W varieties CRCR Hheterozygotes show a distinct Gametes CR CW intermediate phenotype, not seen in homozygotes. Pink This is not blended inheritance F1 Generation CRCW because the traits are separable (particulate) as seen in further 1⁄ 1⁄ crosses. 2 2 Gametes CR CR Offspring of a cross between heterozygotes will show three phenotypes: both parentals and 1⁄ CR 1⁄ CR Sperm Eggs 2 2 the heterozygote. F2 Generation 1⁄ 2 CR The phenotypic and genotypic CR CR CR CW ratios are identical, 1:2:1. 1⁄ 2 Cw CR CW CW CW Codominance Two dominant alleles affect the phenotype in separate, distinguishable ways The human blood group MN Is an example of codominance The M, N, and MN blood groups of humans presence of two specific molecules on the surface of red blood cells Both the M & N molecules are expressed in the heterozygous individual People of group M (genotype MM) have one type of molecule on their red blood cells, people of group N (genotype NN) have the other type, and people of group MN (genotype MN) have both molecules present Dominance/Recessiveness Relationships 1. They range from complete dominance, through various degrees of incomplete dominance, to codominance 2. They reflect the mechanisms by which specific alleles are expressed in the phenotype and do not involve the ability of one allele to subdue another at the level of DNA 3. They do not determine or correlate with the relative abundance of alleles in a population The dominance/recessiveness relationships depend on the level at which we examine the phenotype Tay-Sachs disease lack a functioning enzyme to metabolize gangliosides (a lipid) which accumulate in the brain, harming brain cells, and ultimately leading to death. Children with two Tay-Sachs alleles have the disease. Heterozygotes with one working allele and homozygotes with two working alleles are ―normal‖ at the organismal level, but heterozygotes produce less functional enzyme. However, both the Tay-Sachs and functional alleles produce equal numbers of enzyme molecules, codominant at the molecular level Frequency of Dominant Alleles A dominant allele does not necessarily mean that it is more common in a population than the recessive allele. Polydactyly individuals are born with extra fingers or toes, is due to an allele dominant to the recessive allele for five digits per appendage. However, the recessive allele is far more prevalent than the dominant allele in the population. 399 individuals out of 400 have five digits per appendage. Multiple Alleles Most genes exist in populations in more than two allelic forms The ABO blood group in humans Is determined by multiple alleles Table 14.2 Human Blood Groups The ABO blood groups in humans are determined by three alleles, IA, IB, and I Both the IA and IB alleles are dominant to the i allele The IA and IB alleles are codominant to each other Because each individual carries two alleles, there are six possible genotypes and four possible blood types Human Blood Groups Pleiotropy A gene has multiple phenotypic effects The wide-ranging symptoms of cystic fibrosis & sickle-cell disease are due to a single gene Extending Mendelian Genetics for Two or More Genes Some traits may be determined by two or more genes Epistasis Polygenic inheritance Epistasis A gene at one locus alters the phenotypic expression of a gene at a second locus In mice and many other mammals, coat color depends on two genes. One, the epistatic gene, determines whether pigment will be deposited in hair or not. Presence (C) is dominant to absence (c). The second determines whether the pigment to be deposited is black (B) or brown (b). The black allele is dominant to the brown allele. An individual that is cc has a white (albino) coat regardless of the genotype of the second gene. A cross between two black mice that are heterozygous (BbCc) will follow the law of independent assortment. However, unlike the 9:3:3:1 offspring ratio of an normal Mendelian experiment, the ratio is nine black, three brown, and four white. Polygenic Inheritance Many human characters the additive effects of two or more genes on a single phenotypic character. For example, skin color in humans is controlled by at least three different genes. Each gene has two alleles, one light and one dark, that demonstrate incomplete dominance. An AABBCC individual is dark and aabbcc is light. Vary in the population along a continuum and are called quantitative characters Do not fit the either-or basis that Mendel studied An additive effect of two or more genes on a single phenotype Nature and Nurture Mendelian Inheritance in Humans Humans are not convenient subjects for genetic research However, the study of human genetics continues to advance Pedigree Analysis Is a family tree that describes the interrelationships of parents and children across generations Can also be used to make predictions about future offspring Pedigree Analysis Recessively Inherited Disorders Many genetic disorders are inherited in a recessive manner These range from the relatively mild (albinism) to life-threatening (cystic fibrosis). The recessive behavior of the alleles occurs because the allele codes for either a malfunctioning protein or no protein at all. Heterozygotes have a normal phenotype because one ―normal‖ allele produces enough of the required protein. Individuals who lack the disorder are either homozgyous dominant or heterozygotes. heterozygotes no clear phenotypic effects carriers who may transmit a recessive allele to their offspring. heterozygotes may have no clear phenotypic effects, they are carriers who may transmit a recessive allele to their offspring. Cystic Fibrosis Caucasians, 1in 25 are carriers Symptoms of cystic fibrosis include Mucus buildup in the some internal organs Abnormal absorption of nutrients in the small intestine Treatment untreated death by the age of 5 Treated live to 30yrs antibiotics forced mucus discharge Gene therapy/? Sickle-Cell Disease Sickle-cell disease Affects one out of 400 African-Americans Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include Physical weakness, pain, organ damage, and even paralysis Mating of Close Relatives Matings between relatives Can increase the probability of the appearance of a genetic disease Are called consanguineous matings Prevalence of CF, SC & TS Exist b/c they have benefits Sickle Cell 1 recessive gene gives protection against malaria Tay Sachs prevention from tuberculosis Cyctic Fibrosis protection against cholera/diarrhea driven diseases Dominantly Inherited Disorders Some human disorders Are due to dominant alleles Achondroplasia a form of dwarfism that is lethal when homozygous for the dominant allele Figure 14.15 Huntington’s Disease A degenerative disease of the nervous system Has no obvious phenotypic effects until about 35 to 40 years of age Any child born to a parent who has the allele for Huntington’s disease has a 50% chance of inheriting the disease and the disorder. Molecular geneticists have used pedigree analysis of affected families to track down the Huntington’s allele to a locus near the tip of chromosomes 4. Pedigree Analysis tracking Diseases Multifactorial Disorders Many human diseases Have both genetic and environment components Examples Heart disease Cancer 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 Tests for Identifying Carriers For a growing number of diseases Tests are available that identify carriers and help define the odds more accurately Amniocentesis The liquid that bathes the fetus is removed and tested Chorionic Villus Sampling (CVS) A sample of the placenta is removed and tested Newborn Screening Some genetic disorders can be detected at birth By simple tests that are now routinely performed in most hospitals in the United States Phenyketonuria (PKU) test can detect the presence of a recessively inherited disorder, This disorder occurs in one in 10,000 to 15,000 births. Individuals accumulate the amino acid phenylalanine and its derivative phenypyruvate in the blood to toxic levels. This leads to mental retardation. If the disorder is detected, a special diet low in phenyalalanine usually promotes normal development.
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