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Ch 8: Mendel and Heredity 8.1 The origins of genetics 8.2 Mendel’s theory 8.3 Studying heredity 8.4 Complex patterns of heredity Enduring Understanding Living systems store, retrieve, transmit & respond to information essential to life’s processes. Essential Questions: Why are the sciences of genetics and evolution foundational to modern biology? How is heredity understood by modern scientists? Why might a knowledge of genetics be useful to modern scientists? 8.1 Origins of Genetics From very early times, people have been curious about how traits are passed down through the generations. Within the last one hundred years, since the rediscovery of Mendel's work, we have been able to start to understand the processes of heredity. Mendel's revolutionary work was almost completely ignored when first published. Only after 35 years was it rediscovered and the modern study of genetics began. Heredity Passing of characters ; parents to offspring Ex. Shape of your eyes Ex. Texture of your hair Before DNA and chromosomes were discovered, heredity was once of the biggest mysteries of science. To do! Name 5 characteristics that are passed on in families. Eye, hair and skin color, height etc. Name one characteristic that may be inherited but is also influenced by behavior or environment. Muscle size, body weight, sun tan. Gregor Mendel Gregor Johann Mendel was an Austrian monk. Carried out experiments in which he bred different varieties of the garden pea. First to develop rules that accurately predicted patterns of heredity. The patterns that Mendel discovered formed the basis of genetics - the branch of biology that focuses on heredity. Gregor Mendel Mendel knew much about agriculture. He studied science and math at the University of Vienna. He repeated the experiments of the British farmer T. A. Knight, but Mendel counted the number of each kind of offspring and analyzed the data. Gregor’s Breeding Experiments Used PEAS WHY?? 1. Flower color – purple or white 2. Male and female reproductive parts are enclosed in the same flower – can control mating or can cross pollinate 3. Garden pea is small, grows easily, matures quickly, and produces many offspring. Gregor’s Breeding Experiments See table 1 p. 163 The seven characters that Mendel studied and their contrasting traits. Plant height Mendel’s Monohybrid Cross Phenotype "the form that is shown"; outward, physical appearance of a trait Mendel looked at phenotypes & kept records! Monohybrid cross – cross that involves only one pair of contrasting traits Purple vs white Round vs wrinkled True breeding – offspring would display only one form of the character HOMOZYGOUS P generation – parent generation – first two individuals that are crossed F1 generation – first filial generation – offspring of parent generation F2 generation – second filial generation – offspring of the F1 generation Parental Generation DD Parental Gametes D F1 Genotype x dd d Dd P1 F1 F2 Mendel’s Results For each of the seven traits that Mendel studied, he found the same 3:1 ratio of plants expressing the contrasting traits in the F2 generation. F1 generation all showed the same trait. In the F2 generation the recessive allele reappeared. Mini quiz Describe the contribution of Mendel to the foundation of modern genetics. Mendel was the first to develop rules that accurately predict patterns of heredity. The offspring of true breeding parents are called A. B. C. D. F1 generation F 2 generation Dominant offspring Recessive offspring Which of the following is NOT a good reason why Pisum sativum makes an excellent subject for genetic research? A. B. C. D. Many varieties exist. They require cross-pollination. They grow quickly. The demonstrate complete dominance. Why did Mendel allow the pea plants to self pollinate for several generations before beginning his crosses? To ensure that each variety was true breeding. Define heredity. Heredity is the passing of traits from parents to offspring. 8.2 Mendels’ Theory Mendel’s Hypothesis 1. For each inherited character, an individual has two copies of the same gene – one from each parent. 2. There are alternative versions of genes. Alleles 3. When two different alleles occur together, one of them may be completely expressed, while the other may have no observable effect on the organism’s appearance. Dominant – expressed form of the character present Recessive – trait that is not expressed when dominant form is present. 4. When gametes are formed, the alleles for each gene in an individual separate independently of one another. Gametes only carry one allele When gametes unite during fertilization – each gamete contributes one allele. Mendel’s findings in modern terms Genes and homologous chromosomes 1. Diploid organisms (like peas) have two sets of chromosomes (one from the male, one from the female) and these are called homologous chromosomes. 2. At equivalent positions on these chromosomes exist genes. The location of these "gene pairs" is called the locus. 3. Alternate forms of the gene (ones that differ by mutations in the DNA) are called alleles. 4. Dominant traits we now know are the same as dominant alleles, recessive traits are the same as recessive alleles. 1. Dominant usually capital letter (here G, R, S, Z) and recessive lower case letters (here g, r, s, z). True-breeding traits exist when the organism has two dominant alleles (e.g. TT) or two recessive alleles (tt). homozygous for that gene or those alleles. The F1 individuals above that resulted from a cross between tall and short pea plants have both a dominant (T) allele and a recessive (t) allele. heterozygous for that gene or those alleles. Phenotype = appearance of organism Genotype = genetic makeup of organism the set of alleles that an individual has for a character often represented by letters, genotype Genetic makeup of organism = _________ phenotype Appearance of organism = ____________ The Laws of Heredity The Law of SegregationThe two alleles for a character separate when gametes are formed. The Law of Independent AssortmentThe alleles of different genes separate independently of one another during gamete formation. Mini quiz The color of a dog’s coat is the dog’s A. B. C. D. Dominance Pedigree Phenotype Genotype How are the genotype of a dominant allele and a recessive allele written? Dominant – CAPITAL LETTER Recessive – lowercare letter What is the genotype of a purple-flowered pea plant? PP or Pp What is a dihybrid cross? A cross that considers two pairs of contrasting traits. The scientist whose studies formed the basis of modern genetics? A. B. C. D. T.A. Knight Gregor Mendel Louis Pasteur Robert Hooke 8.3 Studying Heredity Punnett Square A diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in a cross. Named for its inventor – Robert Punnett Types of crosses using Punnett Square Monohybrid cross cross involving a single trait COLOR PLANT HEIGHT Dihybrid cross - cross involving 2 traits COLOR and PLANT HEIGHT Monohybrid cross – homozygous -Produce only red heterozygous offspring - 4/4 =All are heterozygous offspring Monohybrid cross heterozygous Produce ¼ BB (homozygous dominant) 2/4 Bb (heterozygous) ¼ bb (homozygous recessive) Make your own punnett square Exploring further p. 171 – Dihybrid Cross ( 2 characters) Color Shape Determining Unknown Genotypes Test cross- an individual whose phenotype is dominant, but whose genotype is not known, is crossed with a homozygous recessive individual. Test cross Analyzing a Test Cross P. 172 Data Lab Outcomes of Crosses Like Punnett squares, probability calculations can be used to predict the results of genetic crosses. Probability is the likelihood that a specific event will occur. Probabilities can be expressed in words, decimals, percentages, or fractions. Probability Probability = number of one kind of outcome total number of all possible outcomes - Can be used to predict the probability of an allele being present in a gamete. - To find the probability that a combination of two independent events will occur , multiply the separate probabilities of the two events. - ½ x ½ = 1/4 Math Lab p. 174 Predicting the results of crosses using probabilities. Inheritance of traits Pedigree – family history that shows how a trait is inherited over several generations. Helpful in to follow genetic disorders. Carriers do not express the disorder, but can pass the allele for the disorder to their offspring. Sex-linked genes If a gene is autosomal, it will appear in both sexes equally. A sex-linked gene’s allele is located only on the X or Y chromosomes. Most sex-linked genes are carried on the X chromosome and are recessive. See albinism pedigree on p. 175 Mini quiz If smooth peas are dominant over wrinkled peas, the allele for smooth peas should be represented as A. B. C. D. W S w s What is the probability of two parents each carrying a recessive gene for an inherited disease to produce a child that will have that disease? ¼ Simple cross Dd X Dd ¼ chance dd having disease Explain how the parents of an individual who expresses a recessive gene can both not express the gene. Parents must both be heterozygous dominant. 8.4 Complex Patterns of Heredity 8.4 Complex Patterns of Heredity Polygenic inheritance: when several genes influence a character Can be scattered among the same chromosome or located on different chromosomes. Eye color, height, weight, hair color, skin color Incomplete dominance: Snapdragon Incomplete dominance: individual displays a phenotype that is an intermediate between the two parents. Snapdragon- red x white = pink flowers Wavy hair Multiple Alleles Genes with 3 or multiple alleles. Ex: blood types in humans more alleles are said to have ABO blood groups I A, I B, A and B refer to two carbohydrates on surface of red blood cells In i allele, neither carbohydrate is present IA i and IB are both dominant over i, but are not dominant over each other When IA and IB are both present, they are co-dominant The 3 different alleles can code for 4 types of blood Blood Groups Figure 14 p. 178 - The 3 different alleles can code for 4 types of blood When IA and IB are both present, they are codominant There are two special genotypes when it comes to blood transfusions: What are they? OO = universal donors - can donate blood to everybody. - can only receive blood from other type O donors. Frequency in US 46% 4% 40% AB blood =universal recipients - can receive blood from people with all four blood types. 10% Codominance 2 dominant alleles are expressed at the same time. Different than incomplete dominance b/c both traits are displayed. Ex: IA and IB blood alleles Characters influenced by environment An individual’s phenotype often depends on conditions in the environment. Ex: Hydrangea –acidic soil – bloom blue basic soil – bloom pink Ex: Artic fox- temperature affected Summer – enzymes produced = red/brown color Winter – cold temperatures = white color Ex: height in humans = nutrition pH 4.5 pH 7.0 Genetic disorders Genes can be copied wrong or have a mutation. Genetic disorder – inherited genetic mutations Often carried by recessive alleles See Table 2 p. 181 for a list of genetic disorders. Sickle Cell Anemia Recessive genetic disorder. Mutated allele that produces a defective form of hemoglobin – red blood cells bend shape. Sickle-shaped cells rupture easily, resulting in less oxygen being carried by the blood. Poor blood circulation. Recessive allele that causes sickle-cell anemia also helps protect the cell against malaria. 1 in 500 African Americans Sickle Cell- Video Cystic Fibrosis (CF) Most common, fatal, hereditary, recessive disorder among Caucasians. The airway of the lungs becomes clogged with a thick mucus, and the ducts of the liver and pancreas become blocked. Treatments can relieve symptoms, but no known cure. Individuals do not usually live to an old age. 1 in 2,500 Caucasians Hemophilia Recessive genetic disorder, that impairs that blood’s ability to clot. Is a sex-linked trait. Carried on X from female in Hemophilia A and most affects males with only one X. 1 in 10,000 males • In hemophilia, one clotting factor is missing, or the level of that factor is low. • This makes it difficult for the blood to form a clot, so bleeding continues longer than usual, not faster. • Nearly all affected people are male. • An affected male never transmits the trait to his sons. • All daughters of an affected male will be carriers (if the mom is not a carrier). • A carrier female transmits the trait to her sons 50 percent of the time. • No daughters of a carrier female will show the trait, but a daughter in this case (if the dad is not affected) will be a carrier 50 percent of the time. Huntington’s Disease (HD) Dominant allele on an autosome. Gradual deterioration of the brain tissue in middle age – shortened life expectancy Symptoms appear in a person’s 30-40’s so may have already reproduced and passed on the allele. 1 in 10,000 (frequency among human births) The part of the brain most affected by HD is a group of nerve cells at the base of the brain known collectively as the basal ganglia. The basal ganglia organize muscledriven movements of the body, or “motor movement.” Treating Genetic Disorders 1. 2. Most can not be cured, although still searching for cures. Genetic counseling – form of medical guidance before reproducing that informs parents of possible genetic problems. 3. Some can be treated. 4. Gene therapy – isolate defective gene and replace it Human Chromosome Viewer The End