3.a.3- Chromosomal Basis of Life

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1 3.a.3- Chromosomal Basis of Life Big Idea 3: Living systems store, retrieve, transmit and respond to information essential to life processes. EU 3.A: Heritable information provides for continuity of life. EU 3.B: Expression of genetic information involves cellular and molecular mechanisms. EU 3.C: The processing of genetic information is imperfect and is a source of genetic variation. EU 3.D: Cells communicate by generating, transmitting and receiving chemical signals. EU 3.E: Transmission of information results in changes within and between biological systems. A. Rules of probability can be applied to analyze passage of single gene traits from parent to offspring. 1. Gregor Mendel 2. Mendel s Work 3. Chromosomes, Genes, Alleles B. Segregation and independent assortment of chromosomes result in genetic variation. 1. Segregation and independent assortment can be applied to genes that are on different chromosomes. 2. Genes that are adjacent and close to each other on the same chromosome (linked) tend to move as a unit; the probability that they will segregate as a unit is a function of the distance between them. 3. The pattern of inheritance (monohybrid, dihybrid, sex-linked, and genes linked on the same homologous chromosome) can often be predicted from data that gives the parent genotype/phenotype and/or the offspring phenotypes/genotypes. C. Certain human genetic disorders can be attributed to the inheritance of single gene traits or specific chromosomal changes, such as nondisjunction. 1. Sickle cell anemia 2. Tay-Sachs disease 3. Huntington s disease 4. X-linked color blindness 5. Trisomy 21/Down syndrome 6. Klinefelter s syndrome 7. D. Many ethical, social and medical issues surround human genetic disorders. 1. Reproduction issues (should society allow?) 2. Civic issues such as ownership of genetic information, privacy, historical contexts, etc. Hypertrichosis End 1

2 A. An Austrian monk. B. Formulated two fundamental laws of heredity in the early 1860s. C. Previous theories based on the Blending Concept of Inheritance (Tall + Short = Medium)) D. The blending theory did not account for variation, does not explain species diversity E. Mendel's work was unrecognized until 1900; Darwin was never able to use it to support his theory of evolution. Gregor Mendel Back A. Used garden peas 1. Easy to cultivate 2. Short generation time 3. Can be cross-pollinated and self-pollinated B. Mendel cross-pollinated plants. 1. P 1 generation is the parental generation. 2. F 1 generation is the firstgeneration offspring. 3. F 2 generation is the secondgeneration offspring. C. Mendel's results were contrary to those predicted by a blending theory of inheritance. Mendel s Work Mendel s Work D. He found that the F 1 plants resembled only one of the parents. E. Mendel s Conclusions 1. F 1 hybrids contained two factors for each trait 2. One dominant and one recessive factors separated when gametes were formed 3. A gamete carries one copy of each factor 4. Random fusion of all possible gametes occurred upon fertilization. Chromosomes,Genes, Alleles A. Homologous Chromosomes contain genes (locations) for the same traits B. Mendel s factors are called alleles C. Traits are controlled by alleles (alternative forms of a gene). D. Genotype refers to the alleles an individual receives at fertilization. E. Phenotype refers to the physical appearance of the individual. Mendel's Law of Segregation A. Organism contains two factors for each trait. B. Factors segregate during formation of gametes. C. Each gamete contains one factor for each trait. Mendel's Law of Independent Assortment A. Each trait is independent of another. B. Members of one pair of factors assort independently of members of another pair. C. All combinations of factors occur in gametes. D. Meiosis explains these results of independent assortment. 2

3 Genes Are Linked A. Linked genes can show the distance between genes on the chromosomes. B. Percentage of recombinant phenotypes measures distance between genes to map the chromosomes. 1. If 1% of crossing-over equals one map unit, then 6% recombinants reveal 6 map units between genes. 2. If crosses are performed for three alleles on a chromosome, only one map order explains map units. A. Homozygous x Homozygous B. Homozygous x Heterozygous C. Heterozygous x Heterozygous D. Testcross Monohybrid Crosses Dihybrid Cross Feather color = ORANGE or blue CREST or no crest Nondisjunction A. Failure of chromosomes to separate B. More common during meiosis I than meiosis II C. Can occur in mitosis. D. Types 1. Monosomy: missing one chromosome 2. Trisomy: three of one type of chromosome. 3. Polyploidy 1) More than two complete sets of chromosomes 2) Create triploids [3n], tetraploids [4n], etc. 3) A major evolutionary mechanism in plants. Karyotypes A. Picture of chromosomes B. Cells are treated and photographed just prior to dividing. C. Chromosomes are sorted and arranged by homologous pairs D. Organized according to size, shape, and banding pattern in metaphase E. Cells are obtained through chorionic villi sampling and amniocentesis F. Used to diagnose chromosomal abnormalities. Moving Away from Punnett Squares Trihybrid Cross 3

4 Moving Away from Punnett Squares A. Multiplicative Law of Probability 1. The chance of two or more independent events occurring together is the product of the probability of the events occurring separately. 2. Chance of inheriting a specific allele from one parent and a specific allele from another is 1/2 x 1/2 or 1/4. 3. Possible combinations for the alleles Hh x Hh are the following: HH = 1/2 1/2 = ¼ hh = 1/2 1/2 = ¼ Hh = 1/2 1/2 = ¼ hh = 1/2 1/2 = ¼ B. Additive law of probability 1. The probability of an event that occurs in two or more independent ways is the sum of individual probabilities of each way an event can occur 2. In the above example where Tall is dominant (HH, hh, and Hh), chance for Tall is 1/4 + 1/4 + 1/4 = 3/4. C. EX: SsYyAa x SsYyAa; odds of child being SSyyAa (1/32) Sickle Cell Anemia (Chromosome 11) B. One Gene May Control Many Traits 1. Pleiotropy: a single gene effects many aspects of an individual's phenotype. 2. Sickle Cell anemia a) Autosomal Recessive b) Blood cells are abnormal c) Symptoms are anemia, weakness, heart attack. Tay-Sachs Disease (Chromosome 15) A. Autosomal Recessive B. Usually among Jewish people in the U.S. of central and eastern European descent. C. Lipid accumulation in the brain D. Symptoms not initially apparent E. Development slow at 4-8 months, child gradually becomes blind, develops seizures, eventually becomes paralyzed, dies by age of three or four. F. No treatment or cure Huntington Disease (Chromosome 4) A. Autosomal Dominant B. Affects one in 20,000. C. Progressive degeneration of brain cells, which in turn causes severe muscle spasm, personality disorders, and death. D. Most appear normal until they are of middle age. Color Blindness (X Chromosome) A. Sex-Linked Recessive B. Mutations of genes coding for green or red-sensitive cone cells. C. Inability to perceive green or red. Down Syndrome (Trisomy 21) A. Nondisjuction at 21 B. Appearance includes; excess skin at the nape of the neck, flattened nose, single crease in the palm of the hand, small ears, small mouth, upward slanting eyes C. Down syndrome is the most common single cause of human birth defects. 4

5 Klinefelter Syndrome (XXY) A. Nondisjuction of sex chromosomes B. Feminization, sterile, underdeveloped testes. A. Jacob s Syndrome 1. XYY males 2. Learning problems at school 3. Delayed emotional maturity 4. Tall, thin, acne 5. Not overly aggressive B. Turner s Syndrome 1. XO females 2. Short, webbed neck, no puberty. C. Cystic Fibrosis (chromosome 7) 1. Most common lethal genetic disease in Caucasians in U.S. 2. About 1 in 20 Caucasians is a carrier, and about 1 in 2,500 births has this disorder. 3. Production of viscous form of mucus in the lungs and pancreatic ducts. 4. New treatments have raised average life expectancy to 28 years. D. Hemophilia 1. X linked recessive 2. About 1 in 10,000 males. 3. Impaired ability of blood to clot. 4. Hemophiliacs bleed externally after an injury and also suffer internal bleeding around joints. Pedigree Charts A. Show pattern of inheritance within a family. 1. Males are designated by squares, females by circles 2. Shaded individuals are affected 3. A carrier has no apparent abnormality but can pass on an allele for a recessively inherited genetic disorder. 4. Autosomal dominant and autosomal recessive alleles have different patterns of inheritance. Pedigree Charts D. Characteristics of autosomal dominant disorders 1. Affected children must have one affected parent. 2. Heterozygotes are affected 3. Two unaffected parents can produce only unaffected child E. Characteristics of autosomal recessive disorders 1. Affected children can have normal parents 2. Two affected parents always produce an affected child. 3. Close relatives who reproduce together are more likely to have affected children. 5