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Download Chapter 11 Intro to Genetics
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Chapter 11 Intro to Genetics 11-4 Meiosis A. Principle of genetics requires 2 things 1. A single copy of every gene must be inherited 2. When gametes (sperm/egg) are formed a process must separate the 2 sets of each gene so that each gamete has only 1 set of genes B. Chromosome number 1. somatic cell (body cell) has 46 chromosomes a. 23 from female gamete (egg) b. 23 from male gamete (sperm) c. Homologous chromosomes --corresponding chromosomes of male and female --ex. Male has chromosome for hair color/ female has chromosome for hair color 2. Diploid (2 sets of chromosomes) 3. a. Cell has both sets of homologous chromosomes b. Represented by 2N (46 chromosomes) --human somatic (body) cell is 2N c. cell has 2 sets of genes with complete set of chromosomes Haploid (1 set of chromosome) a. Cell has one set of chromosomes and 1 set of genes b. Represented by N (23 chromosomes) --gametes (germ cell) *sperm (N) 23 chromosomes *egg (N) 23 chromosomes *sperm/egg form zygote (46 chromosomes) C. Meiosis 1. process of reduction and division 2. chromosome number is reduced in half by separation of homologous chromosomes in a diploid cell 3. 2 divisions a. Meiosis I b. Meiosis II 4. final cells (sperm or egg) are genetically different from original cell 5. tetrad a. 2 pairs of sister chromatids (4 chromos.) 6. crossing over a. An exchange of genetic material producing new genetic info b. May occur when tetrads are formed 7. handout D. Gamete formation 1. 4 cells formed called gametes 2. male has 4 sperm formed 3. female has 1 egg with 3 polar bodies a. Polar bodies—no reproductive value, absorbed into the body E. Comparing Mitosis/Meiosis 1. Mitosis a. Forms 2 genetically identical cells b. Diploid cells (46) form 2 diploid daughter cells c. Replaces/repairs/healing of body cells d. Asexual reproduction 2. Meiosis a. Begins with diploid (2N) cell to produce 4 haploid (N) cells b. Haploid cells are genetically different from original diploid cell c. By crossing over new genetic material is made d. For sexual reproduction 11-1 Work of Gregor Mendel A. Genetics 1. scientific study of heredity 2. key to species differences B. Gregor Mendel’s Peas 1. Monk trained in math/science 2 Cared for monastery garden 3. Peas are self-pollinators a. Have stamen (male) to produce pollen and pistils (female) to produce egg on same plant b. The pollen self-pollinates the egg on the same plant c. Offspring are identical to parent d. Also called true-breeds or pure breeds 4. Mendel’s experiment a. Removed stamen (left pistal) on plants b. Removed pistal (left stamen) on plants c. Manually dusted pollen from one plant onto pistal of another plant—called crosspollination d. Cross-pollinated offspring are genetically different from parents e. Also called hybrids C. Genes and Dominance 1. trait a. Specific characteristic --seed color, plant height, seed shape, pod color, etc. 2. genes a. Chemical factors which determine traits b. Have contrasting forms 3. alleles a. Different forms of genes --Seed color (trait or gene) --yellow or green (alleles) --hair color (trait/gene) --brown, black, blonde (alleles) 4. Generations a. Parent (P)—original parents b. First Filial (F1)—offspring c. Second Filial (F2)—offspring’s offspring 5. Conclusion of crossings a. Biological inheritance determined by factors passed from generation to generation b. Some alleles are dominant, some are recessive (Principle of dominance) --dominant traits will always be expressed if either parent has the dominant trait *represented by capital letter --recessive traits will only be expressed if both parents have recessive trait *represented by lowercase letter D. Segregation 1. occurs during meiosis 2. alleles are segregated (separated) so each gamete receives only 1 copy of gene (Law of Segregation) 11.2 Probability and Punnett Squares A. Probability 1. Likelihood an event will occur 2. Past events have no bearing on future events 3. coin flip—50% chance or 1:2 of getting heads 4. Principle of Probability a. How will alleles segregate? b. Used to predict genetic crosses Parent alleles segregate across top and side Parent 2 B. Punnett Square 1. Prediction and comparison of genetic variation resulting from a cross 2. Parent 1 offspring offspring offspring offspring Gene combination of offspring inside boxes 3. Trait--Use same letter type --ex. Hair color = T or t 4. Alleles a. Dominant represented by capital (T) b. Recessive represented by lowercase (t) 5. Homozygous (true or pure breeds) a. Homo-same zygous—joined b. Identical alleles --TT (homozygous dominant) --tt (homozygous recessive) 6. Heterozygous (hybrids) a. Hetero—different b. Different alleles --Tt 7. Phenotype a. Physical trait (what can be seen) b. Red hair, blue eyes, tall, red rose 8. Genotype a. Genetic makeup (allele combinations) b. TT, Tt, tt 9. monohybrid cross a. Compares 1 trait only 10. Example Parent 1—heterozygous for tall (Tt) Parent 2—heterozygous for tall (Tt) dominant—tall (T) recessive –short (t) Parent 1 Parent 2 t T T t Offspring 1- Phenotype--tall Homozygous tall -Genotype--TT TT Tt Offspring 2 and 3 -Phenotype—tall Tt tt Heterozygous tall -genotype—Tt Offspring 4 -Phenotype—short Ratios: Homozygous short Phenotype—3 tall, 1 short -genotype—tt 3:1 Genotype– TT--1, Tt—2, tt—1 1:2:1 C. Probabilities Predict Averages 1. Predicts outcomes of large # of events 2. Larger the # of offspring the closer to expected values (prediction of Punnett Square) 3. 100 offspring would expect to see close to a a. 3:1 phenotype ratio or 75:25 actual #’s b. 1:2:1 genotype ratio or 25:50:25 actual #’s 4. 1000 offspring would be? a. 3:1 or 750:250 (phenotype) b. 1:2:1 or 250:500:250 (genotype) 5. Sometimes expressed as percentages not ratios a. 75% tall; 25% short (phenotype) b. 25% TT; 50% Tt; 25% tt (genotype) 11-3 Exploring Mendelian Genetics A. Independent Assortment 1. Alleles and genes segregate independently of each other a. They do not influence each other b. Example: blonde hair/blue eyes do not always appear together 2. Principle of Independent Assortment a. Genes for different traits can segregate independently during gamete formation (Meiosis) b. Accounts for genetic variation of organisms 3. Dihybrid cross a. Use a two-factor (trait) cross (4 alleles) b. Example: --2 different traits/4 alleles *seed shape—round or wrinkled, *seed color—yellow or green 4. Parents are round, yellow seeds (RrYy) (1234) a. Alleles segregate as follows (1234) 1,3 1,4 2,3 2,4 RrYy RY Ry rY ry (1,3) RY (1,3) RY (2,4) ry (2,4) ry RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy (1,4) Ry (2,3) rY (1,4) Ry (2,3) rY B. Summary of Mendel’s Principle 1. Inheritance is determined by genes 2. Some genes are dominant, some recessive 3. Adults have 2 copies of genes from inheritance. Segregation of genes occur during meiosis 4. Alleles for different genes segregate independently. C. Beyond Dominance and Recessive Alleles 1. Some genes are not dominant nor recessive. 2. Many traits controlled by multiple alleles or genes 3. Incomplete dominance a. Neither allele is dominant, heterozygous phenotype falls between homozygous phenotypes. --red 4 o'clock and white 4 o'clock produces pink 4 o'clock 4. Codominance (form of incomplete dom.) a. Both alleles contribute to phenotype b. Red hair cattle/white hair cattle produce a roan (pinkish-brown due to hair combinations) c. Can see red/white hairs d. Chickens w/ black/white feathers 5. Multiple alleles a. Genes have more than 2 alleles b. Individual may inherit 2 alleles only c. Blood types have alleles A,B,O --combination can be AA, AO, BB, BO, AB, OO d. Eye color/hair color 6. Polygenic traits a. Inheritance of several genes --skin color (4 different genes) --foot size --nose length --height D. Applying Mendel’s Principles 1. Applies to all living organisms 2. Fruit fly is model organism for genetics experiments (discovered by Morgan) b/c a. Fly is small –easy lab storage b. Quick reproduction c. Quick life cycle (28 days) E. Genetics and the Environment 1. characteristics are determined by the interactions between genes and environment a. Plant height determined by genes but genes may be affected by climate, soil conditions, and water availability 11-5 Linkage and Gene Maps A. Gene linkage 1. Some genes are almost always inherited together and rarely become separate 2. Genes a linked and called linkage groups 3. The linked groups assort independently but genes on linkage groups are inherited together 4. Chromosomes assort independently not the genes B. Gene Maps 1. genes are located on chromosomes 2. farther apart on chromosomes more likely genes will be separated during crossing over 3. gene maps show location on chromosome 4. human genome a. Gene map of human chromosome b. Can pinpoint hair color, diseases such as cancer, mutations such as Down’s, or anything determined by genes