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Download Chapters 11-13: Classical Genetics
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Chapters 11-12: Classical Genetics I. Mendelian Genetics A. Mendel’s experiments 1. pea plants with seven distinct characteristics or traits over many generations and many years 2. utilized “pure” breeding strains 3. crossed or mated plants by transferring pollen (sperm) from one strain or type to the pistil (ovum) of the other strain or type 4. collected and analyzed data looking for patterns or trends B. Developed the basic laws or principles of genetics 1. Principle of Unit Characters: 2. Principle of Dominance and Recessiveness: 3. Principle of Segregation: 4. Principle of Independent Assortment: C. Key terms and concepts associated with Mendel’s Laws 1. dominant vs recessive 2. homozygous vs heterozygous 3. phenotype vs genotype 4. alleles 5. punnett squares 6. test cross D. Monohybrid vs Dihybrid crosses and punnett square problems – problem solving technique E. Branching Diagrams – problem solving technique II. Non-Mendelian Genetics – Other Patterns of Inheritance A. Incomplete Dominance B. Codominance C. Epistasis – a gene at one locus (position on a chromosome) interferes with a gene at a different locus, both of which affect the same trait 1. both loci must have a functioning (dominant) gene for the dominant phenotype to be expressed 2. AAbb (white) x aaBB (white) all purple (AaBb) 3. AaBb (purple) x AaBb (purple) 9/16 purple + 7/16 white D. Pleiotropy – a single gene affects more that one trait Marfan’s syndrome (tall; thin; long arms, legs, and fingers; near sighted; wall of the aorta is weak – Abraham Lincoln?) E. Multiple Alleles – A,B,AB,O blood type 1. three alleles: IA, IB, i 2. blood type – antigens (on red blood cells) and antibodies (in plasma) 3. O blood is the universal donor, AB blood is the universal recipient. 4. To determine if a blood transfusion is feasible ask if there is a match between the cells of the donor and the antibodies of the recipient. If there is a match, clumping will occur leading to clots and death. F. Polygenic inheritance – many genes affect the same trait (height, hair color, eye color, skin tones, etc.) 1. most human traits are polygenic 2. how do you know: large variation in phenotypes (“bell” curve distribution) G. Sex-linked (X-Linked) Traits – traits whose genes are found only on the sex chromosomes – tend to be more common in males 1. color blindness, hemophilia in humans 2. examples: H. Gene Linkage – genes that are linked are on the same chromosome and therefore inherited together 1. notation: (normal): AaBb x AaBb 9/16 dominant/dominant; 3/16 dominant/recessive; 3/16 recessive/dominant; 1/16 recessive/recessive; __ __ __ __ (linked): AB ab x AB ab 12/16 dominant/dominant; 4/16 recessive/recessive; 2. effect: reduces genetic combinations possible 3. crossing over and gene linkage can be used to determine gene map distance units: in #1: 100 offspring (70 were dominant for both traits; 5 were dominant, recessive; 5 were recessive, dominant; and 20 were both recessive) crossover frequency (recombinant frequency) = 10/100 or 10% = “A/a” and “B/b” genes are 10 map units apart I.Chromosomal Mutations 1. mutation = permanent change in a gene or chromosome that can be passed on to other cells or offspring if they occur in gametes 2. nondisjunction = failure of chromosomes to separate during meiosis, may result in abnormal chromosome numbers a. monosomy = only one chromosome of a pair (2n-1) b. trisomy = three chromosomes instead of a pair (Down’s syndrome – three #21 chromsomes) (2n + 1) c. polyploidy = multiple sets of chromosomes; i.e. 3n, 4n, 5n . . . (uncommon in animals, fairly common in plants – up to 50%, arises during hybridization) 3. individual chromosome abnormalities a. inversion – a portion is turned upside down reversing the order of the genes b. translocation – moving one part of a chromsome to a different, non homologous chromosome c. deletion – part of a chromosome breaks off d. duplication – presence of a chromosomal segment more than once in the same chromosome J. Probabilities and Genetics 1. rules: a. All probabilities fall on a scale from zero to one b. The probabilities of all the outcomes of an event must add up to one c. The probabilities of independent events occurring together is found by multiplying together their separate probabilities 2. Using probabilities in genetic problems a. In a self cross of a pea plant heterozygous for both seed coat and texture (SsYy), what is the probability of an offspring being both smooth and green? Determine the genotype: (S ___ yy) Determine the chance of each separate event: Smooth is dominant and requires at least 1 S (Ss x Ss ¾) Green is recessive and requires two y’s (Yy x Yy - ¼) ¾ x ¼ = 3/16 b. For two separate events each of which has 2 possible outcomes (flipping a coin, dominant vs recessive), the probabilities of the four possible outcomes can be found by using the binomial (p + q)2 (p + q) 2 = p2 + 2pq + q2 For three events: (p + q)3 (p + q)3 = p3 + 3p2q + 3pq2 + q3 c. What is the probability of three peas in a pod from the cross Rr x rr growing into 2 red flowered and one white flowered plant? 3p2q = 3 x (1/2)2 x ½ = 3/8 III. Human Genetics A. Lethal alleles (cause the death of the individual) and other human “genetic” diseases 1. dominant lethal alleles – are rare since they will be eliminated through the death of the individual (example: Huntington’s disease – effects occur later in life – neurological) 2. recessive lethal alleles may be maintained in the population in the heterozygote (example: brachydactyl – shortened digits; in the homozygous recessive, individual dies during embryonic development and is reabsorbed into the uterus – produces a 2:1 ratio) 3. sickle cell anemia – red blood cells are abnormally shaped (recessive, but also codominant) a. Homozygous dominant = normal RBC’s b. Heterozygous dominant = both normal and sickle shaped RBC’s c. Recessive = sickle cell anemia d. Under certain conditions (prevalence of malaria), heterozygotes have an advantage – resistant to malaria 4. Tay Sach’s Disease – metabolic disorder resulting in deterioration of the brain and early death (recessive) 5. Cystic fibrosis – defective in protein in lungs causes mucus to build up, alveoli stick together causing lung damage and early death (recessive) 6. PKU (Phenylketonuria) – individuals lack an enzyme that converts phenylalanine into tyrosine; excessive phenylalanine builds up in the body causing organ damage and mental retardation – can be treated with a diet low in phenylalanine (recessive) 7. Albinism – lack of skin pigmentation (recessive) B. Using Pedigree charts to follow the history of genetic disorders 1. Male = box, female = circle 2. Shaded circles and squares are affected individuals 3. Horizontal line between individuals = mating; vertical line between individuals = children 4. dominant versus recessive patterns, sexlinked (X), blood type and other genetic traits can be traced and determined C. Other human patterns of inheritance 1. polygenic = many genes affect the same trait a. most human traits (eye color, hair color, skin color, height, etc.) b. significant variation in that trait 2. sex determination a. male = XY (heterozygous) b. female = XX (homozygous) c. sex itself is a polygenic trait with at least 19 different sets of genes influencing its expression (some are autosomal) d. in mammals, the Y chromosome carries at least one gene that gives the embryo its first “push” towards becoming male secretes testes determining factor which causes the immature gonads to differentiate into testes e. in birds and some reptiles, female is heterozygous (ZW) and male is homozygous (ZZ) f. abnormal arrangements of sex chromosomes (XO: Turner’s syndrome – abnormal females, infertile; XXY – Klinefelter syndrome: abnormal male) 3. sex-influenced – expression of these traits depends upon the level of sex hormones, genes are usually located on autosomes (male pattern baldness in humans) a. in males, male sex hormones stimulate expression of the baldness allele even if only one allele for baldness is present (acts as if it is dominant) b. in females, female sex hormones reduce the expression of the baldness allele (the same allele acts as if it is recessive). Both alleles must be present for baldness in females, and even if present, the woman’s hair is only “thinner” IV. Genetic Counseling and Fetal Testing A. Blood tests and genetic analysis of chromosomes can help determine whether prospective parents are carriers for genetic disorders; DNA testing B. Chorionic villi sampling and amniocentesis can be used to obtain fetal cells which can then be cultured and examined for genetic defects 1. human karyotype: arrangement of chromosomes in order from largest to smallest (1st 22 pairs – autosomes, 23 rd pair – sex chromosomes)