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Chapter 11 & 14: Fundamentals of Genetics & Human Genetics Section 1: History Genetics: The scientific study of heredity Trait: A characteristic that can be passed from parent to offspring How does this relate to our last chapter? Chromosomes carry the traits and meiosis determines which traits will be carried by the gametes (sex cells)! History Gregor Medel: Austrian Monk who, in the 1860’s, used pea plants and statistics to study inheritance Why Peas? 1. Ideal Flower Structure 2. Presence of distinctive traits 3. Rapid Reproduction Cycle (90 days) How? 1. 1st grew 7 traits "PURE" through self-pollination example: plant height, seed color, seed shape, pod shape, pod color, flower type...etc 2. He crossbred (hybridized) different plants to see what the offspring would look like... i) tall x short ii) yellow x green seeds iii) wrinkled seeds x smooth seeds ...and found he got predictable outcomes Self-fertilization: Pollen from anther fertilizing the pistil of the same flower Purebred: Organism that receives the same genetic traits from both of it’s parents Hybrid: Organism that receives different forms of a genetic trait from each parent Denoting Generations P = Parents F1 = First generation of offspring F2 = Second generation of offspring F3 = Third generation of offspring *** For every trait, a plant must carry a pair of factors which affect each other*** ***When a trait is inherited, the offspring receives 1 factor from each parent*** Factor = Gene: Section of a chromosome that code for a trait Allele: Different and distinct forms of genes Dominant: A form of gene that is fully expressed when two different alleles are present Recessive: A form of gene that is not expressed when paired with a dominant allele ***Two recessives must be present for the recessive gene to be expressed!*** Section 2: Principles of Inheritance Walter S. Sutton (1903): Chromosome Theory of Heredity: The material of inheritance is carried by the genes in chromosomes. Representing Alleles (Pea color is controlled by a single gene with 2 alleles) Dominant = Y = yellow Uppercase = Dominant Recessive = y = green lowercase = recessive Genotype: The genetic make-up of an organism (both genes in a homologous pair of chromosomes) (ex: YY or Yy or yy) Phenotype: The outward expression of the trait (ex: yellow or green) Homozygous: When two alleles in a gene pair are identical (ex: YY or yy) Heterozygous: When two alleles in a gene pair are different (ex: Yy) Mendel’s Laws (Basic Rules of Inheritance) 1. Law of Segregation: A pair of factors is segregated, or separated, during the formation of gametes (each reproductive cell receives only one of a pair of alleles- to be passed on to offspring) 2. Law of Independent Assortment: Factors (genes) separated and are distributed to gametes completely separate from other genes(example: tall (plant height) gene is not connected to pea color gene) Ex: P (YY, RR) F1 X (yy, rr) (Yy , Rr) 3. Law of Dominance: One factor in a pair may mask the other, preventing it from having an effect (only the dominant allele of a gene is expressed, although a recessive allele may be carried) Section 3: Genetics and Predictions Probability: The likelihood of an occurrence or event (ex: coin flip, weather) Represented using: 1. Fractions (1/2) 2. Percentages (50%) 3. Ratio (1:1) ***Used to predict phenotypes/genotypes*** Punnett Squares: Used for predicting the crossing of two organisms, showing the probability of certain phenotypes/genotypes (not actual results!) Monohybrid: Looks at 1 trait at a time Dihybrid: Looks at 2 traits at a time Trihybrid: Looks at 3 traits at a time (extra credit!) Tetrahybrid: Looks at 4 traits at a time (even more extra credit!) How to make a Punnett Square: 1. Make a grid 2. Place alleles of 1 parent on the top of grid, alleles of other parent along left side 3. Fill in the grid a. combine parent alleles inside the boxes (letters show genotypes) b. determine the genotypic ratios c. determine the phenotypic ratios Ex: ***Dihybrid & Trihybrid crosses use the same method as monohybrids, but use a larger grid to accommodate ALL of the possible parental genotype combinations*** Ex: Test Cross: Used to distinguish between homozygous dominant and heterozygous organisms How is it done? Breed an unknown with a Homozygous Recessive ***If unknown is heterozygous, then 1/2 should show recessive ***If unknown is homozygous dominant, then all of offspring show dominant trait Section 4: Predictions and People **Humans & higher organism traits are more complex **Longer time between generations **Fewer offspring Pedigree: Chart that shows how a trait and the genes that control it are inherited within a family **Information is key! **Often blood (DNA) collected (if possible) **Mormons & Ice landers often used in studies due to records Carrier: An individual who carries a recessive trait that is not expressed Example to Charting (for a recessive disorder): Homozygous Recessives I II III Homozygous Dominant Heterozygous = Female showing trait = Normal Female = Female Carrier = Male showing trait = Normal Male = Male Carrier ***Recessive allele can be carried/passed along without affecting carriers health! Ex: Tay-Sachs disease Recessive allele causes disease Homozygous recessive person can not metabolize a certain lipid, which then accumulates in the brain Affected people do not live for more that five years Carriers can pass allele along Other examples: Cystic Fibrosis, Albanism (multiple allele disorder)... Dominant Disorders: Very Rare! Why? Disorders often cause death, so individual dies before procreating Exception: HD (Huntingtons Disorder): Fatal, but onset occurs later in life Genetic Counseling Ethics Down Syndrome PKU (Phenylketonuria) Section 5: Difficult Predictions (Exceptions to the Rules) Intermediate Inheritance: Offspring have trait that is not exactly like that of either parent Three Forms: 1. Incomplete Dominance 2. Codominance 3. Polygenic Inheritance 1. Incomplete Dominance: a. When 2 different alleles for the same trait combine, but neither allele “wins” expression over the other b. Heterozygous offspring show a phenotype that is in between the phenotypes of the 2 homozygous parents Ex: Snapdragons RR (red) X rr (white) Rr (pink) Other examples: Roan Cows Palomino Horses Hypercholesterolemia (humans) HH = Normal Hh = 2X higher cholesterol levels (1 in 500) hh = 5X higher cholesterol levels (1 in 1,000,000) 2. Codominance: Both alleles in a heterozygote express themselves fully Ex: Human Blood Type Antigen (carbohydrate) on RCB Genotype Serum Agglutinin A = A type IAIA or IAi Anti-B B = B type IBIB or IBi Anti-A AB = Both types O = None IAIB ii None (Universal Recipient) Both Anti-A & Anti-B (Universal Donor) 3. Polygenic Traits: A trait controlled by 2 or more genes Ex: Eye color, Skin color, many others Multiple Alleles: 3 or more alleles in a population Pleiotropy: Occurs when a single gene affects more than one trait Environment: May play a role in the expression of certain traits Ex: Himalayan rabbit fur (temperature affects color) Western White butterfly: darker in spring, which help attracts heat and allows it to fly (can only fly in certain temperatures) What other traits do you think are affected by environment? Can you think of any traits in humans that are affected by environment?