Genetics Mendel, Human Genetics, & The Gene Idea Genetics . Genetics -the study of inheritance. -how traits (characteristics) are passed down from parent to offspring. Gregor Mendel- mid 1800’s-Austrian monk -founded modern genetics with data gathered from growing peas. Peas -easy to grow and had many traits. Self-pollinate and cross-pollinate* Flower Anatomy Pollen is here Eggs are here Pollen grains and eggs Pollen is flower sperm Ovules are eggs at the flower base Mendel’s Terms Trait (variant for a character, i.e., brown) True-breeding (all offspring of same variety) -Pure-Breed Hybrid (offspring of 2 different true-breeds) Alleles- different forms of gene P generation (parents) F1 generation (first generation) F2 generation (second generation) Mendel’s Experiments: Self and Cross Pollinated 1. Focused on -garden pea traits by crossing different traits one at a time: Flower/seed coat color: gray vs white Seed color: yellow vs. green Seed shape: smooth vs. wrinkled Pod color: green vs. yellow Pod shape: inflated vs. pinched Stem height: tall vs. short Flower position: axial vs. terminal 2. Counted offspring of each trait form and analyzed the results using math. Vocabulary . Alleles –forms of a gene. Homozygous: pair of identical alleles for a trait . (pure/true-bred) . Heterozygous: two different alleles for a trait . (Hybrid) . Phenotype: an organism’s observed trait . Genotype: an organism’s genetic makeup Law of Segregation . Different alleles account for variations in inherited characteristics . For a trait-an organism inherits 2 alleles, one from each parent . Alleles - segregate (separate) during gamete production (meiosis). Probability and Punnett squares Punnett square- a tool to calculate expected genotypes of offspring of a cross. Principle of Dominance Heterozygous offspring, (Aa) . Dominant allele- fully expressed in the organism’s phenotype . Recessive allele, has no effect on the organism’s phenotype . (* 1 pattern of inheritance) Law of Independent Assortment . Alleles on different chromosomes, segregate independently of each other= . Mendel’s Law of Independent Assortment Dihybrid F1 generation Two traits- Two different chromosomes 2 homozygous Parents with different traits produce Dihybrid offspring Dihybrid cross: F2 generation Phenotype Ratio: 9:3:3:1 Additional Genetic Patterns Mendel’s peas Other Patterns Complete Incomplete Dominance Dominance Codominance Lethal Alleles Two alleles /gene Multiple Alleles One gene affects …Many characteristics one trait (Pleiotropy) Two (or more) genes affect one trait (Polygenic Traits) One gene masking another(Epistasis) Other Inheritance Patterns . Incomplete dominance - appearance between the phenotypes of the 2 parents. Ex: snapdragons CR=red CW=white Pink- heterozygous CRCW Recognize- Different allele representation Incomplete Dominance Problem Snapdragon problem: Use the allele representation C R = Red, C W = white The outcome of a cross between a red- flowered plant and a pink-flowered plant? P = C R C R x C R C W 50% red, C R C R 50% pink, C R C W Sometimes Genotype Written: RR, RW Incomplete Dominance Incomplete dominance: neither allele masks the other and both are observed as a blending in the heterozygote RR x R’R’ Four o’clock flowers Red White R = red, R’ = white RR’ pink Incomplete Dominance RR’ x RR’ Pink x Pink Genotypic Ratio: Phenotypic Ratio: Codominance . Codominance: two alleles equally expressed, but not blended, like incomplete dominance . Ex: Roan coat- cows- both red and white hairs Cross a red cow with a white bull. What is the expected genotype and phenotypes of the offspring? P = RR x WW RW, roan Multiple Alleles -Having more than 2 alleles for a trait. Human Blood alleles- IA ,IB, i codominance & multiple IA = IB > i IA and IB are codominant. IA and IB are completely dominant over i. Antigens on Red Blood Cells IAi IBi IAIB Lethal Alleles Gene capable of causing death- usually in embryo. Example: Manx cat ML = tailless, lethal in homozygote m = tail Tailless male x Tailless female Polygenic Traits . Most of your traits are controlled by the interaction of many genes. Multiple genes working together produce a continuous distribution in a “Bell Shape” curve of degrees. Examples of Polygenic Traits Individual genes of a . Body Type polygenic trait follow . Height Mendel's laws but . Skin Color together do not . Hair color produce Mendelian . Eye color ratios. Intelligence . We often see the famous “Bell Curve” Example of Polygenic Inheritance Two genes affecting skin coloration Number of Skin Color* Genotypes % Pigmentation Dominant (Phenotype) Alleles 0 White aabb 0-11% 1 Light Black Aabb or aaBb 12-25% 2 Medium Black AAbb or AaBb or 26-40% aaBB 3 Dark Black AABb or AaBB 41-55% 4 Darkest Black AABB 56-78% *Based on a study conducted in Jamaica. Polygenic Inheritance Medium Black Woman X Darkest Black Man (mother is white) Pleiotropic Effects One gene affects many phenotypic characteristics Allele S S’ Gene Product Hemoglobin A Hemoglobin S Cell Shape Round Sickled under low O2 tension Response to Susceptible Resistant in SS’ Malaria genotype Pleiotropy Pleiotropy refers to an allele which has more than one effect on the phenotype. This can be seen in human diseases such as cystic fibrosis or sickle cell anemia. In these diseases, multiple symptoms can be traced back to one defective allele. Recent studies show … . Hypertension . Diabetes . Cancers . Allergies . Cardiovascular diseases . Behavioral traits (alcoholism and phobias) …..have some genetic link but also environmental explanation. Environmental Effects Expression of some genes may be impacted by environment Gene for pigment production expressed in cooler regions of body Another example of environmental influence: . Hydrangeas – same genotype, different environments different color flowers Acid pH Alkaline pH Epistasis . An allele of one gene masks the expression of alleles of another gene and expresses its own phenotype instead. Gene that masks = epistatic gene . Gene that is masked = hypostatic gene Epistasis . Action of genes at one location modify expression genes at another location .Effects often complex . Examples . Coat color in Labrador dogs . Snake coloration Epistasis in Labrador Dogs . Bb or BB dark (black) pigment produced . bb light (brown) pigment produced . Ee or EE deposition of melanin . ee deposition of pigment blocked Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ee E_ No dark pigment in fur Dark pigment in fur Yellow Lab E_bb E_B_ eebb eeB_ Chocolate Lab Black Lab Yellow fur; Yellow fur; Brown fur, Black fur, brown nose, black nose, nose, lips, nose, lips, lips, eye rims lips, eye rims eye rims eye rims Interacting Genes Affecting a Single Characteristic eg. Skin coloration in snakes One gene O = orange pigment o = no orange pigment Second gene B = black pigment b = no black pigment Interacting Genes Affecting a Single Characteristic eg. Skin coloration in snakes Oo Bb x Oo Bb OB Ob o B o b OB OO BB OOBb Oo BB Oo Bb Ob OO Bb OO bb Oo Bb Oo bb o B Oo BB Oo Bb o o BB o o Bb o b Oo Bb Oo b b o o Bb o o b b Interacting Genes Affecting a Single Characteristic eg. Skin coloration in snakes OoBb x OoBb 9/16 O_B_ 3/16 O_bb 3/16 ooB_ 1/16 oobb Ch 14 Gene Disorders – Gene Mutations . Gene disorder -the harmful effect a mutated allele produces when it occurs with significant frequency in a population. Can be inherited in several ways Dominant/ recessive Codominance Recessive Disorders Ex PKU - phenylketonuria CF -Cystic fibrosis Tay-Sachs Albinism Prader-Willi Gene Disorders Phenylketonuria (PKU) A recessive disorder that occurs in 1 of 10,000 people. A defective gene on chromosome 12 is responsible. Cannot break down -Amino acid phenylalanine. Recessive Disorders Cystic Fibrosis A recessive disorder - mutated gene on chromosome 17. Excessive secretion of the mucus in the body. Tay-sachs Disease A recessive disorder that occurs in 1 of 5,000 people - European Ashkenazi Jews Defective gene on chromosome 15. Excess lipids in brain Prader Willi Syndrome A recessive disorder that occurs in 1 of 15,000 people. Determined by a set of Mutated genes on chromosome 15. Dominant Disorders Marfan’s Syndrome Huntington’s disease Osteogenesis imperfecta Neurofibromatosis (NF) Huntington Disease A dominant disorder that occurs in about 1 of 10,000 people. A dominant gene on chromosome 4 is responsible. It causes degeneration of neurons producing dementia, and random jerking movements. Other Patterns of Inheritance Multiple Alleles: ABO Blood Group . Codominance - No single allele is dominant, and each allele has its own effect. Blood alleles A and B Recessive . Blood O allele Human Blood Types Phenotype Genotype O i recessive A IA IA or IAi B IB IB or IB i AB IAIB Inherited traits and disorders can be… . Autosomal – non-sex chromosomes(1-22) ex sickle cell, CF, PKU . Sex-Linked – sex chromosomes (23rd) . A sex-linked trait is expressed by a gene on the sex chromosomes . We study traits on the X chromosome. Human Chromosomes . 22 of the pairs are called autosomes and are numbered from largest to smallest. The 23rd pair are the sex chromosomes: . XX females . XY males Chromosomes Sex-Linked Gene Disorders 1- Color Blindness Recessive disorder. Occurs in 1 of 10 males, green –red most frequent 2- Hemophilia A and B Recessive disorders that affect 1 of 5,000 males. Interferes with normal blood clotting and occur at positions on a chromosome on the X chromosome. Sex Linked Genetics problems -must include the chromosomes when doing problems. X Y The allele is superscripted on X H h X Y or X Y H=normal h=hemo Pedigrees Pedigrees - used to study inheritance of trait over generations Ex. hemophilia - inherited condition where blood is slow to clot or does not clot at all . Royal hemophilia - sex-linked You must learn how to analyze a pedigree! Make sure you understand the symbols and shading as we study the pedigrees on your problem sheet.