Transcript Document
Genetics is everywhere these days – and it will continue as a dominant force in biology and society for decades to come. Wouldn’t it be nice if people understood it better? GREGOR MENDEL Biography monk in Austria in mid-1800s studied statistics Teacher Gardener MENDEL’S GARDEN PEAS heredity: transmission of traits from parents to offspring traits: category in which one or more variations an be found (ex: flower color) Allele: value of a trait (purple flowers) Molecular Genetics: study of chromosomes and genes MENDEL’S TRAITS TRAIT VARIANTS TRAIT VARIANTS MENDEL’S METHODS mated known traits together methodically cross-pollination: involves flowers of two separate plants self-pollination: involves flowers of only one plant peas usually self pollinate, but Mendel was able to force cross-pollination Mendel’s Model Organism – the Garden Pea A Method to the Madness – How Mendel Crossed Strains of Pea MENDEL’S EXPERIMENTS Began with “pure” plants Cross-pollinated P generation to create F1 generation Self-pollinated F1 generations to create F2 generation Mendel was able to record the results of every cross MENDEL’S RESULTS AND CONCLUSIONS Recessive and Dominant Traits All members of F1 generations had similar traits, but F2 generation showed variation The Reality of “Round and Wrinkled” – Two Alternative Traits of the Seed Shape Character It’s worth noting that each of these seeds is a new individual of a different generation from the plant that bears the peas. MENDEL’S RESULTS AND CONCLUSIONS Law of Dominance when present, a dominant allele will be expressed DOMINANT AND RECESSIVE TRAITS Phenotypes when dealing with different genotypes MENDEL’S RESULTS AND CONCLUSIONS Law of segregation a pair of factors is segregated, or separated, during the formation of gametes homologous pairs split during anaphase I Monohybrid Crosses and the Principle of Segregation A cross between individuals differing in single character is a monohybrid cross. The analysis of monohybrid crosses allowed Mendel to deduce the Principle of Segregation .... Genes come in pairs that separate in the formation of sex cells (and these sex cells unite randomly at fertilization). MENDEL’S RESULTS AND CONCLUSIONS Law of independent assortment factors for different characteristics are distributed to gametes independently the orientation of one homologous pair does not affect the orientation of other homologous pairs Independent Assortment in Meiosis Linkage Each individual has exactly two alleles for each trait. They may be the same or different. Each gamete has exactly one allele for each trait GENOTYPE AND PHENOTYPE Terms you need to know: Genotype Phenotype Homozygous Heterozygous PROBABILITY: LIKELIHOOD THAT A SPECIFIC EVENT WILL OCCUR numberof positiveoutcomes Probability numberof possibleoutcomes B. PROBABILITY OF A RECESSIVE TRAIT IN F2 GENERATION: 1/4 Mendel’s Monohybrid Cross – P to F1 A Punnett square, something we’ll cover in a moment. Homozygous x homozygous PP P P Pp Pp Pp Pp p pp p Staying the Course – Mendel Continued Crosses to the F2 (the grandchildren) What was learned? The green trait was not lost or altered, even though it disappeared in the F1. One trait is dominant to the other in its expression. The reappearance of the recessive trait in ¼ of the F2, suggests genes come in pairs that separate in the formation of sex cells. Pp x Pp Pp P Heterozygous x heterozygous p PP Pp Pp pp P Pp p PP x Pp or pp x Pp Pp P p Pp pp Pp pp p Homozygous x heterozygous Test crosses pp p Test Cross results for unknown hybrid Pp P p Pp pp Pp pp p pp p Test Cross results for unknown pure bred PP P P Pp Pp Pp Pp p pp p PREDICTING RESULTS OF DIHYBRID CROSSES Dihybrid: cross between individuals that involves comparing two traits Homozygous x homozygous to produce dihybrids RRYY RY rryy Predict the gametes ry ry ry ry Predict RY the gametes RY RY RrYy RrYy RrYy RrYy RrYy RrYy RrYy RrYy Predict the F1 generation RrYy RrYy RrYy RrYy RrYy RrYy RrYy RrYy MAKING THE GAMETES RRYY RY Each gamete has exactly one allele for each trait All possibilities are equally likely RrYy RY Ry rY FOIL (R+r)(Y+y) ry independent assortment in action Heterozygous x heterozygous to produce dihybrids RrYy RrYy Predict the gametes RY Ry rY ry RY Predict Ry the gametes rY ry RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy Predict Predict thethe F1 offspring generation RrYY RrYy rrYY rrYy RrYy rryy Rryy rrYy With Complete Dominance, Different Genotypes Can Produce the Same Phenotype Mendel Truly Stayed the Course, Creating an F3 (great-grandchildren) to Test the Hypothesis of Segregation Are Different Characters Like Color and Shape Inherited Together or Inherited Independently? Mendel performed dihybrid crosses to find out. Mendel’s conclusion: Different characters are inherited independently. What Works for Peas Also Works for Humans In the cross Aa x Aa, where A is a dominant allele for wild type (standard) pigmentation and a is a recessive allele for no pigmentation (albinism), ¾ of offspring will be wild type and ¼ will be albino. An albino woman RULES FOR WRITING GENOTYPES 1. ALWAYS KEEP ALLELES FOR EACH TRAIT TOGETHER 2. ALWAYS PLACE DOMINANT ALLELES BEFORE RECESSIVE ALLELES Some Alleles Are Related Through Incomplete Dominance Incomplete dominance is a partial expression of both alleles Dominance relationships may differ, but the Principle of Segregation remains the same. Pleiotropy – When One Allele Influences Many Traits EXAMPLES OF PLEIOTROPY Marfan Syndrome PKU can cause weakness of the aorta and extremely long thin bones can cause mental retardation and reduced hair and skin pigmentation Polygenic Inheritance – When a Single Trait is Influenced by Many Genes Height is a polygenic trait This slide took too long to make to remove it, even it doesn't make sense Skin Color is polygenic SAsaSBsbSCsc SAsaSBsbSCsc 20 –– 64 Fraction of population 15 –– 64 6 –– 64 1 –– 64 Skin color SUMMARY Pleiotropy Polygenism Trait Gene Trait Trait Gene Trait Gene Trait Trait Trait Gene Gene Gene Gene Multiple Alleles Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles. The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene. Codominance The human ABO blood group illustrates another genetic phenomenon – codominance. Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote. The AB phenotype (genotype IA IB) is an example of codominance Chromosomes and Inheritance Since genes are carried on chromosomes, knowledge of chromosome number and structure has farreaching implications for basic genetics and human health. A normal human male karyotype. X-linked Inheritance X-linked Inheritance – When Men and Woman Play by Different Rules Behind the 8-ball? Colorblindness is an X-linked recessive trait. X-linked Inheritance – When Men and Woman Play by Different Rules What a color blind individual sees. What a normal person sees. Behind the 8-ball? Colorblindness is an X-linked recessive trait. Many Genetic Diseases are Autosomal Recessive Traits What’s an autosome? Sickle cell anemia is a recessive autosomal disease common in areas where malaria is endemic. What Works in Peas Works (genetically speaking) Works in People ¼ of offspring of two carriers of a recessive allele are expected to show the recessive trait; ½ of offspring are expected to be carriers. Many Human Traits are Autosomal Dominant Traits For disease traits, autosomal dominant inheritance is less common than autosomal recessive inheritance. The Fundamental Question What is the relationship between genes (genotype) and observable characteristics (phenotype)? or Nature versus nurture? The answer? Genotype + Environment Phenotype Genes and Environment Determine Characters Genetically identical hydrangeas growing in soils of different acidity (different environments). The phenotype = genotype + environment principle applies equally to human traits. CONCEPT CHECK The figure diagrams one of the genetic crosses that helped Mendel form his theories of inheritance. Which process distributes “P” alleles to approximately one half of the F1 gametes and the “p” allele to the other half? Independent assortment. Mitosis. Meiosis. Chromosome crossover. ANSWER The figure diagrams one of the genetic crosses that helped Mendel form his theories of inheritance. Which process distributes “P” alleles to approximately one half of the F1 gametes and the “p” allele to the other half? 3) Meiosis. CONCEPT CHECK Mendel’s principle of independent assortment holds true only for traits with genes that are on homologous chromosome (linked). are on separate chromosomes (unlinked). have loci far apart on homologous chromosomes. are both “1” and “2”. ANSWER Mendel’s principle of independent assortment holds true only for traits with genes that 4) are both “1” and “2”. CONCEPT CHECK The traits studied by Mendel were mostly discrete traits with two phenotypes. Many traits such as human height vary continuously. Which of the following describe the inheritance of continuously varying traits using Mendel’s rules of inheritance? 1) Two or more genes interact to produce the variation in the trait apparent in the population of the organism. 2) Two or more alleles interact to produce the variation in the trait apparent in the population of the organism. 3) Mendel’s principles do not apply to continuously varying traits. 4) Traits with continuous variability are not influenced by heredity. ANSWER The traits studied by Mendel were mostly discrete traits with two phenotypes. Many traits such as human height vary continuously. Which of the following describe the inheritance of continuously varying traits using Mendel’s rules of inheritance? 1) Two or more genes interact to produce the variation in the trait apparent in the population of the organism.