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Scheman Gregor Mendel • Austrian Monk, mid- 19th century. • Worked with Pea plants. • 1st to succeed in predicting the passage of traits from parent to offspring. • Utilized Scientific Method with controlled experimentation. Both Stamens and Carpels Control cross-pollination • What genetic principles account for the passing of traits from parents to offspring? • The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green) • The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) • Mendel documented a particulate mechanism through his experiments with garden peas EXPERIMENT P Generation (true-breeding parents) F1 Generation (hybrids) F2 Generation Purple flowers White flowers All plants had purple flowers Particulate Hypothesis Proven (no blending) 705 purple-flowered 224 white-flowered plants plants Mendel’s Key Points • Hybrid = The offspring of parents that have different forms for the same trait. • Two Factors that control gene activation for any particular trait. (Alleles) • Dominant vs. Recessive • Law of Segregation • Law of Independent Assortment •What Mendel called a “heritable factor” is what we now call a gene! • Mitosis and meiosis were first described in the late 1800s • The chromosome theory of inheritance states: – Mendelian genes have specific loci (positions) on chromosomes – Chromosomes undergo segregation and independent assortment • The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment All F1 plants produce yellow-round seeds (YyRr) 0.5 mm F1 Generation R R y r Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. y r Y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis r R Y y r R Metaphase I Y y 1 1 r R r R Y y Anaphase I Y y r R Metaphase II R r 2 2 Gametes y Y Y R R 1 4 YR r 1 3 4 yr Y Y y r y Y y Y r r 14 Yr y y R R 14 yR 3 Punnett Squares • Technique for predicting offspring from one generation to the next. • Homozygous vs. Heterozygous The Testcross • How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a testcross: breeding the mystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings EXPERIMENT YYRR P Generation yyrr Gametes YR F1 Generation YyRr Hypothesis of dependent assortment Predictions yr Hypothesis of independent assortment Sperm or Predicted offspring of F2 generation 1/ 4 Sperm 1/ YR 1/ 2 2 yr 1/ 4 1/ 2 YR 1/ 4 1/ 4 Yr yR 1/ 4 yr YR YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr YR YYRR Eggs 1/ 2 YyRr 1/ 4 Yr Eggs yr YyRr 3/ 4 yyrr 1/ 4 yR 1/ 4 Phenotypic ratio 3:1 1/ 4 yr 9/ 16 3/ 16 3/ 16 1/ 16 Phenotypic ratio 9:3:3:1 RESULTS 315 108 101 32 Phenotypic ratio approximately 9:3:3:1 Concept 14.3: Inheritance patterns are often more complex than predicted by simple Mendelian genetics • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied • Many heritable characters are not determined by only one gene with two alleles • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: – When alleles are not completely dominant or recessive – When a gene has more than two alleles – When a gene produces multiple phenotypes Degrees of Dominance • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical • In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties • In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings P Generation Red CRCR White CWCW CR Gametes CW incomplete dominance Pink CRCW F1 Generation Gametes 1/2 CR 1/ CW 2 Sperm 1/ 2 CR 1/ 2 CW F2 Generation 1/ 2 CR Eggs 1/ 2 CRCR CRCW CRCW CWCW CW Multiple Alleles • Most genes exist in populations in more than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Blood Typing Allele IA IB Carbohydrate A B i none (a) The three alleles for the ABO blood groups and their associated carbohydrates Genotype Red blood cell appearance Phenotype (blood group) IAIA or IA i A IBIB or IB i B IAIB AB ii O (b) Blood group genotypes and phenotypes • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • For example, in mice and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles B for black and b for brown) • The other gene (with alleles C for color and c for no color) determines whether the pigment will be deposited in the hair • Quantitative characters are those that vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color in humans is an example of polygenic inheritance • A pedigree is a family tree that describes the interrelationships of parents and children across generations • Inheritance patterns of particular traits can be traced and described using pedigrees Punnett Square Practice Great Punnett Square Problem Web Site www.biology.arizona.edu Go to Mendelian Genetics Meiosis • The process of making gametes (sex cells such as sperm and egg). • Purpose = to reduce the chromosome number in half! • 1 Diploid(2N) cell becomes 4 Haploid(N) cells. Human Meiosis • Spermatogenesis produces 4 viable sperm every meiosis. • Oogenesis produces 1 viable egg every meiosis and 4 polar bodies. • Zygote = fertilized egg which contains the standard number of chromosomes. 46 Zygote • Most of the time, meiosis occurs flawless. • However, sometimes, chromosomes, parts of chromosomes, or specific nucleotides get messed up! • Genetic disorders are inherited from the parents and can be found in the DNA of every cell. Nondisjunction Trisomy 21 • Non disjunction results when an entire chromosome does not separate from its sister chromosome. • Thus, both chromosomes travel together. Trisomy 21 Cystic Fibrosis • Common disorder in Caucasians. • Mutation in amino acids that make up a specific transport protein. • Thick mucus builds up in respiratory and digestive systems. Sickle Cell Anemia & PKU • Minor changes in the sequence of nucleotides can lead to extreme genetic disorders. • Sickle cell anemia • PKU Sex-Linked Inheritance • Some inherited traits are located on the X and Y (sex) chromosomes. • Hemophilia • Pattern baldness • Color blindness • Duchenne muscular dystrophy Dominant Inheritance • 6-Fingers • Huntington’s Disease Morgan’s Experimental Evidence: Scientific Inquiry • The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist • Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair • In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) – The F1 generation all had red eyes – The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes • Morgan determined that the white-eyed mutant allele must be located on the X chromosome • Morgan’s finding supported the chromosome theory of inheritance CONCLUSION P Generation w+ X X w+ X Y w Eggs F1 Generation Sperm w+ w+ w+ w w+ Eggs F2 Generation w w+ Sperm w+ w+ w w w w+ 44 + XY 44 + XX Parents 22 + 22 + or Y X Sperm + 44 + XX or 22 + X Egg 44 + XY Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X 76 + ZW 76 + ZZ 32 (Diploid) 16 (Haploid) (b) The X-0 system (c) The Z-W system (d) The haplo-diploid system Early embryo: X chromosomes Two cell populations in adult cat: Allele for orange fur Allele for black fur • In mammalian females, one of the two X chromosomes Cell division and in each cell is X chromosome randomly inactivated inactivation during embryonic Active X development Inactive X • The inactive X condenses into a Black fur Orange fur Barr body Active X • If a female is heterozygous for a particular gene located on the X X Inactivation in chromosome, she will Female Mammals be a mosaic for that character Alterations of Chromosome Structure • Breakage of a chromosome can lead to four types of changes in chromosome structure: – – – – Deletion removes a chromosomal segment Duplication repeats a segment Inversion reverses a segment within a chromosome Translocation moves a segment from one chromosome to another Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings (a) (b) (c) (d) A B C D E F G H A B C D E F G H A B C D E F G H A B C D E F G H Deletion Duplication A B C E F G H A B C B C D E Inversion A D C B E R F G H M N O C D E Reciprocal translocation M N O P Q F G H A B P Q R F G H DNA Technology TRANSFORMATION Protein Synthesis • The making of Proteins! • DNA carries the “blueprints” for the making of every molecule and cellular structure. • Transcription • Translation