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Transcript
Patterns of Inheritance Ancestry and Obesity In The News Historical Views of Inheritance • Hippocrates (~ 400 B.C.) – Particles given off from bodies of father and mother – Offspring was mixture • Homunculus (pre-1900) – Each sperm contained a tiny preformed human Gregor Mendel • Austrian Monk • Worked with garden peas • Discovered process of heredity Suitability for Using Peas • Each flower has male & female parts • Sex organs are enclosed & protected • Pollination can be controlled True-breeding • Generations of self-fertilization produce consistent offspring • Mendel used true-breeding varieties Mendel’s Experiments • Artificially crossed true-breeding plants • Recorded offspring from cross-fertilization Characters Used by Mendel • 7 true-breeding traits • Monohybrid crosses Mendel’s Crosses • Example: – Tall plant crossed with short plant – Hybrid offspring (F1) were not intermediate – Resembled only one parent F1 of Hybrid Cross • Dominant – form expressed in F1 • Recessive – form not expressed in F1 Mendel’s Next Step • Allowed F1 to self fertilize • F2 expressed dominant & recessive forms • Ratio in F2 = 3:1 (dominant:recessive) Mendel’s Monohybrid Crosses Conclusions of Mendel’s Work • Traits are inherited as genes • Alleles are alternate forms of genes • Gametes receive only 1 allele of each pair • Alleles may differ or may be identical Mendel’s Law of Segregation Each gamete receives only one of an organisms pair of alleles, and which one it receives is determined by chance Alleles of an Individual • Homozygous – identical alleles for a trait • Heterozygous – different alleles for a trait Genotype • An organisms allelic makeup • Dominant designated by capital letter • Recessive designated by small letter Phenotype • Expression of alleles • Dominant gene is expressed if present Punnet Square • Diagram to predict possible combinations Test Cross Determining Genotype Dihybrid Crosses Are Traits Linked? Law of Independent Assortment The transmission of alleles for one trait into gametes does not affect the transmission of alleles for other traits Location of Mendel’s Factors • Mendel did not know where traits were located • Chromosomes were discovered 22 years after Mendel’s work was published • Walter Sutton suggested that Mendel’s factors were on chromosomes Thomas Hunt Morgan • Studied fruit flies, Drosophila melanogaster • Discovered different traits in flies • Experiments to test Mendelian inheritance Morgan’s Cross • • • • White-eyed male Red-eyed female (normal eye color) Eye color followed Mendel’s 3:1 ratio However, all white-eyed F2 were males Conclusion of Morgan’s Experiment • White eyes were linked to males • Trait was found on X chromosome • Females have two X chromosomes Conclusion of Morgan’s Experiment • Traits are located on chromosomes • Some traits are sex-linked End chapter 13