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Understanding the Basic Principles of Heredity and Genetics

Explore Mendel's principles of inheritance, genetic ratios, rules of probability, linkage, sex determination, and various genetic interactions such as incomplete dominance and epistasis.

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Understanding the Basic Principles of Heredity and Genetics

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  1. The Basic Principles of Heredity

  2. Phenotype Genotype Locus Allele Dominant allele Recessive allele Homozygous Heterozygous Terminology

  3. Mendel’s principles of inheritance • Segregation • During meiosis, alleles for each locus segregate • Independent assortment • Alleles of different loci distributed randomly into the gamete • Results in recombination • Production of new gene combinations not found in parent

  4. Seven characters in Mendel’s study of pea plants

  5. Oneof Mendel’spea crosses

  6. Genelociand theiralleles

  7. Monohybrid cross • Cross between homozygous parents • Differ at one locus • Dihybrid cross • Differ at two loci • Test cross • Between individual of unknown genotype and homozygous recessive individual

  8. Monohybridcross in guinea pigs

  9. Dihybridcrossinguinea pigs

  10. Test cross inguinea pigs

  11. Genetic ratios can be expressed as probabilities • Product rule predicts combined probability of independent events • Sum rule predicts combined probability of mutually exclusive events

  12. Rulesof probability

  13. Segregation • Results from homologous chromosomes separating during meiosis • Independent assortment • Orientation of homologous chromosomes on the metaphase plate determines how chromosomes are distributed

  14. Chromosomalbasis forsegregation

  15. Linkage • Tendency for a group of genes on same chromosome to be inherited together • Recombination of linked genes • Results from crossing-over in meiotic prophase I • By measuring frequency of recombination, can construct linkage map of chromosome

  16. Two-pointtest cross todetect linkagein fruit flies

  17. Crossing-over

  18. Sex chromosomes • Cells of females of many species contain two X chromosomes • Males have single X chromosome and single, smaller Y chromosome • Y chromosome determines male sex in most species of mammals

  19. Sex determinationin mammals

  20. X-linked red-green colorblindness

  21. Dosage compensation in female mammals

  22. Incomplete dominance • Heterozygote is intermediate in phenotype • Codominance • Heterozygote simultaneously expresses the phenotypes of both homozygotes

  23. Incompletedominancein four o’clocks

  24. Multiple alleles in rabbits

  25. Geneinteraction in chickens

  26. Epistasis in Labrador retrievers

  27. Norm of reaction • Range of phenotypic possibilities from a single genotype under different environmental conditions • Example is height in human • Can be modified by factors such as diet • Genes that affect height set norm of reaction • Environment molds phenotype within norm of reaction

  28. Polygenic inheritance in human skin pigmentation

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