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Introduction to Genetics

Introduction to Genetics. Chapter 19 Human Biology, eighth edition Cecie Starr et al . Impacts, Issues: The Color of Skin. Skin color comes from the pigment melanin Produced by melanocytes in skin cells

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Introduction to Genetics

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  1. Introduction to Genetics Chapter 19 Human Biology, eighth edition Cecie Starr et al

  2. Impacts, Issues: The Color of Skin • Skin color comes from the pigment melanin • Produced by melanocytes in skin cells • More than 100 genes directly or indirectly influence amount of melanin in an individual’s skin • Lead to many variations in skin color

  3. Video: ABC News: All in the family: Mixed race twins

  4. 19.1 Basic Concepts of Heredity • Genes provide the instructions for all human traits, including physical features and how body parts function • Each person inherits a particular mix of maternal and paternal genes

  5. Basic Concepts of Heredity (1) • Genes • Humans have ~21,500 • Chemical instructions for building proteins • Locus: specific location on a chromosome • Diploid cells contain two copies of each gene on pairs of homologous chromosomes • Allele: each version of a gene

  6. A Few Basic Genetic Terms

  7. Many Genetic Traits Have Dominant and Recessive Forms

  8. Basic Concepts of Heredity (2) • Homozygous condition: identical alleles • Heterozygous condition: different alleles • Dominant allele • Effect masks recessive allele paired with it

  9. Basic Concepts of Heredity (3) • Genetic representations • Homozygous dominant (AA) • Homozygous recessive (aa) • Heterozygous (Aa) • Genotype • Inherited alleles • Phenotype • Observable functional or physical traits

  10. Genotype and Phenotype Compared

  11. 19.2 One Chromosome, One Copy of a Gene • We inherit pairs of a genes (alleles) on pairs of chromosomes, but a gamete receives only one gene from each pair

  12. One Chromosome, One Copy of a Gene • Monohybrid cross • Learn more about genotypes • Segregation • Pairs of alleles separated during gamete formation

  13. The Trait Called a Chin Fissure Arises from One Allele of a Gene

  14. Animation: Chromosome segregation

  15. Each Pair of Gene Alleles Is Separated and Two Alleles End Up in Different Gametes

  16. 19.3 Genetic Tools: Testcrosses and Probability • When potential parents are concerned about passing a harmful trait to a child, genetic counselors must try to predict the likely outcome of the mating

  17. Probability • Measure of the chance that some particular outcome will occur • Factor in the inheritance of single-gene traits • Cross CC x cc • All of the offspring will be heterozygous, Cc • Cross Cc x Cc • ¼ CC, ½ Cc, and ¼ cc

  18. A Punnett Square Can Be Used to Predict the Result of a Genetic Cross • Punnett square • Grid used to determine possible outcomes of genetic crosses • Rules of probability apply because fertilization is a chance event • Possibility can be expressed mathematically, e.g., between 0% and 100% • Most probable outcome does not have to occur • In a given situation, probability does not change

  19. Making a Punnett Square Is One Way to Determine Likely Outcome of Genetic Cross

  20. Different Genetic Results Possible in SecondGeneration after Monohybrid Mating

  21. Use Multiplication to Figure the Probability of the Inheritance of Alleles

  22. A Testcross Also Can Reveal Genotypes • Testcross • Learn the genotype of a (nonhuman) organism • Cross organism with homozygous recessive organism (aa) • If all offspring are Aa, parent was probably AA • If some of the offspring have the dominant trait and some have the recessive trait, parent was Aa

  23. 19.4 How Genes for Different Traits Are Sorted into Gametes • When we consider more than one trait, we see that the gene for each trait is inherited independently of the gene of other traits

  24. How Genes for Different Traits Are Sorted into Gametes • Independent assortment • Occurs during meiosis • A given chromosome and its genes move randomly into gametes • Metaphase I • Metaphase II • Crosses between individuals heterozygous for two traits yields sixteen different gamete unions • Probability displayed using a Punnett square

  25. Independent Assortment: Chromosomes Moved at Random into Forming Gametes

  26. One of two possible alignments The only other possible alignment a Initial chromo- some alignments (at metaphase I): b The resulting alignments at metaphase II: c Possible combinations of alleles in gametes: AB ab Ab aB Stepped Art Fig. 19-8, p. 378

  27. Tracking Two Traits Shows the Results of Independent Assortment

  28. CcDd CcDd meiosis, gamete formation meiosis, gamete formation 1/4 CD 1/4 Cd 1/4 cD 1/4 cd 1/4 CD 1/16 CCDd 1/16 CcDD 1/16 CcDd 1/16 CCDD 1/4 Cd 1/16 CCDd 1/16 CCdd 1/16 CcDd 1/16 Ccdd 1/4 cD 1/16 CcDD 1/16 CcDd 1/16 ccDD 1/16 ccDd 1/16 CcDd 1/4 cd 1/16 Ccdd 1/16 ccDd 1/16 ccdd Adding up the combinations possible: 9/16 or 9 chin fissure, dimples 3/16 or 3 chin fissure, no dimples 3/16 or 3 smooth chin, dimples 1/16 or 1 smooth chin, no dimples Fig. 19-9, p. 379

  29. Probability Rules Apply to Independent Assortment

  30. 19.5 Single Genes, Varying Effects • Some traits have clearly dominant and recessive forms • For most traits, however, the story is not so simple

  31. One Gene May Affect Several Traits • Pleiotropy • Wide-ranging effect of one gene • Sickle-cell anemia • One amino acid substitution in hemoglobin • Val instead of glu • Pleiotropic effects • Treatments

  32. Single Genetic Change Leads to Many Physical Effects of Sickle-Cell Anemia

  33. Fig. 19-11a, p. 380

  34. Fig. 19-11b, p. 380

  35. In Codominance, More Than One Allele of a Gene Is Expressed • Codominance • Heterozygous for a trait, but both alleles are expressed • Example: alleles for blood type determine presence or absence of polysaccharides on surface of red blood cells • IA and IB; codominant when paired with each other • Multiple allele system • A gene that has three or more alleles

  36. There Are Several Possible Allele Combinations for ABO Blood Types

  37. 19.6 Other Gene Effects and Interactions • Many phenotypes, such as eye color, can’t be predicted with certainty • Biologists have uncovered several underlying causes for these variations

  38. Other Gene Effects and Interactions • Penetrance • Probability that someone who inherits an allele will have the phenotype associated with it • Cystic fibrosis • Homozygous recessive • 100% penetrant • Polydactyly • Dominant allele for extra digits • Incompletely penetrant

  39. People with Polydactyly Have Extra Digits on Their Hands or Feet

  40. Polygenic Traits Come from Several Genes Combined (1) • Polygenic traits • Combined expression of several genes • Skin and eye color; many variations due to the amount and distribution of melanin • Continuous variation • Populations show a range of continuous differences • Most evident in traits that can be measured, e.g., height

  41. Polygenic Traits Come from Several Genes Combined (2) • Multifactorial trait • Phenotypes shaped by more than one gene and affected by the environment, e.g., height

  42. Eye Color Is Just One of Many Human Polygenic Traits

  43. The Environment Can Affect Phenotypes • Height • Diet low in protein • Disease or injury that prevents the normal release of growth hormone • Skin color • Tanning • Good lifestyle choices • May limit the chances that a harmful gene(s) will be expressed

  44. “Tongue-Roller” Trait Is Due to a Dominant Allele

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