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Splash. Chapter Introduction Genes and Chromosomes 13.1 Heredity and Environment 13.2 Mendel and the Idea of Alleles 13.3 Genes and Chromosomes Mendelian Patterns of Inheritance 13.4 Probability and Genetics 13.5 Inheritance of Alleles 13.6 Sex Determination Other Patterns of Inheritance

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  1. Splash

  2. Chapter Introduction Genes and Chromosomes 13.1 Heredity and Environment 13.2 Mendel and the Idea of Alleles 13.3 Genes and Chromosomes Mendelian Patterns of Inheritance 13.4 Probability and Genetics 13.5 Inheritance of Alleles 13.6 Sex Determination Other Patterns of Inheritance 13.7 Multiple Alleles and Alleles without Dominance 13.8 Linked Genes 13.9 X-Linked Traits 13.10 Nondisjunction 13.11 Multigene Traits Chapter Highlights Chapter Animations Chapter Menu Contents

  3. Learning Outcomes By the end of this chapter you will be able to: A Explain the roles of heredity and environment in organism development. BExplain the relationship among alleles, genes, and chromosomes. C Apply the principles of probability to genetics. D Explain Mendel’s principles of segregation and independent assortment. Learning Outcomes 1

  4. Learning Outcomes By the end of this chapter you will be able to: E Give examples of how sex is determined in several organisms. F Explain the role of multiple alleles in inheritance. G Explain linked genes and recognize how X-linked traits differ from other linked traits. H Summarize the effects of nondisjunction. Learning Outcomes 2

  5. This photo shows a group of starfish in a tide pool in Big Sur, California. Patterns of Inheritance • What important biological concept is illustrated in this photo? • What differences among these starfish do you see? Chapter Introduction 1

  6. This photo shows a group of starfish in a tide pool in Big Sur, California. Patterns of Inheritance • Genetics is the branch of biology that seeks to explain inherited variation. • Some of the differences among individuals are passed from parent to offspring. • Such inherited features are the building blocks of evolution. Chapter Introduction 2

  7. End of the Introduction

  8. Genes and Chromosomes 13.1 Heredity and Environment • Organisms are products of their heredity and of their surroundings. Siamese cats have a gene for dark fur, but the enzyme produced by the gene functions best at low temperatures. The cat’s extremities—its ears, paws, and tail—are cooler than the rest of its body. 13.1 Heredity and Environment 1

  9. Genes and Chromosomes 13.1 Heredity and Environment (cont.) • Studies of twins can help separate the effects of inheritance and the environment. • Identical twins have the same genetic information while fraternal twins are no more genetically similar than other siblings. • If identical twins exhibit the same trait more often than fraternal twins, the trait is probably influenced by genetic factors. • If the trait differs in identical twins, the environment must have a strong influence on it. 13.1 Heredity and Environment 2

  10. Genes and Chromosomes 13.1 Heredity and Environment (cont.) • According to the once popular theory of “blending”, an individual’s genetic makeup was formed when the parents’ genes mixed at fertilization. The result was a sort of averaging of the parents’ genes. Genetics no longer includes the theory of blending inheritance, but it was a logical explanation for many visible differences. 13.1 Heredity and Environment 3

  11. Gregor Mendel (1822–1884) By experimenting with peas in his monastery garden, Mendel developed the fundamental principles of heredity that became the foundation of modern genetics. Genes and Chromosomes 13.2 Mendel and the Idea of Alleles • In the 1860s, Gregor Mendel used garden peas to study heredity. Peas are very easy to grow, are self-fertilizing, and Mendel could study many generations during his eight-years of experiments. 13.2 Mendel and the Idea of Alleles 1

  12. Self-fertilization in a pea flower 13.2 Mendel and the Idea of Alleles 2

  13. Genes and Chromosomes 13.2 Mendel and the Idea of Alleles (cont.) • Mendel concentrated on traits that did not fit the blending theory. Mendel found seven different characteristics of pea plants that he could study in an either-or form. 13.2 Mendel and the Idea of Alleles 3

  14. Genes and Chromosomes 13.2 Mendel and the Idea of Alleles (cont.) • Mendel classified and isolated true-breeding plants which produce offspring identical to themselves generation after generation. Mendel then crossbred his plants, classified all the offspring, and looked for patterns of inheritance. 13.2 Mendel and the Idea of Alleles 4

  15. Genes and Chromosomes 13.2 Mendel and the Idea of Alleles (cont.) • When Mendel mated a plant with round seeds with a plant with wrinkled seeds, the next generation of plants produced only round seeds. When those plants were allowed to self-fertilize, however, some plants in the next generation produced round seeds and other plants produced wrinkled seeds. 13.2 Mendel and the Idea of Alleles 5

  16. Genes and Chromosomes 13.2 Mendel and the Idea of Alleles (cont.) • Mendel demonstrated with pea plants that both parents pass on to their offspring genetic factors that remain separate generation after generation. Today the concept of genes has replaced Mendel’s vague idea of factors. A gene is now defined as a segment of DNA whose sequence of nucleotides codes for a specific functional product. 13.2 Mendel and the Idea of Alleles 6

  17. Genes and Chromosomes 13.2 Mendel and the Idea of Alleles (cont.) • Most genes exist in more than one form, or allele. Each allele of a particular gene has a different base sequence. All organisms have genes that exist as several different alleles. Most traits—such as hair color, skin color, nose shape, and handedness—result from the complex interaction of several genes with each other and the environment. 13.2 Mendel and the Idea of Alleles 7

  18. Genes and Chromosomes 13.3 Genes and Chromosomes • The arrangement of genes in chromosomes differs in eukaryotes and prokaryotes. Eukaryotic chromosomes are long molecules of DNA wrapped around proteins. Only part of this DNA codes for proteins; noncoding DNA is not translated. 13.3 Genes and Chromosomes 1

  19. A scanning electron micrograph (x79,000) of a bacterial plasmid. These small circular DNA molecules are common in some types of bacteria. Their ability to transfer genes between cells makes plasmids useful in genetic research. Genes and Chromosomes 13.3 Genes and Chromosomes (cont.) • Prokaryotes have a single circular chromosome with little associated protein. An estimated 90% of prokaryotic DNA is translated. Many bacteria also have plasmids—small circles of DNA that contain additional genes. 13.3 Genes and Chromosomes 2

  20. Genes and Chromosomes 13.3 Genes and Chromosomes (cont.) • Homologous chromosomes carry the same genes, although their genes may be present as different alleles. Stains help identify homologous chromosomes by binding to specific regions of chromosomes to create unique banding patterns. Chromosome painting can also be used to identify all of an organism’s chromosomes. 13.3 Genes and Chromosomes 3

  21. Genes and Chromosomes 13.3 Genes and Chromosomes (cont.) Human chromosomes 7 and X are similar in size and shape. The banding patterns evident in the photographs show more clearly in the drawings. These two chromosomes, which have almost the same length and centromere location, can be distinguished by their banding patterns. In chromosome painting, fluorescent dyes of different colors are chemically bonded to short pieces of DNA (probes) that bind to genes on different chromosomes. This process stains each homologous pair of chromosomes a different color. 13.3 Genes and Chromosomes 4

  22. Genes and Chromosomes 13.3 Genes and Chromosomes (cont.) • Chromosomes are easiest to study in the condensed form during cell division. • Chemicals are added to stop cell division during metaphase. • The cells are treated with water on a microscope slide causing them to swell and their chromosomes to spread apart. • Stains applied to the resulting chromosome spread produce the banding patterns. 13.3 Genes and Chromosomes 5

  23. Genes and Chromosomes 13.3 Genes and Chromosomes (cont.) In the human chromosome spread, the banding patterns result from differences in stain absorption by different regions of the chromosomes. Banded chromosomes enable biologists to detect missing or extra chromosome parts more easily than uniformly stained chromosomes do. The banding patterns also have made the mapping of genes on chromosomes more accurate. 13.3 Genes and Chromosomes 6

  24. Genes and Chromosomes 13.3 Genes and Chromosomes (cont.) • Stained chromosomes can be photographed under the microscope. Individual chromosomes can be cut out of an enlarged photograph and arranged by size and shape to form a display called a karyotype. Karyotypes of fetal cells can be used to check for suspected chromosomal abnormalities in developing fetuses. 13.3 Genes and Chromosomes 7

  25. In this karyotype of a human cell, the chromosomes are arranged as homologous pairs. The larger groups (for example, pairs 1–3) include chromosome pairs with similar size and centromere position. A karyotype is prepared by cutting out individual chromosomes from a photograph and matching them, pair by pair. The pair of X chromosomes determines this individual’s sex. 13.3 Genes and Chromosomes 8

  26. End of Section 1

  27. Mendelian Patterns of Inheritance 13.4 Probability and Genetics • Diploid organisms usually carry different alleles of many genes. Geneticists use probability, a branch of mathematics that predicts the chances that a certain event will occur, to predict the results of matings. Probability is usually expressed as a fraction and works when each event is independent. The chance that two or more independent events will occur together is the multiplication product of their chances of occurring separately. 13.4 Probability and Genetics 1

  28. Mendelian Patterns of Inheritance 13.4 Probability and Genetics (cont.) • Geneticists use probability to predict the alleles of the offspring of various crosses, or matings. Mathematical tests can help show whether a difference between observed and predicted results is significant. Genetic ratios are estimates of probability, not absolute numbers. The larger the sample size, the less deviation is expected from predicted ratios. 13.4 Probability and Genetics 2

  29. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles • The principles of probability enable you to explain the results of Mendel’s experiment. Mendel performed a monohybrid cross, or crossed plants that differed in only one trait: seed shape. The plants involved in this first cross are called the parental (P) generation. 13.5 Inheritance of Alleles 1

  30. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles (cont.) • The generation of plants that grew from the seeds of the parental crosses is called the first filial, or F1, generation. The F1 generation plants self-fertilized producing the second filial, or F2, generation. 13.5 Inheritance of Alleles 2

  31. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles (cont.) • Only one trait of each pair was visible in the F1 generation. • The visible trait is the dominant trait. • The alternative trait, which is not visible in the F1 generation, is called the recessive trait. In the F2 generation produced by self-fertilization of F1 plants, the two traits appear in a ratio of approximately 3:1, dominant to recessive. 13.5 Inheritance of Alleles 3

  32. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles (cont.) • Mendel used mathematics to conclude that each true-breeding plant has two identical copies of the factor for a particular trait. • When gametes formed during meiosis, only one copy of the factor went into each pollen or egg cell. • At fertilization, the F1 generation received a factor from each parent. • Only one of these factors went into each gamete formed by the F1 plants. 13.5 Inheritance of Alleles 4

  33. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles (cont.) • Mendel called this separation of factors the principle of segregation. The alleles (the modern term for factors) of each gene separate during meiosis when homologous chromosomes are divided among the gametes. 13.5 Inheritance of Alleles 5

  34. This example of a monohybrid cross demonstrates Mendel’s principle of segregation. Each plant has two factors (alleles) for each trait, which segregate during the formation of gametes. Fertilization restores paired alleles. The grid, or Punnett square, shows how gametes of the F1 generation combine to form the genotypes of the F2 generation. 13.5 Inheritance of Alleles 6

  35. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles (cont.) • The allele for a dominant trait is commonly represented by a capital letter, and the allele for the recessive trait is represented by the lowercase letter. Most multicellular organisms are diploid, so their genotype, or genetic makeup, has two alleles for each gene. • In Mendel’s experiment, round-seeded plant in the P generation would have been RR. • The wrinkled-seeded parental plant had the genotype rr. 13.5 Inheritance of Alleles 7

  36. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles (cont.) • If both alleles are the same (RR or rr), the genotype is homozygous. If the alleles are different (Rr), the genotype is heterozygous. The genotype of each individual is responsible for its phenotype—its appearance or observable characteristics. Because the round-seeded phenotype is dominant, both the homozygous RR and the heterozygous Rr genotypes produce plants with round seeds. 13.5 Inheritance of Alleles 8

  37. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles (cont.) • A Punnett square is useful for calculating probable ratios of genotypes and phenotypes. All the possible genotypes of the gametes one parent can produce are at the top of the square, and the genotypes of the other parent’s gametes are at the side. The square is then filled in like a multiplication table with the genotypes that would result from the union of those gametes. 13.5 Inheritance of Alleles 9

  38. Mendelian Patterns of Inheritance 13.5 Inheritance of Alleles (cont.) • To follow the inheritance of two characteristics at once, Mendel made dihybrid crosses—crosses between individuals that differ in two traits. The ratio 9:3:3:1 is characteristic of the F2 generation in a dihybrid cross. The results of dihybrid crosses formed the basis for Mendel’s principle of independent assortment. Alleles for one characteristic assort, or divide up among the gametes during meiosis, independently of alleles for other characteristics. 13.5 Inheritance of Alleles 10

  39. A dihybrid cross illustrates the principle of independent assortment. Note that there are 12 round to every 4 wrinkled seeds and 12 yellow to every 4 green seeds. Each trait independently shows the 3:1 ratio typical of a monohybrid cross. R = round seed; r = wrinkled seed; Y = yellow embryo; y = green embryo. 13.5 Inheritance of Alleles 11

  40. Mendelian Patterns of Inheritance 13.6 Sex Determination • Chromosomes come in matching pairs except for the sex chromosomes, which may be different. This sex chromosomes determine the sex of the individual. In humans, other mammals, and the fruit fly Drosophila melanogaster, the sex chromosomes are labeled X and Y. Females have two X chromosomes; males have one X chromosome and one Y chromosome. 13.6 Sex Determination 1

  41. Sex determination 13.6 Sex Determination 2

  42. Mendelian Patterns of Inheritance 13.6 Sex Determination (cont.) • Some insects, such as grasshoppers, crickets, and roaches, females have two X chromosomes, males have one; there is no Y chromosome. Birds, some fish, and some insects have what is called a Z-W system where the male has two matching sex chromosomes (ZZ), and the female has one Z chromosome and one W chromosome. 13.6 Sex Determination 3

  43. Mendelian Patterns of Inheritance 13.6 Sex Determination (cont.) • Some plants, such as spinach and date palms, have separate female and male plants and have sex chromosomes that follow the X-Y system of sex determination. Most plants and some animals have no sex chromosomes. 13.6 Sex Determination 4

  44. End of Section 2

  45. Other Patterns of Inheritance 13.7 Multiple Alleles and Alleles without Dominance • Some genes do not follow an either dominant or recessive pattern. A type of inheritance is known as incomplete dominance occurs when the phenotype is intermediate between those of the parents. 13.7 Multiple Alleles and Alleles without Dominance 1

  46. Incomplete dominance of flower color in snapdragons 13.7 Multiple Alleles and Alleles without Dominance 2

  47. Other Patterns of Inheritance 13.7 Multiple Alleles and Alleles without Dominance (cont.) • Human blood type depends on the presence or absence of type A or type B carbohydrates on the surface of red blood cells. • The alleles IA or IB code for different forms of an enzyme that add different sugars to the carbohydrate bound to the plasma membrane. • When a person has the IAIB genotype, both types of carbohydrates are produced and the individual’s blood type is AB. When alleles are codominant, both phenotypes appear in heterozygous individuals. 13.7 Multiple Alleles and Alleles without Dominance 3

  48. Other Patterns of Inheritance 13.7 Multiple Alleles and Alleles without Dominance (cont.) • Blood types also involve multiple alleles. • In addition to the IA and IB alleles, the allele i codes for no active enzyme. • An ii genotype produces type O blood. • The symbols IA , IB, and i are used to show that the A and B traits are codominant and the O trait is recessive. 13.7 Multiple Alleles and Alleles without Dominance 4

  49. Other Patterns of Inheritance 13.7 Multiple Alleles and Alleles without Dominance (cont.) • If someone receives a transfusion of a different type of blood, antibodies, which are defensive proteins found in the blood, cause the donated red cells to clump together, clogging the blood vessels. If a person with type B blood receives a transfusion of type A blood, the anti-A antibodies that are produced attack the A surface carbohydrates and vice versa. 13.7 Multiple Alleles and Alleles without Dominance 5

  50. Other Patterns of Inheritance 13.7 Multiple Alleles and Alleles without Dominance (cont.) 13.7 Multiple Alleles and Alleles without Dominance 6

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