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CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE

CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE. Section A: Relating Mendelism to Chromosomes. 1. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles 2. Morgan traced a gene to a specific chromosome

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CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE

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  1. CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Section A: Relating Mendelism to Chromosomes • 1. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles • 2. Morgan traced a gene to a specific chromosome • 3. Linked genes tend to be inherited together because they are located on the same chromosome • 4. Independent assortment of chromosomes and crossing over produce genetic recombinants • Geneticists use recombination data to map a chromosome’s genetic loci • Mendel’s hereditary factors are the genes located on chromosomes.

  2. Around 1900, cytologists and geneticists began to see parallels between the behavior of chromosomes and the behavior of Mendel’s factors. Chromosomes and genes are both present in pairs in diploid cells. Homologous chromosomes separate and alleles segregate during meiosis. Fertilization restores the paired condition for both chromosomes and genes. 1. Mendelian inheritance has its physical basis in the behavior of chromosomes during sexual life cycles

  3. Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and a chromosome theory of inheritance began to take form.

  4. Chromosome theory of Inheritance ·Mendelian factors or genes are located on chromosomes ·It is the chromosome that segregates and independently assort

  5. Drosophila melongaster are fruit flies which are good to use in population studies because 1.easily cultured in the lab 2.have a short generation time 3.have few chromosomes 4.easily distinguishable traits

  6. Thomas Hunt Morgan was the first to associate a specific gene with a specific chromosome in the early 20th century. Drosophila melanogaster Fruit flies are prolific breeders and have a generation time of two weeks. Fruit flies have three pairs of autosomes and a pair of sex chromosomes (XX in females, XY in males). 2. Morgan traced a gene to a specific chromosome

  7. Morgan spent a year looking for variant individuals among the flies he was breeding. He discovered a single male fly with white eyes instead of the usual red. The normal character phenotype is the wild type. Alternative traits are mutant phenotypes. Morgan studied fly traits. A gene’s symbol is based on the first mutant – not normal or wild type discovered If a mutant is dominant curly =Cy, the wild-type is straight= Cy+ (Wild Type is the most frequent type)

  8. Surprisingly, the white-eyed trait appeared only in males. All the females and half the males had red eyes. Morgan concluded that a fly’s eye color was linked to its sex. Morgan deduced that the gene with the white-eyed mutation is on the X chromosome alone, a sex-linked gene. Females (XX) may have two red-eyed alleles and have red eyes or may be heterozygous and have red eyes. Males (XY) have only a single allele and will be red eyed if they have a red-eyed allele or white-eyed if they have a white-eyed allele.

  9. Each chromosome has hundreds or thousands of genes. Genes located on the same chromosome, linked genes, tend to be inherited together because the chromosome is passed along as a unit. Results of crosses with linked genes deviate from those expected according to independent assortment. 3. Linked genes tend to be inherited together because they are located on the same chromosome

  10. Morgan observed this linkage and its deviations when he followed the inheritance of characters for body color and wing size. The wild-type body color is gray (b+) and the mutant black (b). The wild-type wing size is normal (vg+) and the mutant has vestigial wings (vg). Morgan crossed F1 heterozygous females (b+bvg+vg) with homozygous recessive males (bbvgvg).

  11. According to independent assortment, this should produce 4 phenotypes in a 1:1:1:1 ratio. Surprisingly, Morgan observed a large number of wild-type (gray-normal) and double-mutant (black-vestigial) flies among the offspring. These phenotypes correspond to those of the parents. Much like our example – blonde haired, blue eyed mother x brown hair brown eyed parent having offspring being either or.

  12. Morgan reasoned that body color and wing shape are usually inherited together because their genes are on the same chromosome.

  13. The production of offspring with new combinations of traits inherited from two parents is genetic recombination. Genetic recombination can result from independent assortment of genes located on nonhomologous chromosomes or from crossing over of genes located on homologous chromosomes. 4. Independent assortment of chromosomes and crossing over produce genetic recombinants

  14. Mendel’s dihybrid cross experiments produced some offspring that had a combination of traits that did not match either parent in the P generation. If the P generation consists of a yellow-round parent (YYRR) crossed with a green-wrinkled seed parent (yyrr), all F1 plants have yellow-round seeds (YyRr). A cross between an F1 plant and a homozygous recessive plant (a test-cross) produces four phenotypes (1:1:1:1). DO THIS CROSS ON YOUR PAPER Half are be parental types, with phenotypes that match the original P parents, either with yellow-round seeds or green-wrinkled seeds. Half are recombinants, new combination of parental traits, with yellow-wrinkled or green-round seeds.

  15. A 50% frequency of recombination is observed for any two genes located on different (nonhomologous) chromosomes. Under normal Mendelian genetic rules, we would not expect linked genes to recombine into assortments of alleles not found in the parents.

  16. The results of Morgan’s testcross for body color and wing shape did not conform to either independent assortment or complete linkage. Under independent assortment the testcross should produce a 1:1:1:1 phenotypic ratio. If completely linked, we should expect to see a 1:1:0:0 ratio with only parental phenotypes among offspring. Most of the offspring had parental phenotypes, suggesting linkage between the genes. A low % of flies being recombinants suggests incomplete linkage.

  17. The occasional production of recombinant gametes during prophase I accounts for the occurrence of recombinant phenotypes in Morgan’s testcross. Recombinants x 100 . total

  18. A wild-type fruit fly (heterozygous for gray body color and normal wings) was mated with a black fly with vestigial wings. The offspring had the following phenotypic distribution: wild type, 778; black-vestigial, 785; black-normal, 158; gray-vestigial, 162. What is the recombination frequency between those genes for body color and wing type? PROBLEMS

  19. In another cross, a wild type fruit fly (heterozygous for gray body color and red eyes) was mated with a black gruit fly with purple eyes. The offspring were as follows: wild type, 721; black-purple, 751, gray-purple, 49; black-red, 45. What is the recombination frequency between these genes for body color and eye color?

  20. One of Morgan’s students, Alfred Sturtevant, used crossing over of linked genes to develop a method for constructing a genetic map. This map is an ordered list of the genetic loci along a particular chromosome. 5. Geneticists can use recombination data to map a chromosome’s genetic loci

  21. Frequency of recombinant offspring reflected the distances between genes on a chromosome. The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore a higher recombinationfrequency. relative position of genes along chromosomes give you a linkage map.

  22. Sturtevant used the testcross design to map the relative position of three fruit fly genes, body color (b), wing size (vg), and eye color (cn). The recombination frequency between cn and b is 9%. The recombination frequency between cn and vg is 9.5%. The recombination frequency between b and vg is 17%. The only possible arrangement of these three genes places the eye color gene between the other two. Fig. 15.6

  23. Sturtevant expressed the distance between genes, the recombination frequency, as map units. One map unit (sometimes called a centimorgan) is equivalent to a 1% recombination frequency. You may notice that the three recombination frequencies in our mapping example are not quite additive: 9% (b-cn) + 9.5% (cn-vg) > 17% (b-vg). This results from multiple crossing over events. A second crossing over “cancels out” the first and reduces the observed number of recombinant offspring. Genes father apart (for example, b-vg) are more likely to experience multiple crossing over events.

  24. Some genes on a chromosome are so far apart that a crossover between them is virtually certain. In this case, the frequency of recombination reaches is its maximum value of 50% and the genes act as if found on separate chromosomes and are inherited independently. In fact, several genes studies by Mendel are located on the same chromosome.

  25. Determine the sequence of genes along a chromosome based on the following recombination frequencies: A-B 8mu A-C 28 mu A-D 25 mu B-C 20mu B-D 33 mu PROBLEMS:

  26. Determine the sequence of genes along a chromosome based on the following recombination frequencies: LOCI RECOMB Freq B-vg 18.5% cn-b 9.0% cn – vg 9.5%

  27. CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Section B: Sex Chromosomes 1. The chromosomal basis of sex varies with the organism 2. Sex-linked genes have unique patterns of inheritance

  28. Although the anatomical and physiological differences between women and men are numerous, the chromosomal basis of sex is rather simple. In human and other mammals, there are two varieties of sex chromosomes, X and Y. XX = female XY = male 1. The chromosomal basis of sex varies with the organism

  29. This X-Y system of mammals is not the only chromosomal mechanism of determining sex. Other options include the X-0 system, the Z-W system, and the haplo-diploid system. Fig. 15.8

  30. In the X-Y system, Y and X chromosomes behave as homologous chromosomes during meiosis. In reality, they are only partially homologous and rarely undergo crossing over. Each egg receives an X chromosome. Half the sperm receive an X chromosome and half receive a Y chromosome. Because of this, each conception has about a fifty-fifty chance of producing a particular sex.

  31. In humans, the anatomical signs of sex first appear when the embryo is about two months old. In individuals with the SRY gene (sex-determining region of the Y chromosome), the generic embryonic gonads are modified into testes. In addition, other genes on the Y chromosome are necessary for the production of functional sperm. In individuals lacking the SRY gene, the generic embryonic gonads develop into ovaries.

  32. In addition to their role in determining sex, the sex chromosomes, especially the X chromosome, have genes for many characters unrelated to sex. These sex-linked genes follow the same pattern of inheritance as the white-eye locus in Drosophila. 2. Sex-linked genes have unique patterns of inheritance

  33. If a sex-linked trait is due to a recessive allele, a female will have this phenotype only if homozygous. Heterozygous females will be carriers. Because males have only one X chromosome (hemizygous), any male receiving the recessive allele from his mother will express the trait. The chance of a female inheriting a double dose of the mutant allele is much less than the chance of a male inheriting a single dose. Therefore, males are far more likely to inherit sex-linked recessive disorders than are females.

  34. Several serious human disorders are sex-linked. Duchenne muscular dystrophy affects one in 3,500 males born in the United States. Affected individuals rarely live past their early 20s. This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin. The disease is characterized by a progressive weakening of the muscles and a loss of coordination.

  35. Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors. These proteins normally slow and then stop bleeding. Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly. Bleeding in muscles and joints can be painful and lead to serious damage. Individuals can be treated with intravenous injections of the missing protein.

  36. Although female mammals inherit two X chromosomes, only one X chromosome is active. Therefore, males and females have the same effective dose (one copy ) of genes on the X chromosome. During female development, one X chromosome per cell condenses into a compact object, a Barr body. This inactivates most of its genes. The condensed Barr body chromosome is reactivated in ovarian cells that produce ova. If a female is heterozygous for a sex-linked trait, approximately half her cells will express one allele and the other half will express the other allele.

  37. Similarly, the orange and black pattern on tortoiseshell cats is due to patches of cells expressing an orange allele while others have a nonorange allele. Fig. 15.10

  38. A man with hemophilia has a daughter of normal phenotype. She marries a man who is normal for the trait. What is the probability that a daughter of this mating will be a hemophilia? A son? If the couple has four sons, what is the probability that all four will be born with hemophilia? PROBLEMS

  39. Red-green colorblindness is caused by a sex-linked recessive allele. A color-blind man marries a woman with normal vision whose father was color-blind. What is the probability that they will have a color-blind daughter? What is the probability that their first son will be color-blind? (NOTE: the two questions are worded differently)

  40. What is the order of these genes on a chromosome?

  41. Multifactorial- many factors, both genetic and environmental, contribute to the disease. heart disease, high blood pressure, Alzheimer’s disease, arthritis, diabetes, cancer, and obesity Huntington's disease (autosomal dominant) - This is caused by a dominant single gene defect and generally does not appear until the individual is 35-45 years of age. Uncontrolled movements, loss of intellectual faculties, and emotional disturbance Tay-Sachs disease (autosomal recessive)- Individuals with this disorder are unable to metabolize certain lipids, affecting proper brain development. Affected individuals die in early childhood. phenylketonuria (autosomal recessive)- Effects of this recessive disorder can be completely overcome by regulating the diet of the affected individual.

  42. cystic fibrosis (autosomal recessive)- This results from a defect in membrane proteins that normally function in chloride ion transport. • sickle-cell disease (autosomal recessive)- Substitution of the "wrong" amino acid in the hemoglobin protein results in this disorder. • Colorblindness (sex linked) Red green - A form of colorblindness in which red and green are perceived as identical. • Duchenne Muscular Dystrophy (sex linked recessive) An absence of dystrophin, a protein that helps keep muscle cells intact. Generalized weakness and muscle wasting first affecting the muscles of the hips, pelvic area, thighs and shoulders. • Hemophilia (sex linked) is a rare, inherited bleeding disorder in which your blood doesn’t clot normally

  43. Alleles that cause an organism to die are called lethal alleles So complete the cross for a homozygous recessive lethal trait. Cross two heterozygotes and answer the following. What % of offspring would be homozygous dominant? What percentage of offspring would have the dominant phenotype. What percentage of offspring would be heterozygous? LETHAL TRAITS

  44. CHAPTER 15 THE CHROMOSOMAL BASIS OF INHERITANCE Section C: Errors and Exceptions in Chromosomal Inheritance 1. Alterations of chromosome number or structure cause some genetic disorders 2. The phenotypic effects of some mammalian genes depend on whether they are inherited from the mother or the father (imprinting) 3. Extranuclear genes exhibit a non-Mendelian pattern of inheritance

  45. Sex-linked traits are not the only notable deviation from the inheritance patterns observed by Mendel. Also, gene mutations are not the only kind of changes to the genome that can affect phenotype. Major chromosomal aberrations and their consequences produce exceptions to standard chromosome theory. In addition, two types of normal inheritance also deviate from the standard pattern. Introduction

  46. Nondisjunction occurs when problems with the meiotic spindle cause errors in daughter cells. This may occur if tetrad chromosomes do not separate properly during meiosis I. Alternatively, sister chromatids may fail to separate during meiosis II. 1. Alterations of chromosome number or structure cause some genetic disorders

  47. As a consequence of nondisjunction, some gametes receive two of the same type of chromosome and another gamete receives no copy. Offspring results from fertilization of a normal gamete with one after nondisjunction will have an abnormal chromosome number or aneuploidy. Trisomic cells have three copies of a particular chromosome type and have 2n + 1 total chromosomes. Monosomic cells have only one copy of a particular chromosome type and have 2n - 1 chromosomes. If the organism survives, aneuploidy typically leads to a distinct phenotype.

  48. Organisms with more than two complete sets of chromosomes, have undergone polypoidy. This may occur when a normal gamete fertilizes another gamete in which there has been nondisjunction of all its chromosomes. The resulting zygote would be triploid (3n). Alternatively, if a 2n zygote failed to divide after replicating its chromosomes, a tetraploid (4n) embryo would result from subsequent successful cycles of mitosis.

  49. Polyploidy is relatively common among plants and much less common among animals. The spontaneous origin of polyploid individuals plays an important role in the evolution of plants. Both fishes and amphibians have polyploid species. Recently, researchers in Chile have identified a new rodent species that may be the product of polyploidy.

  50. Polyploids are more nearly normal in phenotype than aneuploids. One extra or missing chromosome apparently upsets the genetic balance during development more than does an entire extra set of chromosomes.

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