Introduction. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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2 Introduction It was not until 1900 that biology finally caught up with Gregor Mendel. Let s read Short History guys made him famous. Factors = genes located on chromosomes, which are made of the chemical DNA, whose function was not known until 1951.

3 1. Mendel and meiosis What do chromosomes do that sound like what Mendel said factors do?

4 Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels and proposed a chromosome theory of inheritance. Fig. 15.1

5 Sex chromosome Autosome Sex-linked trait Linked gene Chromosome map Germ-cell mutation Somatic-cell mutation Chromosome mutations Nondisjunction Pedigree Carrier Polygenic Multiple alleles (blood type groups A, B, and O)

6 3. Assortment is not always independent Each chromosome has hundreds or thousands of genes, much like recipes (the genes) in a recipe book (the chromosome). What are linked genes? These genes won t do that Mendel said factors do?

7 2. Another Columbia University (NYC) guy becomes famous Thomas Hunt Morgan made an insightful choice as an experimental animal, Drosophila melanogaster, a fruit fly species that eats fungi on fruit. What makes them good subjects for heredity experiments?

8 Morgan followed the inheritance of genes for body color and wing size in Drosophila. The dominant body color is gray (b + ) and the recessive black (b). Notice the difference in symbols. The dominant wing size is normal (vg + ) and the recessive has small vestigial wings (vg). Morgan crossed F 1 heterozygous females (b + bvg + vg) with homozygous recessive males (bbvgvg). This would be a dihybrid cross, yes? But not a classic Mendelian one, yes? What kids would they have?

9 Morgan s results were surprising to him. Let s look at the numbers.

10 Fig. 15.4

11 Morgan concluded that body color and wing shape are usually inherited together because their genes are on the same chromosome, or linked.

12 Describe in details the chromosomal basis of inheritance by clearly relating independent assortment, linkage, and nondisjunction to specific events in meiosis.

13 So linkage would result in results different from Mendel s, but not quite like Morgan s either. So what gives? It was proposed that an event called crossing over produces recombinant chromosomes which combine genes inherited from each parent. Fig

14 Crossing over begins during what phase? In crossing over, homologous portions (same size and type of genes) of two non-sister chromatids break and trade places. For humans, this is a common event. Let s look at exactly how Morgan s results came about.

15 How often does it look like these recombinant gametes form??? Fig. 15.5b

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

17 The farther apart two genes are, the higher the probability that a crossover will occur between them. The greater the distance between two genes, the more points between them where crossing over can occur.

18 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.

19 The three sources of genetic variability in a sexually reproducing organism are, then: All three mechanisms reshuffle the various genes carried by individual members of a population. What, then, ultimately creates different forms of genes (alleles)? And the three events noted above do what to these different alleles?

20 Chalk another one up for Morgan 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. Imagine the excitement in the lab!! The normal (wild type) red phenotype is dominant. Alternative traits are mutant phenotypes, and are usually recessive. Fig. 15.2

21 Let s diagram these crosses.

22 Sex chromosomes vs. Autosomes In 1905, Nettie Stevens had discovered that in many animals, cells from males and females differed in the appearance of one set of chromosomes. How are these sex chromosomes different from autosomes? This creates yet another non-mendelian result.

23 Morgan deduced that the gene with the white-eyed mutation is on the X chromosome alone, a sex-linked gene. Did we get the crosses right? Fig

24 In the X-Y system, Y and X chromosomes behave as homologous chromosomes during meiosis. In reality, they have very few of the same genes, and rarely undergo crossing over. So who determines the gender of a child? Can we Punnett square that one?

25 Around 1995, someone figured out more about sex determination. Let s see what you used to look like. The key is the SRY gene (Sex determining Region of the Y chromosome). Then again, take a look at this

26 Gynandromorph chicken. What???????

27 2. Sex-linked genes have unique patterns of inheritance Sex-linked genes follow the same pattern of inheritance as the eye color gene in Drosophila. Fig. 15.9

28 Use Punnett Squares to predict inheritance patterns of traits that are sex-linked, and/or have three or more alleles.

29 What are carriers? Why are males far more likely to have sex-linked recessive disorders than are females? How about some famous examples?

30 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 loss of coordination.

31 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. Red-green color blindness is also a sex-linked recessive trait.

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33 Some traits do not fit the either-or basis that Mendel studied. These are usually due to polygenic inheritance, the additive effects of two or more gene pairs on a single phenotypic trait. For example, skin color in humans is controlled by at least three different gene pairs. Imagine that each gene has two alleles, one light and one dark, that demonstrate incomplete dominance. An AABBCC individual is the darkest and aabbcc is the lightest.

34 A cross between two AaBbCc individuals (intermediate skin shade) would produce offspring covering a wide range of shades. Individuals with intermediate skin shades would be the most likely offspring, but very light and very dark individuals are possible as well. The range of phenotypes forms a normal distribution. Fig

35 Compare and contrast Hemophilia, Sickle Cell, ABO Blood Types Polygenic Inheritance, Multiple Alleles

36 Phenotype depends on environment and genes. A single tree has leaves that vary in size, shape, and greenness, depending on exposure to wind and sun. For humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests. Even identical twins, genetic equals, accumulate phenotypic differences as a result of their unique experiences. The relative importance of genes and the environment in influencing human characteristics is a very old and hotly contested debate.

37 Explain how pedigrees track inheritance of human traits.

38 1. Pedigree analysis reveals Mendelian patterns in human inheritance Rather than manipulate mating patterns of people (a bit impractical and unethical), geneticists analyze the results of matings that have already occurred. In a pedigree analysis, information about the presence/absence of a particular phenotypic trait is collected from as many individuals in a family as possible and across generations. The distribution of these characters is then mapped on the family tree.

39 Now let s look at some commonly used human traits as examples. Later we will see that THESE TRAITS ARE NOT WHAT TEXTBOOKS HAVE MADE THEM OUT TO BE FOR MANY YEARS!!!! For example, the occurrence of widows peak (W) is dominant to a straight hairline (w). The relationship among alleles can be integrated with the phenotypic appearance of these traits to predict the genotypes of members of this family.

40 Let s analyze what these two pedigree examples tell us. Notice that in the one on the left the dominant phenotype individuals are shaded in and in the one on the right it is the recessive ones. Don t be confused. Fig

41 We can use the same family tree to trace the distribution of attached earlobes (f), a recessive characteristic. NOW FOR THE REAL STORY Check out this link for some fun with cats! Fig

42 How about some pedigree practice Now that we have studied a number of different inheritance patterns, let s practice some pedigree problems ees/

43 2. Many human disorders follow Mendelian patterns of inheritance How about examples of autosomal bad genes? These range from the relatively mild (albinism) to lifethreatening (cystic fibrosis). Heterozygotes have a normal phenotype because one normal allele produces enough of the required protein.

44 One such disease is cystic fibrosis which strikes one of every 2,500 whites of European descent. One in 25 whites is a carrier. The normal allele codes for a membrane protein that transports Cl - between cells and the environment.

45 The most common inherited disease among African Americans is sicklecell disease. You will see this one again! Here is its story. 1 min. This disease is ALWAYS on tests because it is a good example of several concepts in biology.

46 This sickling creates a number of symptoms, such as clogging of capillaries and reduced oxygen transport. Doctors can use regular blood transfusions to prevent brain damage and new drugs to prevent or treat other problems. Fig

47 Carriers (heterozygotes) are said to have the sickle-cell trait. These individuals are usually healthy, although some suffer some symptoms of sickle-cell disease under oxygen stress, such as at altitude and during exercise. At the molecular level, the two alleles are codominant as both normal and abnormal hemoglobins are synthesized.

48 Why African Americans have a higher incidence of this disease is a classic story of how evolution works.

49 Now here is the inbreeding link. Most societies and cultures have laws or taboos forbidding marriages between close relatives, although this practice is common among royal families. The spread of hemophilia in the royal families of England and Russia is a famous example of inbreeding.

50 How about an example of a lethal dominant gene. Folk singer Woody Guthrie (This Land Is My Land) died of Huntington s disease.

51 3. Technology is providing new tools for genetic testing and counseling Many hospitals have genetic counselors to provide information to prospective parents who are concerned about a family history of a specific disease. How can they figure it out?

52 Amniocentesis is one way. A karyotype pairs same size and shape chromosomes to create a chart of homologues. Fig a

53 A second one is chorionic villus sampling (CVS). This technique extracts a sample of fetal tissue from the chorionic villi of the placenta. Fig b

54 PKU, routinely tested for, can be easily fixed.

55 1. Alterations of chromosome number or structure cause some genetic disorders Look at non-disjunction. Watch here. Fig

56 Explain how errors in meiosis can result in chromosome mutations and nondisjunction.

57 Trisomy or monosomy can result. Let s sketch them.

58 Then there is polyploidy. Ever eat a banana or a seedless orange?

59 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 which may be the product of polyploidy. Fig

60 Famous non-disjunction related disease. Fig

61 What if non-disjunction occurs in sex chromosomes? Turner? Klinefelter?

62 Mutagens are chemical or physical agents that interact with DNA to cause mutations. What can cause this change in the chemical DNA? What are Insertions? Deletions? Inversions?

63 Types of Mutations Gene vs. Chromosome Somatic vs. Germ