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Copyright © 2009 Pearson Education, Inc. PowerPoint ® Lecture Presentation for Concepts of Genetics Ninth Edition Klug, Cummings, Spencer, Palladino Chapter.

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Presentation on theme: "Copyright © 2009 Pearson Education, Inc. PowerPoint ® Lecture Presentation for Concepts of Genetics Ninth Edition Klug, Cummings, Spencer, Palladino Chapter."— Presentation transcript:

1 Copyright © 2009 Pearson Education, Inc. PowerPoint ® Lecture Presentation for Concepts of Genetics Ninth Edition Klug, Cummings, Spencer, Palladino Chapter 4 Extensions of Mendelian Genetics Lectures by David Kass with contributions from John C. Osterman. Copyright © 2009 Pearson Education, Inc.

2 Alternative forms of a gene are called alleles. Mutation is the source of alleles. The wild-type allele is the one that occurs most frequently in nature and is usually, but not always, dominant. Alleles

3 Copyright © 2009 Pearson Education, Inc. Mutations Loss-of-function mutations Null alleles Gain-of-function mutations Neutral mutations

4 Copyright © 2009 Pearson Education, Inc. Phenotypic traits may be influenced by more than one gene and the allelic forms of each gene involved.

5 Copyright © 2009 Pearson Education, Inc. Dominant alleles are usually indicated either by: an italic uppercase letter (D) Recessive alleles are usually indicated either by: an italic lowercase letter (d) Allelic Symbols Used

6 Copyright © 2009 Pearson Education, Inc. System Used for Drosophila melanogaster e+/e+ gray homozygote (wild type) e+/egray heterozygote (wild type) e/eebony homozygote (mutant) +/+gray homozygote (wild type) +/egray heterozygote (wild type) e/eebony homozygote (mutant) Wr/Wrwrinkled-wing homozygote (mutant) Wr/Wr+wrinkled-wing heterozygotes (mutant) Wr+/Wr+normal wings (wild type) Allelic Symbols Used

7 Copyright © 2009 Pearson Education, Inc. If no dominance exists, italic uppercase letters and superscripts are used to denote alternative alleles (R 1, R 2, C W, C R ). Allelic Symbols Used

8 Copyright © 2009 Pearson Education, Inc. In incomplete dominance: neither trait is dominant offspring from a cross between parents with contrasting traits may have an intermediate phenotype Incomplete Dominance

9 Copyright © 2009 Pearson Education, Inc. Figure 4.1 The phenotypic ratio is identical to the genotypic ratio in cases of incomplete dominance.

10 Copyright © 2009 Pearson Education, Inc. The threshold effect comes about if normal phenotypic expression occurs whenever a certain level (usually 50% or less) of gene product is attained. Ex. Tay-Sachs Disease Incomplete Dominance

11 Copyright © 2009 Pearson Education, Inc. Codominance both alleles are expressed in the heterozygote One example is the MN blood group. Codominance

12 Copyright © 2009 Pearson Education, Inc. Multiple alleles (>2) can be studied only in populations, because any individual will have at most two alleles of the same gene. Multiple Allelism

13 Copyright © 2009 Pearson Education, Inc. Alleles present in population: A, B, O alleles Each individual has the A, B, AB, or O phenotype I A and I B alleles are dominant to the I O allele I A and I B alleles are codominant Multiple Allelism – ABO Blood Group

14 Copyright © 2009 Pearson Education, Inc. Figure 4.2

15 Copyright © 2009 Pearson Education, Inc. Figure 4.3 Bombay Phenotype

16 Copyright © 2009 Pearson Education, Inc. Loss-of-function mutation can sometimes be tolerated in the heterozygous state but may behave as a recessive lethal allele in the homozygous state. In this case, homozygous recessive individuals will not survive. Recessive Lethal Alleles

17 Copyright © 2009 Pearson Education, Inc. Figure 4.4

18 Copyright © 2009 Pearson Education, Inc. In some cases, a mutation can be a dominant lethal allele, in which case the heterozygote will not survive. Ex. Huntington disease For dominant lethal alleles to exist, the affected individual must reproduce before dying. Dominant Lethal Alleles

19 Copyright © 2009 Pearson Education, Inc. Mendel’s principle of independent assortment applies to situations in which two modes of inheritance occur simultaneously, provided that the genes controlling each character are not linked on the same chromosome. Mendel – Independent Assortment

20 Copyright © 2009 Pearson Education, Inc. Figure 4.5

21 Copyright © 2009 Pearson Education, Inc. In gene interaction, the cellular function of numerous gene products contributes to the development of a common phenotype. Epigenesis – often a phenotype occurs due to many steps in a developmental process that are influenced and controlled by many genes. Ex. Development of organs Phenotypes Affected by Many Genes

22 Copyright © 2009 Pearson Education, Inc. Epistasis occurs when: one gene masks the effect of another gene, or two gene pairs complement each other such that one dominant allele is required at each locus to express a certain phenotype Ex. Bombay effect Epistasis

23 Copyright © 2009 Pearson Education, Inc. Figure 4.6

24 Copyright © 2009 Pearson Education, Inc. Figure 4.7

25 Copyright © 2009 Pearson Education, Inc. Eight cases of epistasis are described in Figure 4.8. These include recessive epistasis (case 1), dominant epistasis (case 2), and complementary gene interaction (case 3). Section 4.8

26 Copyright © 2009 Pearson Education, Inc. Figure 4.8

27 Copyright © 2009 Pearson Education, Inc. Figure 4.9

28 Copyright © 2009 Pearson Education, Inc. Figure 4.10 Eye color in Drosophila. Interaction of two gene products result in the wild- type eye color, which is brick red.

29 Copyright © 2009 Pearson Education, Inc. Two cases of mutation in Drosophila (Figure 4.11) Case 1: All offspring develop normal wings Case 2: All offspring fail to develop normal wings Complementation Analysis

30 Copyright © 2009 Pearson Education, Inc. Figure 4.11

31 Copyright © 2009 Pearson Education, Inc. Pleiotropy occurs when expression of a single gene has multiple phenotypic effects, and it is quite common. Examples of pleiotropy are Marfan syndrome and porphyria variegata. Pleiotropy Abraham_Lincoln_standing_portr ait_1863.jpg Flo Hyman

32 Copyright © 2009 Pearson Education, Inc. Genes present on X chromosome exhibit unique patterns of inheritance due to presence of only one X chromosome in males. X-Linkage

33 Copyright © 2009 Pearson Education, Inc. Drosophila eye color one of the first examples of X- linkage described X-Linkage

34 Copyright © 2009 Pearson Education, Inc. Hemizygosity Occurs in males due to the inability of males to be homozygous or heterozygous for an X- linked gene Have only one copy of that gene despite having diploid cells

35 Copyright © 2009 Pearson Education, Inc. Figure 4.14

36 Copyright © 2009 Pearson Education, Inc.

37 Lethal X-linked recessive disorders are observed only in males. Usually never reproduce Females can only be heterozygous carriers that do not develop the disorders. Lethal X-Linked Recessive Disorders

38 Copyright © 2009 Pearson Education, Inc. Sex-limited inheritance occurs in cases where the expression of a specific phenotype is absolutely limited to one sex. In sex-influenced inheritance, the sex of an individual influences the expression of a phenotype that is not limited to one sex or the other. Individual’s Sex Can Influence Phenotype

39 Copyright © 2009 Pearson Education, Inc. Figure 4.15

40 Copyright © 2009 Pearson Education, Inc. Figure 4.16

41 Copyright © 2009 Pearson Education, Inc. Phenotypic expression of a trait may be influenced by environment as well as by genotype. Epigenetics

42 Copyright © 2009 Pearson Education, Inc. Figure 4.19

43 Copyright © 2009 Pearson Education, Inc. In cases of genomic (parental) imprinting, phenotypic expression may depend on the parental origin of the chromosome. Imprinting is thought to occur before or during gamete formation and may involve DNA methylation. Genomic (Parental) Imprinting

44 Copyright © 2009 Pearson Education, Inc. The End


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