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Benjamin BenjaminA. A. Pierce Pierce •GENETICS Essentials •Concepts and Connections • SECOND EDITION CHAPTER 4 Extensions and Modifications of Basic Principles PART I © 2013 W. H. Freeman and Company CHAPTER 4 OUTLINE • 4.1 Sex Is Determined by a Number of Different Mechanisms, 74 • 4.2 Sex-Linked Characteristics Are Determined by Genes on the Sex Chromosomes, 79 • 4.3 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses, 87 • 4.4 Gene Interaction Takes Place When Genes at Multiple Loci Determine a Single Phenotype, 91 • 4.5 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways, 97 • 4.6 The Expression of a Genotype May Be Influenced by Environmental Effects THE CASE OF YELLOW MICE • All of the sudden Mendel’s rules do not work as predicted • What happened?? CHROMOSOMAL SEX-DETERMINATION SYSTEMS: SEX CHROMOSOMES AND NON-SEX CHROMOSOMES (AUTOSOMES) • XX-XO system: • XX – female (homogametic sex) • XO – male (heterogametic sex) • grasshoppers • XX-XY system: • XX – female (homogametic sex) • XY – male (heterogametic sex) • Mammals • ZZ-ZW system: • ZZ – male • ZW – female • Birds, snakes, butterflies, some amphibians, and fishes • Autosomes- remaining of the chromosomes that are same in both sexes GENIC SEX-DETERMINING SYSTEM • No sex chromosomes, only the sex-determining genes at certain loci • Plants, fungi, protists ENVIRONMENTAL SEX DETERMINATION • Environmental factors • Temperature in turtles SEX DETERMINATION IN DROSOPHILA MELANOGASTER • Genic balance system • X:A ratio • X, number of X chromosomes; • A, number of haploid sets of autosomes SEX DETERMINATION IN HUMANS XX-XY • SRY gene on the Y chromosome determines maleness 1/1000 1/3000 1/1000 THE ROLE OF SEX CHROMOSOMES • The X chromosome contains genetic information essential for both sexes; at least one copy of an X is required. • The male-determining gene is located on the Y chromosome. A single Y, even in the presence of several X, still produces a male phenotype. • The absence of Y results in a female phenotype. THE MALE-DETERMINING GENE IN HUMANS • Sex-determining region Y (SRY) gene • Androgen-insensitivity syndrome • Caused by the defective androgen receptor 4.2 SEX-LINKED CHARACTERISTICS ARE DETERMINED BY GENES ON THE SEX CHROMOSOMES • X-linked characteristics • X-linked white eye in Drosophila • X-linked color blindness in humans Morgan’s Experiments Drosophila eye color • Growing Drosophila found white eyed male • Cross of wild type red female with the white eyed male • F1 all red eyed for that cross Morgan’s Experiments Drosophila eye color F1 cross • The F1 generation offspring all had red eyes • So far looks like Mendel’s dominant/recessive red over white • Cross females and males of the F1 generation Morgan’s Experiments Drosophila eye color F2 generation • As expected 3:1 red:white HOWEVER only males are white eyed • Embryonic lethal? Morgan’s Experiments Drosophila eye color the Reciprocal cross • Cross of white eyed female with wild type male • F1 ratio 1:1 red eyed females/white eyed males Morgan’s Experiments Drosophila eye color F1 cross • The F1 generation offspring represents departure from Mendel’s rules • Cross females and males of the F1 generation Morgan’s Experiments Drosophila eye color F2 generation • 1:1 red:white but now also white eyed females COLOR BLINDNESS IN HUMANS ALLELES AND CHROMOSOME LABELING • There are rules for lableing of alleles in sex linked traits: • • • • X+ X+ is a female with both wild type alleles X+Y is a male with wild type allele X+ Xw is a female heterozygous for the white allele Xw Y is a male hemizygous for the white allele and affected • Since Y does not carry the homologous gene the labeling can be expressed as X+/ X-LINKED CHARACTERISTICS: DOSAGE COMPENSATION • The amount of protein produced by X-linked genes is equal in both sexes regardless to the fact that females have two copies of those genes • Solutions vary • Fruit flies: dosage compensation is achieved by a doubling of the activity of the genes on the X chromosome of the male. • The worm C. elegans: it is achieved by a halving of the activity of genes on both of the X chromosomes in the female • Placental mammals: genes on one of the X chromosomes in the female are inactivated creating Barr body BARR BODY X-INACTIVATION: THE CATS • X linked locus for fur color: • X+, which produces nonorange (usually black) fur, • Xo, which produces orange fur • Tortoise cats • Calico cats Y-CHROMOSOME TRAITS 4.3 ADDITIONAL FACTORS AT A SINGLE LOCUS CAN AFFECT THE RESULTS OF GENETIC CROSSES • Incomplete dominance • Codominance • Table 4.1 INCOMPLETE DOMINANCE CODOMINANCE: BLOOD TYPES 4.3 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses • Penetrance: the percentage of individuals having a particular genotype that express the expected phenotype. • Expressivity: The degree to which a character is expressed. PENETRANCE • Penetrance: the percentage of individuals having a particular genotype that express the expected phenotype. • The genotypes of the population have to be known • If 83 individuals out of 100 with genotype that should produce a certain phenotype express the phenotype the penetrance is 83% • Polydactyly EXPRESSIVITY • Expressivity: The degree to which a character is expressed. • Polydactyly again: • Some express extra finger and tows that are functional • Others have small extra skin tags PENETRANCE AND EXPRESSIVITY Another example • Neurofibromatosis • Autosomal dominant • 50-80% penetrance • Individuals with the allele show a wide range of phenotypes: • Mild few pigmented areas on the skin (café-au-lait spots). • severe) • • • • • • • • (1) Neurofibroma tumors of various sizes. (2) High blood pressure. (3) Speech impediments. (4) Headaches. (5) Large head. (6) Short stature. (7) Tumors of eye, brain, or spinal cord. (8) Curvature of the spine 4.3 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses • A lethal allele: causes death at an early stage of development, and so some genotypes may not appear among the progeny. • The example from the intro of this chapter the yellow coat and the allele for it • Yellow is dominant Y • Viable in heterozygous Yy • Lethal In homozygous YY