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THE CHROMOSOMAL BASIS OF INHERITANCE CHAPTER 15 Warm up 1.What is the probability of the following? a.) Aabb X AaBb -> Aabb b.) AaBB X AaBb -> aaBB c.) AABbcc X aabbCC -> AaBbCc d.) AaBbCc X AaBbcc -> aabbcc 2. Explain what a linkage map is and a map unit. 3. Map the following gene. (pg 281 7th ed.) • J-k 12% k-L 6% • J-m 9% L-m 15% Warm-Up 1. What is the pattern of inheritance of the trait (shaded square/circle) shown in the pedigree? 1. How many chromosomes are in a human cell that is: a) Diploid? b) Triploid? c) Monosomic? d) Trisomic? What you must know: • How the chromosome theory of inheritance connects the physical movement of chromosomes in meiosis to Mendel’s laws of inheritance. • The unique pattern of inheritance in sexlinked genes. • How alteration of chromosome number or structurally altered chromosomes (deletions, duplications, etc.) can cause genetic disorders. • How genetic imprinting and inheritance of mitochondrial DNA are exceptions to standard Mendelian inheritance. Chromosome theory of inheritance: • Genes have specific locations (loci) on chromosomes • Chromosomes segregate and assort independently Chromosomes tagged to reveal a specific gene (yellow). Thomas Hunt Morgan • Drosophila melanogaster – fruit fly – Fast breeding, 4 prs. chromosomes (XX/XY) • Sex-linked gene: located on X or Y chromosome – Red-eyes = wild-type; white-eyes = mutant – Specific gene carried on specific chromosome Sex determination varies between animals Sex-linked genes • Sex-linked gene on X or Y • Females (XX), male (XY) – Eggs = X, sperm = X or Y • Fathers pass X-linked genes to daughters, but not sons • Males express recessive trait on the only X (hemizygous) • Females can be affected or carrier Transmission of sex-linked recessive traits Sperm Ova Sperm Ova Sperm Ova Sex-linked disorders • Colorblindness • Duchenne muscular dystrophy • Hemophilia X-Inactivation Barr body = inactive X chromosome; regulate gene dosage in females during embryonic development Because of this only female cats can be tortoiseshell or calico. Human development • • • • Y chromosome required for development of testes Embryo gonads indifferent at 2 months SRY gene: sex-determining region of Y Codes for protein that regulates other genes Linked genes: located on same chromosome and tend to be inherited together during cell division Genetic Recombination: production of offspring with new combo of genes from parents • Unlinked genes: follow law of independent assortment – 50% frequency of recombination observed for any 2 genes on different chromosomes Crossing over: explains why some linked genes get separated during meiosis • the further apart 2 genes on same chromosome, the higher the probability of crossing over and the higher the recombination frequency Geneticists can use recombination data to map a chromosome's genetic loci Linkage Map: genetic map that is based on % of cross-over events • 1 map unit = 1% recombination frequency • Express relative distances along chromosome • 50% = far apart on same chromosome or on different chromosomes Nondisjunction: chromosomes fail to separate properly in Meiosis I or Meiosis II • Aneuploidy: incorrect # chromosomes – Monosomy (1x) or Trisomy (3x) • Polyploidy: 2+ complete sets of chromosomes; 3n or 4n – Rare in animals, frequent in plants A tetraploid mammal. Scientists think this species may have arisen when an ancestor doubled its chromosome # by errors in mitosis or meiosis. Nondisjunction The structure of an individual chromosome can be altered during DNA replication Karyotyping can detect nondisjunctions. Down Syndrome = Trisomy 21 Karyotyping can detect nondisjunctions. Klinefelters Syndrome: 47XYY, 47XXY Karyotyping can detect nondisjunctions. Turners Syndrome = 45XO Extranuclear Genes • Some genes located in organelles – Mitochondria, chloroplasts, plastids – Contain small circular DNA – Do not display Mendelian inheritance • Mitochondria = maternal inheritance (eggs) Variegated (striped or spotted) leaves result from mutations in pigment genes in plastids, which generally are inherited from the maternal parent.