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The Chromosomal Basis of Inheritance Chromosome theory of inheritance:-genes have specific locations (loci) on chromosomes-chromosomes segregate and assort independently Thomas Hunt Morgan-Drosophila melanogaster – fruit fly -fast breeding, 4 pairs 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-X-Y system, X-O system, Z-W system, haplo-diploid system 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 single X (hemizygous)-females can be affected or carrier-sex-linked disorders: color-blindness, Duchenne muscular Dystrophy, hemophilia X-inactiviation-Barr body = inactive X chromosome; regulate gene dosage in females during embryonic development-ex: cats – allele for fur color is on X, only F 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 Genetic Recombination: production of offspring with new combination of genes from parents-if offspring look like aprents à parental types-if different from parents à recombinants(what you would expect to see in Mendelian inheritance) -if results do not follow Mendel’s Law of Independent Assortment, then the genes are probably linked(like if you see that 2 parental alleles are always inherited together or there is little or no recombination between genes and expected ratios are not observed) Linked genes – located on same chromosome and tend to be inherited together during cell division 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 Calculating recombination frequencyPerform a test cross and observe the resulting offspring Recombination freq = (# recombinants / total offspring) x 100 = % Linkage Map: genetic map that is based on % of cross-over events-1 map unit = 1% recombination frequency-express relative distances along chromosome-50% recombination = far apart on same chromosome or on 2 different chromosomes ***Chi-Square Analysis Practice*** Exceptions to Mendelian Inheritance-Genomic imprinting – phenotypic effect of gene depends on whether from M or F parent-Methylation – silences genes by adding methyl groups to DNA-Non-nuclear DNA -some genes located in organelles (mitoshondris, chloroplasts, plastids) -contain small circular DNA -mitochondria = maternal inheritance (eggs)-ex: variegated (striped or spotted) leaves result from mutations in pigment genes in plastids, which generally are inherited from the maternal parent. Genetic testingReasons for genetic tests:-Diagnostic testing (genetic disorders)-Pre-symptomatic and predictive testing-Carrier testing (before having children-Pharmacogenetics (medication and dosage)-Prenatal testing-Newborn screening-Preimplantation testing (embryos) Prenatal testing-May be used on a fetus to detect genetic disorders-Amniocentesis: remove amniotic fluid around fetus to culture for karyotype-Chorionic villus sampling: insert narrow tube in cervix to extract sample of placenta with fetal cells for karyotype Nondisjunction – chromosomes fail to separate properly in meiosis I or meiosis II-karyotyping can detect nondisjunctions-ex: down syndrome = trisomy 21 ex: Kleinfelter Syndrome = 47XYY, 47XXY ex: Turner Syndrome = 45XO Chromosomal mutations-Deletion = removes a chromosomal segment-Duplication = repeats a segment-Inversion = reverses a segment within a chromosome-Translocation = moves a segment from one chromosome to a nonhomologous chromosome Nondisjunction-Aneuploidy: incorrect # chromosomes – monoploidy (1 copy) or trisomy (3 copies)-Polyploidy: 2+ complete sets of chromosomes; 3n or 4n – rare in animals, frequent in plants
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