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Unformatted text preview: Chapter 15-Chromosomal Basis of Inheritance CHROMOSOMAL THEORY OF INHERITANCE Early 1900s cytology and genetics merge ~ parallels between chromo s o m e behavior and Mendels factors MENDELs hereditary factors = gene s on chromo s o m e s Gene s have specific loci (positions) on chromo s o m e s SEGREGATION and INDEPENDENT ASSORTMENT of chromo s o m e s during meiosis explains patterns of inheritance (ratios) seen by Mendel GENE LINKAGE 1 st linked gene discovered by THOMAS HUNT MORGAN = white eyes in fruit flies Genes on same chromosomes dont sort independently More likely to be inherited together (unless separated by crossing over) GENETIC MAP = ordered list of genetic loci along a particular chromo s o m e Frequency of crossing over ~ related to the distance between genes Gene s farther apart on a chromo s o m e = more likely to be separated during crossing over Linked gene s have recombination frequencie s less than 50% Gene s very far apart on sam e chromo s o m e may appear to be unlinked Max value 50%= genes on different chromo s o m e s OR so far apart on sam e chromo s o m e appear unlinked Many fruit fly genes were mapped using recombination frequencie s ( LINKAGE MAP ) 1% recombination frequency = 1 map unit (centimorgans) PEDIGREE Show how trait is pass ed through generations in a family Square s = males; circles = females; Individuals with trait are colored in; carriers can be shown as half/half DETERMINING SEX HUMANS/other MAMMALS USE XY SYSTEM: XX= female; XY = male: All humans start as females; SRY gene code s for malenes s Som e INSECTS (grasshopp ers, cockroach e s ) use XO SYSTEM: XX = female; X0 = male BIRDS, som e fish, som e insects use ZW SYSTEM: ZW =female; ZZ = male BEES and ANTS use HAPLOID/DIPLOID SYSTEM: Diploid = females; Haploid (unfertilized eggs) = males AUTOSOMAL RECESSIVE Parents are generally unaffected; Need two reces sive alleles for trait to show two unaffected parents can have affected offspring Males and females both can be carriers for autosom al reces sive traits Heterozygotes = normal but carriers Dont show trait but can pass allele on to offspring...
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- Fall '08
- Zoology, Mental retardation, Sex linkage, Phenylketonuria, gen er ation, bin ation frequ, SY S TEM