Chapter 15 Genes and chromosomes are present in pairs in diploid cells Homologous chromosomes separate and alleles segregate during meiosis 1 Paired condition for both chromosomes and genes is restored during fertilization 1 Thomas Hunt Morgan Mapping genes a Used drosophila melanogaster in research fruit fly species that eats fungi on fruit 2 Linked genes short generation time b 8 chromosomes i 3 pairs of autosomes ii 1 pair of sex chromosomes XX in females XY in males c After 1 year of looking Morgan found a single white eyes male i A mutant phenotype d Wild type normal phenotype red eyes e White eyes ale crossed with wild female i F1 had red eyes F2 had classic ratio only males were white eyed ii Conclusion the gene that coded for eye color was on the X chromosome f Female has 2 copies of gene on the X chromosome Male has 1 copy of a gene on the X chromosome a On the same chromosome b Typically inherited together because the chromosome is passed along as a unit c Do not follow exhibit probabilities associated with Mendel s law of independent assortment d Body color of fruit flies wild type gray body mutant black i Wild type normal wings mutant vestigial wings ii If independent assortment GENES ON DIFFERENT CHROMOSOMES THE RATIO WOULD BE 1 1 1 1 If genes are linked genes on the same chromosome 1 1 0 0 iii e Genes for body color and wing shape are on the same chromosome inherited together i Other phenotypes gray vestigial wings and black normal wings resulted from crossing over ii Recombinants new combination of genes parental from crossing over 1 Crossing over during prophase 1 results in production of more types of gametes than one would predict by the Mendelian rules alone f Recombinant frequency 391 2300 17 i You do the recombinant offspring over the total offspring times 100 g Chromosome map i Ordered list of the genetic loci along a particular chromosome ii Frequency of recombinant offspring reflected the distances between genes on a chromosome iii Frequency of crossing over used to construct map h Recombination frequencies are not additive i WHY Second crossing over event cancels out the first and reduced to the observed number of recombinant offspring ii The greater the distance the greater the chance of multiple crossing over events the smaller the percentage the closer together the traits are on the chromosome i Maximum frequency 50 i Same as if genes are on different chromosomes ii Far enough apart recombinant from crossing over is almost assured j Mendel s traits i Seed color and flower color in peas ii On the same chromosome but far enough apart that linkage is 3 Sex Linked Genes a Genetic chromosomal difference between males and females i Humans have 22 autosomes and males XY females XX b Thus for a gene located on the X chromosome males have one allele only one X chromosome HEMIZYGOUS Females have 2 alleles one on each X chromosome 4 Other systems the X 0 system the ZW system and the haplo diploid system 5 Value of Having Two X chromosomes a Diseases caused by X linked recessive alleles are more common in males than females b X chromosome that male gets from his mother determines phenotype he only has one allele normal condition c Women have two X chromosomes so they can be heterozygous but still may have 6 Only one Chromosome is active in Females In each cell X chromosome condenses into a Barr body compact inactive form mostly a b Selection of the X turned off is random 50 50 probability i If female is heterozygous 1 cells will express one allele 2 will express the other c Example differently colored fur in cats 7 X linked disorders a Duchenne muscular dystrophy lack a key muscle protein dystrophin progressive weakening of the muscles loss of coordination b Red green color blindness ability to absorb the different colored wavelengths in light c Hemophilia blood clotting protein i Color Blindness 1 Versions vary often an inability to distinguish between red and green 2 XCXc XCXC normal female XCY normal male XcXc colorblind female 8 Hemophilia another recessive X linked disorder XcY colorblind male a When there is a cut in blood vessel a blood clot will form to prevent loss of blood process involves several proteins that form fibers trapping blood cells and platelets b Mutation in any of the proteins may prevent the fiber formation and thus prevent the blood from clotting Most proteins are from genes on autosomes but 2 are on the X chromosome c Mutant recessive allele of either may produce hemophilia if a normal allele is not also present 9 Chromosome aberrations d Males only get one copy if they get the mutant allele they will have it a Polyploidy aberrations in the number of chromosome sets common in plants because they can self fertilize 2n to 4n 10 Aneuploidy incorrect chromosome number caused by nondisjunction failure to separate of homologous chromosomes during Meiosis 1 or sister chromatids in meiosis 2 Meiosis 1 is worse because then you have 4 incorrect cells a Gamete gets aberrant number of chromosomes so offspring has an aberrant chromosome number 11 Trisomic cells have three copies of a particular chromosome type and have 2n 1 total chromosomes chromosomes 12 Monosomic cells have only one copy of a particular chromosome type and have 2n 1 a One extra or missing chromosome appears to be more harmful during development than an entire set of chromosomes b Down syndrome extra 21st chromosome the mother s age 13 Aneuploid zygotes may be common in humans most are likely spontaneously aborted long before birth a Aneuploidy in sex chromosomes i Upsets the genetic balance less severely ii Y chromosome contains fewer genes iii Extra copies of the x chromosome are inactivated as barr bodies in somatic cells 14 Abnormal Numbers of Sex Chromosomes a Klinefelter s sterile males with XXY syndromes normal IQ 1 2000 births b Extra Y chromosome tend to somewhat taller than average c Trisomy X XXX which occurs once in every 2000 live births produces healthy females d Turner s syndrome X0 which occurs once in every 5000 births produces phenotypic but immature females 15 Chromosome aberrations a Deletion fragment of chromo lacking a centromere is lost during cell division b Duplication fragment becomes attached as an extra segment to a sister chromatid i Cri Du Chat deletion in specific part chromo 5 individuals are mentally disabled unusual facial features and small head cry like a distressed cat death in infancy or early childhood Inversion fragment reattaches to the