X LINKAGE 4-14-08 chap4 revised

X LINKAGE 4-14-08 chap4 revised - Specific traits are...

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Unformatted text preview: Specific traits are transmitted with specific chromosomes. A test of the chromosome theory If genes are on specific chromosomes, then traits determined by the gene should be transmitted with the chromosome. T.H. Morgan's experiments demonstrating sexlinked inheritance of a gene determining eye-color demonstrate the transmission of traits with chromosomes. 1910 T.H. Morgan discovered a white eyed male, Drosophila melanogaster, among his stocks. Nomenclature for Drosophila genetics Wild-type allele - allele that is found in high frequency in a population Denoted with a "+" (usually superscript, or just + when we get lazy) Mutant allele - allele found in low frequency. NAMES GENE! May be denoted with superscript - symbol or no symbol Recessive mutation - gene symbol is in lower case (ie, w for the `white' gene for the white eye mutant found by Morgan) Dominant mutation - gene symbol is in upper case (ie, N for the dominant `Notch' gene mutant) Examples of notations for Drosophila Gene symbol is chosen arbitrarily e.g., Cy is curly winged, v is vermilion eyed, etc. Cy, Sb, D are dominant (upper case letter). vg, y, e, are recessive (lower case letter). vg+ - wild-type dominant allele for vestigial gene locus Cy+ - wild-type recessive allele for curly gene locus Criss-cross inheritance of the white gene demonstrates X-linkage. Fig. 4.20 Autosomes pairs of nonsex chromosomes Sex chromosomes and autosomes are arranged in homologous pairs Note 22 pairs of autosomes and 1 pair of sex chromosomes Sex chromosome Provide basis for sex determination One sex has matching pair. Other sex has one of each type of chromosome. Photomicrograph of human X and Y chromosome Fig. 4.6a After fertilization Cells with XX were females. Cells with XY were males. Great lubber grasshopper (Brachystola magna) Fig. 4.5 Sex determination in humans Children receive only an X chromosome from mother but X or Y from father. Fig. 4.6b X and Y chromosomes pair at meiosis I (one tip of each touch) de la Fuente et al 2007 PLoS Genetics vol 3: 30198 X and Y chromosomes are homologs since they segregate from each other in meiosis. Criss-cross inheritance of the white gene demonstrates X-linkage. Fig. 4.20 Draw chromosome figures and add allele markers at locus to show X linked inheritance by reciprocal crosses w w w + + w w + + w + + X and Y linked traits in humans are identified by pedigree analysis. X-linked traits exhibit five characteristics seen in pedigrees. Trait appears in more males than females. Mutation and trait never pass from father to son. Affected male does pass X-linked mutation to all daughters, who are heterozygous carriers. Trait often skips a generation. Trait only appears in successive generations if sister of an affected male is a carrier. If so, one half of her sons will show trait. Example of sex-linked recessive trait in human pedigree hemophilia Fig. 4.23 a Example of sex-linked dominant trait in human pedigree hypophosphatemia Fig. 4.23 b There is variation between species in how chromosomes determine an individual's sex. __________________________________________________ Chromosome Females Males Organism __________________________________________________ XX-XY XX XY Mammals, Drosophila XX-XO XX XO Grasshoppers ZZ-ZW ZW ZZ Fish, Birds, Moths __________________________________________________ Table 4.1 Rare events of nondisjunction in XX female produce XX and O eggs. Segregation in an XX female Fig. 4.21 a Complement of sex chromosomes Humans presence of Y determines sex Drosophila ratio of autosomes to X chromosomes determines sex XXX XX XXY XO XY XYY OY Drosophila Dies Normal female Normal female Sterile male Normal male Normal male Dies Humans Nearly normal female female Normal female Kleinfelter male (sterile); tall, thin Turner female (sterile); webbed neck Normal male Normal or nearly normal male Dies Trisomy- Non-disjunction in germ cell results in three of a given chromosome. Most trisomies in humans are lethal. Most famous- Down syndrome. Other viable trisomies can occur with chromosomes 13, 18 and the X chromosome. After Fig 5.1 Interpretation of hemophila and colorblindness segregating in family Both hemophilia (A) and colorblindness are X-linked. Appear mostly in males, inherited through mothers. The founder male (I-1) of this clan has both hemophilia and colorblindness. His daughters are all doubly heterozygous h c / + +. Their sons are mostly either h c or + + but there is one + c son. Product of crossover. Crossing over means alleles on a chromosome that come in from same parent don't always have to go out together. Genetic recombination between car and Bar genes on the Drosophila X chromosome represents actual physical exchange Fig. 5.6 Recombination frequencies for pairs of genes reflect distance between them. Alfred H. Sturtevant Percentage of recombination, or recombination frequency (RF) reflects the physical distance separating two genes. 1 RF = 1 map unit (or 1 centiMorgan) Fig. 5.8 ...
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