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University of Alberta
Gregory Wagner

BIOL207 B01 Lec05 2014-01-13 CHAPTER 3 cont’d: A. Sex-linkage a. Genes on the X chromosome are present as only one possible allele in males (=hemizygous) b. A single recessive allele can give a mutant phenotype in a hemizygote c. Sex linkage is revealed by reciprocal crosses (i.e. test each phenotype in M and F parents) d. Sex linkage is an exception to Mendel’s First Law e. One (of many) genes that affect Drosophila eye color is White (W) i. Note that the normal function of White is to make red eyes ii. W is sex-linked, located on X-chromosome f. All significant cases of sex -linked genes involve loci on the X-chromosome; Y chromosome probably encodes only some of the genes only required males g. Red-green color blindness is X -linked in humans, so this trait is more common in males – but not exclusive to males B. Sex-determination (from Chapter 2) a. Some organisms have sex chromosomes (although process of sex determination varies widely between species). b. Drosophila superficially similar to humans: (XX = female, XY = male) c. Birds, moths, butterflies different: (ZZ = male, ZW = female) d. ZZ, XX = “homogametic”; ZW, XY = “heterogametic” e. X and Y pair like homologous chromosomes during meiosis; BUT they do not contain the same loci C. Dosage compensation (from Chapter 2) a. To maintain gene balance, one X chromosome in most cells of XX females mammals is (randomly) inactivated (X ); i i. X is replicated, and is transmitted at mitosis, but does not express most i of its genes (n.b. a few genes do remain active on X ) i b. Red gene in cats ii. red makes orange pigment; red does not make pigment O o iii. female hetz (red /red ) are a mosaic of different colors (“tortiseshell”) depending which allele is inactivated; 1. red /red cells make orange fur O o 2. red /red cells do not make orange fur iv. homozygotes and hemizygotes al l have uniform color c. birds, flies have sex chromosomes but do not inactivate whole chromosomes for dosage compensation (see turkey example, 3.5.3) Cat fur phenotypes: optional assignment (fill out survey on e-class) 1. Score the cat(s) you know based on the following fur phenotypes, each controlled by these different genes. (from Christensen (2000) Genetics 155:999-1004)
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