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Lecture 5

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University of Alberta
Biology (Biological Sciences)
Mike Deyholos

BIOL207 B01 Lec05 2014-01-13 CHAPTER 3 cont’d: Thomas Hunt Morgan crossed red-eyed fruit flies and red-eyed fruit flies. • The gene that controls red eyes… called wildtype white. The white locus is on the X-chromosome, but not on the Y chromosome. In flies, the X and Y chromosomes determine the sex of the fly. • Flies can be homozygous, heterozygous, or hemizygous 1 + • Males are hemizygous for either w / Y (white) or w / Y (wildtype) A. Sex-linkage a. Genes on the X chromosome are present as only one possible allele in males (=hemizygous); only if you use the XY chromosome pattern for gender; b. Asingle recessive allele can give a mutant phenotype in a hemizygote; a recessive allele can give the mutant; can exist on its own as well c. Sex linkage is revealed by reciprocal crosses (i.e. test each phenotype in M and F parents)  REDFEMALE X WHITEMALE vs WHITEFEMALE X REDMALE; since we get different results in both, this is considered sex linkage. X+ X+ Xw X+XW X+XW Y X+Y X+Y Xw Xw X X+XW X+XW + Y XwY XwY Why is the gene called white when it makes red eyes? Genes that make mutant screens will be named based on the phenotype, but it is opposite to the wildtype. 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; you will see a lot of stuff in brown Males are more likely to have this disease; if you’re female, this is an X-linked recessive trait, so you have to inherit the bad allele from both parents. More common in 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); maies are missing an extra X chromosomes, so the females also shut off their second X chromosomes into a barr body, it’s very heterochromatic, it’s so tightly packed that there’s almost no transcription from it. One of the homologs is randomly inactivated in an early stage of the embryo, that inactivated homolog becomes a barr body, remains tightly packed through the life of the individual. Still gets replicated during mitosis, but becomes incompatible during the germ line in meiosis. i. X ii replicated, and is transmitted at mitosis, but does not express most of its genes (n.b. a few genes do remain active on X) i b. Red gene in cats O o ii. red makes orange pigment; red does not make pigment; the capital O is the functional version, making orange pigment. The small o does not make orange pigment. Every tortoise shell cat you see/ calico cat you see is female, because the pattern comes from a being hetz with both colors. O o iii. female hetz (red /red ) are a mosaic of different colors (“tortiseshell”) depending which allele is inactivated; each cell in the ball decides whether to make the orange pigment/ black pigment 1. red /red cells make orange fur o o 2. red /red cells do not make orange fur iv. homozygotes and hemizygotes all have uniform color; males will be uniformly orange/ uniformly black in absence of other genes; c. birds, flies have sex chromosomes but do not inactivate whole chromosomes for dosage compensation (see turkey example, 3.5.3) Not all organisms worry about Barr bodies; birds don't have the patchy  all homozygotes are uniform. The male carries 2 different alleles, but they are not mosaics with a quarter shell pattern. Orange  XOXO or XOY Black  XoXo or XoY Combine  XOXo Non-white  ww; recessive at the white locus; White  WW or Ww 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. (fro
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