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HMB265H1 (54)

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University of Toronto St. George
Human Biology
Stephen Wright

Pages 214-218 When a pure-breeding wild-type four-o’clock plant line with red petals is crossed with a pure line having white petals, the F 1eneration has pink petals - If the F1generation is selfed, the F2generation results in ¼ red, ½ pink, ¼ white - This inheritance pattern is based on 2 alleles of 1 gene; however, the heterozygotes (F and1half the F2) are intermediate in phenotype o Genotypes – c /c (red), c/c (white), c /c (pink) - Occurrence of the intermediate phenotype suggests incomplete dominance – general case in which the phenotype of a heterozygote is intermediate btw those of the two homozygotes - At the molecular level, each wild-type allele produces a set dose of its protein product; the # of doses determines concentration of a chemical made by the protein, e.g. pigment o 2 doses = most pigment, red o 1 dose = less pigment, pink o 0 doses = no pigment, white Codominance – the expression of both alleles of a heterozygote, e.g. in ABO human blood groups, there is codominance of antigen alleles (3 alleles of 1 gene) - 3 major alleles are i, I and I , but someone can only have 2 of the 3 alleles or 2 copies of 1 alleles; this results in 6 different genotypes (3 homozygotes and 3 different types of heterozygotes) o I / I , I /i – blood type A o I / I , I /i – blood type B o I / I – blood type AB o i/i – blood type O A B - I and I determine 2 forms of cell-surface molecule, and i is a null allele, so it doesn’t result in a cell-surface molecule o In genotypes I /i and I /i, the I and I alleles are fully dominant over I, but in the I / IB genotype, each allele produces its own form of the cell-surface molecule, so both the A and B alleles are codominant - Sickle cell anemia – 2 main alleles (Hb and Hb ) and 3 possible genotypes o Hb /Hb – normal, RBCs never sickle o Hb /Hb – severe anemia, often fatal; abnormal haemoglobin causes RBCs to have sickle shape o Hb /Hb – no anemia, RBCs only sickle under low O concen2rations - Presence/absence of anemia – Hb allele is dominant; a single Hb allele produces enough functioning haemoglobin to prevent anemia - Blood cell shape – incomplete dominance, b/c many of the cells in heterozygotes have a slight sickle shape - Haemoglobin itself – codominance b/c the 2 main alleles encode 2 different forms of haemoglobin that differ by one amino acid; both forms synthesized in the heterozygote The type of dominance inferred depends on the molecular functions of the alleles of a gene and by the investigative level of analysis (organismal, cellular, molecular) Recessive lethal alleles – alleles that are capable of causing the death of an organism - In the characterization of a set of newly discovered mutant alleles, recessive mutation is sometimes found to be lethal - This information can be useful, in that it shows that the newly discovered gene (whose function isn’t known yet) is essential to the organism’s operation - Using modern technology, we can make null alleles/make them homozygous to see if they’re lethal and under what conditions - Lethal alleles are also used to determine the developmental stage in which the gene normally acts; geneticists look for whether death from a lethal allele occurs early or late in development Diagnostic test for lethality – mouse coat colour - Wild-type mice have dark coat colour, while the mutant is “yellow” (lighter coat colour) - Yellow x homozygous wild-type – 1:1 ratio of yellow to wild-type; suggests that a yellow mouse is always heterozygous for the yellow allele, which is dominant over the wild-type allele - Yellow x yellow – result is always 2/3 yellow, 1/3 wild type (a 2:1 ratio) o In order for this ratio to make sense, the yellow allele can be assumed to be lethal when homozygous; the yellow allele is called A o ¼ A /A – lethal; these genotype don’t survive to be co
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