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Chapter 11 CHROMOSOMES
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A. What Is a Chromosome? A long, continuous strand of DNA, plus several types of associated proteins, and RNA.
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Chromosome pairs are distinguished by: size, banding pattern & centromere position.
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B. Linked Genes Genes carried on the same chromosome. Linked genes violate Mendel’s law of independent assortment because they may not separate during crossing-over of meiosis I.
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What types of gametes are expected from this individual? ¼ PL ¼ pl ¼ Pl ¼ pL What types of gametes are expected from this individual? ½ PL ½ pl
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The further apart two linked genes are, the more likely they will separate during gamete formation.
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Parental gametes retain the gene combinations from the parents. Recombinant gametes result from the mixing of maternal & paternal alleles during crossing- over. Closely linked genes yield few recombinant chromosomes - will NOT obtain expected 9:3:3:1 phenotypic ratio.
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Knowing allele arrangement is important in predicting trait transmission. Ex. Two allele combinations are possible for a pea plant with genotype PpLl. ] Alleles in coupling tend to be transmitted together. ] Alleles in repulsion separate with each generation.
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C. Sex Determination Mechanism by which an individual develops as a male or a female. 1. Total chromosome number is diploid (develops from a fertilized ovum) is haploid (develops from an unfertilized ovum) Ex. bees
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2. X-O System (number of X chromosomes determines sex) is XX is XO Ex. grasshoppers, crickets & roaches 3. X-Y System (presence of Y chromosome determines sex) SRY gene is XX is XY Ex. all mammals
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In X-Y system, determines sex of offspring.
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D. Inheritance of Sex-Linked Traits Most sex-linked traits are carried on the X chromosome (X-linked) & are recessive. Ex. colorblindness, hemophilia ] more common in ] cannot be a carrier ( is hemizygous) ] inherits condition from his mother, NOT his father
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Hemophilia: recessive X-linked trait GenotypePhenotype X H X H non-carrier X H X h carrier X h X h with hemophilia X H Ynormal X h Y with hemophilia
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What is the probability that a carrier and a normal will have a son with hemophilia? ¼ or 25% What is the probability that a non- carrier and a hemophiliac will have a son with hemophilia? ZERO
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E. X Inactivation Female mammals have 2 alleles for every gene on the X chromosome, while males have only 1. This inequality is balanced by “turning off” one X chromosome in each cell of a 3 week old embryo. F some cells turn off paternal X F some cells turn off maternal X
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Inactivated X appears as a dark- staining structure called a Barr body. How many Barr bodies would cells of a male possess?
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X inactivation is responsible for the appearance of calico cats. The earlier X inactivation occurs, the larger the patches.
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F. Chromosome Abnormalities 1. Polyploidy - extra full sets of chromosomes. F animal polyploids spontaneously abort or die shortly after birth F plant polyploids are relatively common (wheat, lilies)
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2. Aneuploidy - an extra (trisomy) or missing (monosomy) chromosome. Aneuploidy is usually due to a meiotic error called nondisjunction.
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] Autosomal aneuploids F trisomy 13 F trisomy 18 F trisomy 21 (Down syndrome) ] Sex chromosome aneuploids F Turner syndromeXO F Triplo-X XXX F Klinefelter syndromeXXY F Jacobs syndromeXYY
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3. Deletion - part of a chromosome is missing. 4. Duplication - part of a chromosome is present twice. 5. Inversion - part of a chromosome is reversed.
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6. Translocation - nonhomologous chromosomes exchange parts (reciprocal translocation) or combine (Robertsonian translocation).
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