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Changes in Chromosome Number Chromosome Number Mutations Type of Mutation Aneuploidy Difference from wild type chromosome number Abnormal number of individual chromosomes (addition or loss of a small number of chromosomes) Polyploidy Abnormal number of chromosome sets Euploid = organism with multiples of the number of chromosomes in a basic set Aneuploid Numbers Nullisomic 2n - 2 Monosomic 2n - 1 Trisomic 2n + 1 Tetrasomic 2n + 2 Missing both copies of a homolog Missing one copy of a homolog Having an extra copy of one homolog Having two extra copies of one homolog n = haploid number of chromosomes 2n = diploid number of chromosomes Causes of Aneuploidy 1. Loss of a chromosome that has a centromeric deletion 2. Loss of the small chromosome produced by Robertsonian translocation 3. Nondisjunction = failure of homologs to separate during meiosis or chromatids to separate during meiosis or mitosis Nondisjunction Aneuploidy in Plants: Seed Capsule Changes Due to Trisomy in Jimson Weed Human Autosomal Aneuploidy •Most common type is Down Syndrome: Trisomy 21 •Arises in two ways •Primary Down Syndrome •Due to nondisjunction in egg formation • Egg with 2 copies of #21 (24 chromosomes) + Sperm with 1 copy of #21 (23 chromosomes) = Embryo with 3 copies of #21 (47 chromosomes) •Familial Down Syndrome •Inherited from a carrier of a Robertsonian Translocation involving chromosome 21 Down Syndrome Karyotype and Physical Features Eye fold Palm Crease Incidence of Primary Down Syndrome Increases with Maternal Age Robertsonian Translocation Carrier of Down Syndrome Translocation of 21q to 15q Chromosome Segregation in a Translocation Carrier Expected Result: 1/3 carrier + 1/3 normal + 1/3 Down Syndrome Human Sex Chromosome Aneuploidy Condition Turner Syndrome Klinefelter Syndrome Triple-X Female Genotype Phenotype XO Sterile Female XXY Sterile Male XXX Fertile Female XYY Fertile Male Taller Human Sex Chromosome Aneuploidy Turner Syndrome One Copy of X chromosome No second sex chromosome Eg. Egg with 0 copies of X (22 chromosomes) +Sperm with 1 copy of X (23 chromosomes) = Embryo with 1 copy of X (45 chromosomes) Turner Syndrome Karyotype and Physical Features Non-functional Ovaries From Adult Female with Turner Syndrome Child with Turner Syndrome Normal uterus, tubes and ovaries Human Sex Chromosome Aneuploidy Klinefelter’s Syndrome Two Copies of the X chromosome One Copy of the Y chromosome Eg. Egg with 2 copies of X (24 chromosomes) + Sperm with 1 copy of Y (23 chromosomes) = Embryo with XXY (47 chromosomes) Klinefelter Syndrome Karyotype and Physical Features X-Chromosome Inactivation in Females •Inactivation of one of the X chromosomes in each cell of an adult female balances the sex chromosome/autosome ratio. • Either the maternal or paternal chromosome is inactivated. •The arrow shows a Barr body, or inactivated X. •The number of Barr bodies equals the number of X chromosomes minus one. female male Applying Your Knowledge 1. 2. 3. 4. Nullisomic Monosomic Disomic Trisomic Which term describes the chromosome number change for individuals with A. Down Syndrome? B. Turner Syndrome? Applying Your Knowledge Zero = fist One Two Three How many Barr bodies would be found in cells of individuals with A. Klinefelter Syndrome? B. Turner Syndrome? C. Triple-X Syndrome? Polyploid Numbers Three chromosome sets Triploid Four chromosome sets Tetraploid Five chromosome sets Pentaploid Six chromosome sets Hexaploid Polyploid = euploid organism with more than two sets of chromosomes Types of Polyploids Autopolyploid Multiple chromosome sets from within one species Allopolyploid Multiple chromosome sets from different species Polyploidy Arises by Nondisjunction Chromosome Segregation in Polyploids Leads to Unbalanced Gametes Production of modern wheat Is T. aestivum 1. Autopolyploid? 2. Allopolyploid? 3. Euploid? 4. Aneuploid?