Changes in Chromosome Number

Chromosome Number Mutations Type of Mutation Difference from wild type chromosome number Aneuploidy 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 of chromosomes in a basic set

Aneuploid Numbers Nullisomic 2n - 2 Missing both copies of a homolog Monosomic 2n - 1 Missing one copy of a homolog Trisomic 2n + 1 Having an extra copy of one homolog Tetrasomic 2n + 2 Having two extra copies of one homolog n = haploid number of chromosomes 2n = diploid number of chromosomes

Causes of Aneuploidy 1.Loss 1.Loss of a chromosome that has a centromeric deletion 2.Loss 2.Loss of the small chromosome produced by Robertsonian translocation 3.Nondisjunction 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 MostMost common type is Down Syndrome: Trisomy 21 ArisesArises in two ways PrimaryPrimary Down Syndrome DueDue to nondisjunction in egg formation Egg with 2 copies of #21 #21 (24 chromosomes) + Sperm with 1 copy of (23 chromosomes) = Embryo with 3 copies of (47 chromosomes) FamilialFamilial Down Syndrome InheritedInherited 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 ConditionGenotypePhenotype TurnerSyndrome Klinefelter Syndrome Triple-X Female Sterile Female Sterile Male Fertile Female Fertile Male Taller XO XXY XXX XYY

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

Child with Turner Syndrome Non-functional Ovaries From Adult Female with Turner Syndrome Normal uterus, tubes and ovaries 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 XXY (47 chromosomes)

Klinefelter Syndrome Karyotype and Physical Features

X-Chromosome Inactivation in Females InactivationInactivation 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. TheThe arrow shows a Barr body, or inactivated X. number of Barr bodies equals the number of X chromosomes minus one. female male

Applying Your Knowledge Which term describes the chromosome number change for individuals with A. Down Syndrome? B. Turner Syndrome? 1.Nullisomic 2.Monosomic 3.Disomic 4.Trisomic

Applying Your Knowledge How many Barr bodies would be found in cells of individuals with A. Klinefelter Syndrome? B. Turner Syndrome? C. Triple-X Syndrome? Zero = fist OneTwoThree

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 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?