Changes in Chromosome Changes in Chromosome Number Number

Chromosome Number MutationsChromosome Number Mutations

Type of MutationType of Mutation Difference from wild type Difference from wild type chromosome number chromosome number

AneuploidyAneuploidy Abnormal number of Abnormal number of individual chromosomesindividual chromosomes(addition or loss of a small number (addition or loss of a small number

of chromosomes)of chromosomes)

PolyploidyPolyploidy Abnormal number of Abnormal number of chromosome sets chromosome sets

Euploid = organism with multiples of the number Euploid = organism with multiples of the number of chromosomes in a basic set of chromosomes in a basic set

Aneuploid NumbersAneuploid Numbers

Nullisomic Nullisomic 2n - 22n - 2 Missing both copies of Missing both copies of a homologa homolog

MonosomicMonosomic 2n - 12n - 1 Missing one copy of a Missing one copy of a homologhomolog

TrisomicTrisomic 2n + 12n + 1 Having an extra copy Having an extra copy of one homologof one homolog

TetrasomicTetrasomic 2n + 22n + 2 Having two extra Having two extra copies of one homologcopies of one homolog

n = haploid number of chromosomesn = haploid number of chromosomes2n = diploid number of chromosomes2n = diploid number of chromosomes

Causes of AneuploidyCauses of Aneuploidy

1.1. Loss of a chromosome that has a Loss of a chromosome that has a centromeric deletioncentromeric deletion

2.2. Loss of the small chromosome produced Loss of the small chromosome produced by Robertsonian translocationby Robertsonian translocation

3.3. Nondisjunction = failure of homologs to Nondisjunction = failure of homologs to separate during meiosis or chromatids to separate during meiosis or chromatids to separate during meiosis or mitosisseparate during meiosis or mitosis

Nondisjunction

Aneuploidy in Plants: Seed Aneuploidy in Plants: Seed Capsule Changes Due to Capsule Changes Due to Trisomy in Jimson WeedTrisomy in Jimson Weed

Human Autosomal AneuploidyHuman Autosomal Aneuploidy

•Most common type is Down Syndrome: Trisomy 21Most common type is Down Syndrome: Trisomy 21

•Arises in two waysArises in two ways•Primary Down SyndromePrimary Down Syndrome

•Due to nondisjunction in egg formation Due to nondisjunction in egg formation • Egg with 2 copies of #21Egg with 2 copies of #21 (24 chromosomes)(24 chromosomes)

+ Sperm with 1 copy of #21+ Sperm with 1 copy of #21 (23 chromosomes)(23 chromosomes)

= Embryo with 3 copies of #21= Embryo with 3 copies of #21 (47 chromosomes)(47 chromosomes)

•Familial Down SyndromeFamilial Down Syndrome•Inherited from a carrier of a Robertsonian Inherited from a carrier of a Robertsonian Translocation involving chromosome 21Translocation involving chromosome 21

Down Syndrome Down Syndrome Karyotype and Karyotype and

Physical FeaturesPhysical Features

Eye foldEye fold Palm CreasePalm Crease

Incidence of Primary Down Syndrome Incidence of Primary Down Syndrome Increases with Maternal AgeIncreases with Maternal Age

Robertsonian Translocation Carrier Robertsonian Translocation Carrier of Down Syndromeof Down Syndrome

Translocation of

21q to 15q

Chromosome Segregation Chromosome Segregation in a Translocation Carrier in a Translocation Carrier

Expected Result: 1/3 carrier + 1/3 normal + 1/3 Down SyndromeExpected Result: 1/3 carrier + 1/3 normal + 1/3 Down Syndrome

Human Sex Chromosome AneuploidyHuman Sex Chromosome Aneuploidy

ConditionCondition GenotypeGenotype PhenotypePhenotype

TurnerTurner

SyndromeSyndrome

Klinefelter Klinefelter SyndromeSyndrome

Triple-X Triple-X FemaleFemale

XOXO

XXYXXY

XXXXXX

XYYXYY

Human Sex Chromosome Human Sex Chromosome AneuploidyAneuploidy

Turner SyndromeTurner Syndrome

One Copy of X chromosomeOne Copy of X chromosome No second sex chromosomeNo second sex chromosome

Eg. Egg with 0 copies of XEg. Egg with 0 copies of X (22 chromosomes)(22 chromosomes) +Sperm with 1 copy of X+Sperm with 1 copy of X (23 chromosomes)(23 chromosomes)

= Embryo with 1 copy of X= Embryo with 1 copy of X (45 chromosomes)(45 chromosomes)

Turner Syndrome Turner Syndrome Karyotype and Karyotype and

Physical FeaturesPhysical Features

Child with Child with Turner SyndromeTurner Syndrome

Non-functional OvariesNon-functional OvariesFrom Adult Female with From Adult Female with Turner SyndromeTurner Syndrome

Normal uterus, tubesNormal uterus, tubes and ovariesand ovaries

Human Sex Chromosome Human Sex Chromosome AneuploidyAneuploidy

Klinefelter’s SyndromeKlinefelter’s Syndrome

Two Copies of the X chromosomeTwo Copies of the X chromosome One Copy of the Y chromosomeOne Copy of the Y chromosome

Eg. Egg with 2 copies of XEg. Egg with 2 copies of X (24 chromosomes)(24 chromosomes) + Sperm with 1 copy of Y+ Sperm with 1 copy of Y (23 chromosomes)(23 chromosomes)

= Embryo with XXY= Embryo with XXY (47 chromosomes)(47 chromosomes)

Klinefelter Syndrome Klinefelter Syndrome Karyotype and Karyotype and

Physical FeaturesPhysical Features

X-Chromosome Inactivation in FemalesX-Chromosome Inactivation in Females

•Inactivation of one of the X Inactivation of one of the X chromosomes in each cell of an chromosomes in each cell of an adult female balances the adult female balances the sex chromosome/autosome ratio. sex chromosome/autosome ratio.• Either the maternal or paternal Either the maternal or paternal chromosome is inactivated. chromosome is inactivated. •The arrow shows a Barr body, or The arrow shows a Barr body, or inactivated X. inactivated X. •The number of Barr bodies The number of Barr bodies equals the number of equals the number of X chromosomes minus one. X chromosomes minus one.

femalefemale

malemale

Polyploid NumbersPolyploid Numbers

Three chromosome setsThree chromosome sets TriploidTriploid

Four chromosome setsFour chromosome sets TetraploidTetraploid

Five chromosome setsFive chromosome sets PentaploidPentaploid

Six chromosome setsSix chromosome sets HexaploidHexaploid

Polyploid = euploid organism with more thanPolyploid = euploid organism with more than two sets of chromosomestwo sets of chromosomes

Types of PolyploidsTypes of Polyploids

AutopolyploidAutopolyploid Multiple chromosome Multiple chromosome sets from within one sets from within one speciesspecies

Allopolyploid Allopolyploid Multiple chromosome Multiple chromosome sets from different sets from different speciesspecies

Polyploidy Arises by NondisjunctionPolyploidy Arises by Nondisjunction

Chromosome Segregation in Polyploids Chromosome Segregation in Polyploids Leads to Unbalanced GametesLeads to Unbalanced Gametes

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