1. Changes in Chromosome Number

2. 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

3. 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

4. Causes of Aneuploidy
Loss of a chromosome that has a centromeric deletion
Loss of the small chromosome produced by Robertsonian translocation
Nondisjunction = failure of homologs to separate during meiosis or chromatids to separate during meiosis or mitosis

5. Nondisjunction

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

7. 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

8. Down Syndrome Karyotype and Physical Features
Eye fold
Palm Crease

9. Incidence of Primary Down Syndrome Increases with Maternal Age

10. Robertsonian Translocation Carrier of Down Syndrome
Translocation of 21q to 15q

11. Chromosome Segregation in a Translocation Carrier
Expected Result: 1/3 carrier + 1/3 normal + 1/3 Down Syndrome

12. Human Sex Chromosome Aneuploidy
Condition
Genotype
Phenotype
Turner
Syndrome
Klinefelter Syndrome
Triple-X Female XO
XXY
XXX
XYY

13. 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)

14. Turner Syndrome Karyotype and Physical Features

15. Non-functional Ovaries
From Adult Female with
Turner Syndrome
Child with
Turner Syndrome
Normal uterus, tubes
and ovaries

16. 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)

17. Klinefelter Syndrome Karyotype and Physical Features

18. 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

19. 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

20. Types of Polyploids Autopolyploid
Multiple chromosome sets from within one species
Allopolyploid
Multiple chromosome sets from different species

21. Polyploidy Arises by Nondisjunction

22. Chromosome Segregation in Polyploids Leads to Unbalanced Gametes

23. Production of
modern wheat