1. Chromosomal Mutations
Changes in Chromosome Number or Structure

2. Alterations in Chromosome Number
Polyploidy: one or more extra sets of chromosomes
Aneuploidy: gain or loss of one chromosome or a small number of chromosomes

3. Aneuploidy Arises by Non-disjunction
Non-disjunction = failure of homologues or chromatids to separate during meiosis
Normal
Meiosis
Non-disjunction
in Meiosis I
in Meiosis II

4. Human Chromosomal Aneuploids
Autosomal Aneuploids
Down Syndrome
Trisomy 21
Edward Syndrome
Trisomy 18
Patau Syndrome
Trisomy 13
Trisomy: three copies of one chromosome

5. Human Autosomal Abnormality
Down Syndrome
Trisomy 21 Three copies of chromosome 21
How can Down Syndrome occur?
Eg. Egg with 2 copies of #21 (24 chromosomes)
+ Sperm with 1 copy of #21 (23 chromosomes)
= Embryo with 3 copies of #21 (47 chromosomes)

6. Karyotype for Down Syndrome
Physical Features
Eye fold
Palm Crease

7. Incidence of Down Syndrome Increases with Maternal Age
100
200
300
400 10 20 30 40 50
Number per 1000 Births
Figure: FIGURE 12.20
Title:
Down syndrome frequency increases with maternal age
Caption:
The increase in frequency of Down syndrome after maternal age 35 is quite dramatic.
Age of Mother (years)

8. Human Chromosomal Aneuploids
Sex Chromosome Aneuploids
Turner Syndrome
45, XO
Triplo-X
47, XXX
Klinefelter Syndrome
47, XXY
XYY Syndrome
47, XYY
Sterile female
Fertile female
Sterile male
Fertile male

9. Human Sex Chromosome Abnormality
Turner Syndrome XO
One copy of X
No second sex chromosome
How can Turner Syndrome occur?
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)

10. Karyotype for Turner’s Syndrome
Non-functional Ovaries From Adult Female with Turner’s Syndrome
Normal uterus, tubes
and ovaries

11. Human Chromosomal Aneuploids
How can XYY Syndrome occur?
One Copy of the X chromosome
Two Copies of the Y chromosome
Eg. Egg with 1 copy of X (23 chromosomes) Sperm with 2 copies of Y (24 chromosomes) = Embryo with XYY (47 chromosomes)

12. Chromosome Structure Changes
Description
Deletion
Loss of a chromosomal segment
Duplication
Repeat of a chromosomal segment
Translocation
Movement of chromosomal segment to non-homologous chromosome
Inversion
Reversal of a chromosomal segment (rotated 180o)

13. Chromosome Deletion in Humans
Cri-du-chat syndrome
is correlated with
a deletion at the end
of chromosome 5.

14. Chromosome Duplication in Humans
Small duplications in chromosome 15 cause no symptoms
Large duplication (with inversion) causes seizures and mental retardation

15. Chromosome Translocation in Humans
Reciprocal Translocation involves exchange between two non-homologous chromosomes
Reciprocal translocation between chromosomes 2 and 20 causes Alagille Syndrome

16. Chromosome Translocation in Humans
Robertsonian Translocation involves a fusion of the long arms of two different chromosomes
Translocation Down Syndrome involves a Robertsonian Translocation between chromosomes 14 and 21

17. Chromosome Inversions Lead to Unbalanced Meiotic Products
A paracentric inversion does not include the centromere
A pericentric inversion includes the centromere