1. Chromosomes and Karyotypes

2. The Human Chromosome Set
Chromosome analysis is a powerful and useful technique in human genetics
Human chromosomes exist in pairs, with most cells having 23 homologous pairs, or 46 chromosomes.
This is the diploid, 2n, number
Eggs and sperm (gametes), contain only one copy of each chromosome, which is haploid, or n number.

3. Chromosome Number
Chromosome number in selected organisms

4. Human Chromosomes
Replicated chromosomes at metaphase consist of sister chromatids joined by a single centromere

5. Types of Chromosomes Sex chromosomes Autosomes
In humans, the X and Y chromosomes are involved in sex determination.
These have different sizes and shapes
Autosomes
Chromosomes other than the sex chromosomes
In humans, chromosomes 1 to 22 are autosomes
Human chromosomes are analyzed by construction of karyotypes
Karyotype - A complete set of chromosomes from a cell that has been photographed during cell division and arranged in a standard sequence

6. Making a Karyotype
Allows any region to be identified by a descriptive address (chromosome number, arm, region, and band)
Different stains and dyes produce banding patterns specific to each chromosome
Karyotypes reveal variations in chromosomal structure and number
1959: Discovery that Down syndrome is caused by an extra copy of chromosome 21

7. A Human Karyotype

8. Karyogram: Chromosome Banding Patterns3 6 1 4 5 2 11 7 8 9 10 12 16 17 18 13 14 15 19 20 21 22 Y X

9. Metaphase Chromosomes (a) Arranged Into a Karyotype (b)

10. Information Obtained from a Karyotype
Number of chromosomes
Sex chromosome content
Presence or absence of individual chromosomes
Nature and extent of large structural abnormalities

11. Chromosome Painting
New techniques using fluorescent dyes generate unique patterns for each chromosome

12. Variations in Chromosome Number
Changes in chromosome number or chromosome structure can cause genetic disorders
A karyotype can detect
A change in chromosomal number
A change in chromosomal arrangement

13. Causes Nondisjunction
The failure of homologous chromosomes to separate properly during anaphase in meiosis.
Two cell divisions in meiosis, and nondisjunction can occur in either the first or second