Chromosomes and Karyotypes

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.

Chromosome Number Chromosome number in selected organisms

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

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

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

A Human Karyotype

Karyogram: Chromosome Banding Patterns 3 6 1 4 5 2 11 7 8 9 10 12 16 17 18 13 14 15 19 20 21 22 Y X

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

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

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

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

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