A karyotype is an organized picture of a cell's chromosomes, usually arranged from largest to smallest and grouped into matching pairs. It lets scientists see the number, size, and shape of chromosomes in a person or organism. In humans, a typical body cell has 46 chromosomes, which are arranged as 23 pairs.
Karyotypes matter because changes in chromosome number or structure can affect growth, development, fertility, and health.
To make a karyotype, cells are collected, stimulated to divide, stopped during metaphase, stained, photographed, and arranged into pairs. The first 22 pairs are autosomes, and the 23rd pair is the sex chromosomes, usually XX or XY. Homologous chromosomes have the same genes in the same locations, but they may carry different versions of those genes.
By comparing chromosome pairs, scientists can detect disorders such as trisomy 21, in which a person has three copies of chromosome 21.
Key Facts
- A typical human body cell has 46 chromosomes arranged in 23 pairs.
- Human karyotype notation uses total chromosome number plus sex chromosomes, such as 46,XX or 46,XY.
- Autosomes are chromosome pairs 1 through 22, and sex chromosomes are pair 23.
- A homologous pair contains one chromosome inherited from the mother and one from the father.
- Trisomy means three copies of one chromosome, such as trisomy 21: 47,XX,+21 or 47,XY,+21.
- Diploid chromosome number is written as 2n = 46 in humans, while gametes are haploid with n = 23.
Vocabulary
- Karyotype
- A karyotype is an ordered display of an organism's chromosomes arranged by size, shape, and banding pattern.
- Chromosome
- A chromosome is a long DNA molecule packaged with proteins that carries many genes.
- Homologous pair
- A homologous pair is a pair of chromosomes with the same genes in the same positions, one inherited from each parent.
- Autosome
- An autosome is any chromosome that is not a sex chromosome.
- Trisomy
- Trisomy is a chromosome condition in which a cell has three copies of a particular chromosome instead of two.
Common Mistakes to Avoid
- Counting chromatids as chromosomes, which is wrong because a duplicated chromosome still counts as one chromosome until sister chromatids separate.
- Calling every chromosome pair identical, which is wrong because homologous chromosomes have the same gene locations but can carry different alleles.
- Mixing up autosomes and sex chromosomes, which is wrong because autosomes are pairs 1 through 22 and the sex chromosomes form pair 23.
- Assuming a normal karyotype proves a person has no genetic disease, which is wrong because many gene mutations are too small to see on a karyotype.
Practice Questions
- 1 A karyotype shows 22 pairs of autosomes and two X chromosomes. Write the standard karyotype notation and state the biological sex usually associated with it.
- 2 A cell has 47 chromosomes, including three copies of chromosome 21 and XY sex chromosomes. Write the karyotype notation and name the chromosomal condition.
- 3 Explain why karyotypes are usually made from cells stopped in metaphase rather than from nondividing cells.