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.
Understanding Biology: Karyotypes and Chromosomes
Chromosomes are not normally visible as neat rods inside a living cell. For most of the cell cycle, DNA is spread out as loose chromatin in the nucleus. This loose form helps the cell read genes and make proteins.
Before cell division, every chromosome is copied. Each copied chromosome has two identical sister chromatids joined at a centromere. During metaphase, the DNA coils tightly.
This makes chromosomes easier to stain and separate in an image. A karyotype therefore shows chromosomes at a particular moment in cell division, not their usual working appearance.
Scientists identify matching chromosomes by more than length. Each chromosome has a centromere in a characteristic position, creating a short arm and a long arm. Stains produce light and dark bands along the chromosome.
The band pattern acts like a map. It helps a laboratory worker tell chromosome 7 from chromosome 8, even when their sizes are similar. Banding can reveal a deletion, where a segment is missing, or a duplication, where a segment is repeated.
It can sometimes reveal a translocation, where a piece has attached to a different chromosome. Such changes may alter gene dosage or interrupt a gene at the joining point.
Extra or missing chromosomes often begin when cells make eggs or sperm. In meiosis, chromosome pairs must separate so each gamete receives one member of every pair. If a pair does not separate properly, a process called nondisjunction, one gamete can receive an extra chromosome while another receives none.
After fertilization, the resulting cells may carry the unusual number. The chance of some nondisjunction events rises with increasing maternal age, though chromosome differences can occur in any pregnancy.
A change can be present in every cell or only some cells. When two or more cell groups with different chromosome sets occur in one person, the condition is called mosaicism.
A karyotype has important limits. It is strong at finding whole chromosome gains, losses, and large rearrangements. It usually cannot detect a tiny DNA change within one gene.
A person can have a genetic condition despite a normal-looking karyotype. Other tests, including chromosome microarrays or DNA sequencing, can examine smaller changes. When studying karyotypes, keep three levels separate.
Chromosome number concerns how many whole chromosomes are present. Chromosome structure concerns the arrangement of chromosome pieces. Gene variants concern changes in the DNA sequence itself.
These levels can influence one another, but they require different tools and explanations. In real medical settings, results are interpreted with family history, symptoms, and other tests rather than used alone.
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.