A karyotype describes the number and visible characteristics of chromosomes, while a karyogram is an arranged image of those chromosomes. They are used to classify chromosomes, identify chromosomal abnormalities, determine chromosomal sex and compare chromosomes between species.
This content is required at both IB Biology SL and HL in A3.1 Diversity of organisms.
| Term | Precise meaning |
|---|---|
| Karyotype | The number and visible characteristics of the chromosomes in a cell or organism |
| Karyogram | A photograph or diagram in which chromosomes are arranged into homologous pairs |
To produce a karyogram, chromosomes are usually observed when they are condensed and individually visible. They are paired and classified using three main features:
- Banding pattern: homologous chromosomes have corresponding light and dark bands.
- Length: chromosomes in a homologous pair are approximately the same length.
- Centromere position: the centromere occurs in a corresponding location on each homologous chromosome.
Once arranged, a karyogram can reveal changes in chromosome number. For example, trisomy 21 occurs when three copies of chromosome 21 are present and is associated with Down syndrome. It can also show some large structural changes, such as chromosome deletions or fusions, although small gene mutations cannot normally be detected.
Karyograms can additionally provide evolutionary evidence. In the IB syllabus, students evaluate the hypothesis that human chromosome 2 resulted from the fusion of two chromosomes corresponding to chromosomes 12 and 13 in a shared primate ancestor. The chromosome number therefore differs between humans, with 46 chromosomes, and chimpanzees, with 48.
A common misconception is that “karyotype” and “karyogram” mean exactly the same thing. Strictly, the karyotype is the chromosome complement and its characteristics; the karyogram is its arranged visual representation.
Exam technique: When asked to classify chromosomes, refer explicitly to banding pattern, length and centromere position. When interpreting a karyogram, count chromosomes carefully and identify the specific pair affected rather than merely stating that an abnormality exists.
