Interpret A Karyotype
Count chromosomes in a karyotype to spot a normal set (46), an extra chromosome (trisomy), or a missing one.
- Chromosomes come in pairs: A normal human karyotype has 23 pairs (46 total), so each numbered slot should hold two.
- Count and compare: Reading a karyotype means counting how many chromosomes sit in each numbered pair.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Interpret the count in a slot: two chromosomes is a normal pair, three is a trisomy (an extra chromosome), and one is a missing chromosome.
A karyotype key says chromosomes should be in pairs. One numbered slot shows three copies of the same chromosome. What does that suggest?
Approved| What you see in the karyotype | What it means |
|---|---|
| 23 matched pairs, 46 chromosomes total | A normal human set |
| Two chromosomes in a numbered slot | A normal pair for that number |
| Three chromosomes in one numbered slot | Trisomy - one extra chromosome (for example, trisomy 21) |
| Only one chromosome in a numbered slot | Monosomy - a missing chromosome |
| The last pair (sex chromosomes) | XX = female, XY = male |
- A.Trisomy: an extra chromosome (for example, trisomy 21)
- B.A normal pair
- C.Monosomy: a missing chromosome
- D.A normal sex-chromosome pair
Show the worked solution ▾
Answer: A. Trisomy: an extra chromosome (for example, trisomy 21)
- Step 1: Compare to the pair: A normal slot has two chromosomes. This slot has three, which is one more than expected.
- Step 2: Name it from the table: The table says three chromosomes in one slot is a trisomy: an extra chromosome, like trisomy 21.
Why it's right: Three copies in one numbered slot is one more than the normal pair, which the table defines as a trisomy (an extra chromosome).
- B: A normal pair is two chromosomes; three is one too many.
- C: Monosomy is one chromosome (missing one), the opposite of an extra one.
- D: The sex-chromosome pair (XX or XY) is two chromosomes, not three.
Aligned to BRE: interpret chromosome number (trisomy) · reading level ~grade 9
- In Unit 2.2 to 2.3 Genetic Risk, this skill turns class evidence into a result another person can check.
Fill these in as you work through the lesson.
- Karyotype (a sorted picture of all the chromosomes):
- Chromosome (a packaged length of DNA):
- Pair (the two chromosomes of one number):
- Trisomy (three copies (one extra) of a chromosome):
- Monosomy (one copy (one missing) of a chromosome):
- Sex chromosomes (XX female, XY male):
A normal human karyotype has chromosomes in pairs. Three copies in one numbered slot is a . Only one copy in a slot is a .
- How many chromosomes are in a normal human karyotype, and in how many pairs?
- What is it called when one numbered slot holds three chromosomes instead of two?
- Which pair tells you whether the karyotype is XX or XY?
A karyotype shows three chromosomes in slot number 21 and two in every other slot. Name what this shows. (Answer: trisomy 21: an extra copy of chromosome 21.)