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1.6.3Inquiry Process

Karyograms: Skills — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 17 of 160

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Part 1 of 5Making a Karyogram

1.6.3 Karyograms: Skills

A karyogram is an organised picture of all the chromosomes of one cell.

1
Cells are collected, for example from amniotic fluid or blood, and encouraged to divide in culture.
2
A chemical such as colchicine stops the cells in metaphase, when chromosomes are most condensed.
3
The cells are burst, stained and photographed under the microscope.
4
The chromosomes in the picture are cut out or arranged by computer into homologous pairs.
5
Pairs are sorted by size, centromere position and banding pattern, with the sex chromosomes placed last.

Part 2 of 5Reading a Karyogram

Pairing
  • Homologous chromosomes are matched by length, centromere position and the pattern of dark bands.
Numbering
  • Human autosomes are numbered 1 to 22 in descending size, with the sex chromosomes shown as pair 23.
Sex
  • Two X chromosomes indicate a female, while one X and one Y indicate a male.
Abnormal number
  • Any pair with three chromosomes, or a missing chromosome, shows an error of chromosome number.
Down syndrome
  • Three copies of chromosome 21, trisomy 21, leads to 47 chromosomes and characteristic developmental features.

Part 3 of 5Uses of Karyograms

Uses of Karyograms
Prenatal diagnosis
  • Cells from the fetus, obtained by amniocentesis or chorionic villus sampling, are tested for abnormal chromosome number.
Sex determination
  • The karyogram gives certain information on the sex chromosomes of an individual.
Evolution
  • Comparing the karyograms of related species shows fused or rearranged chromosomes.
Limits
  • Karyograms detect only large changes in number or structure, not single gene mutations.
KaryotypeDescription
46,XXNormal female
46,XYNormal male
47,XY,+21Male with Down syndrome
45,XFemale with Turner syndrome

Part 4 of 5Worked Example

Question

  • A karyogram of a male patient shows 47 chromosomes, with three copies of the chromosome 21 and one X and one Y.
  • State the sex and describe the condition.

  1. Count the sex chromosomesone X and one Y means the patient is male.
  2. Normal diploid number is 46, so 47 shows one extra chromosome.
  3. Three copies of chromosome 21 is trisomy 21, which is Down syndrome.
  4. The condition arises from non-disjunction in meiosis in a parent.

Answer Male with Down syndrome (trisomy 21), written 47,XY,+21.

Part 5 of 5Exam Checklist & Tips

1.6.3 – Key Points for the Exam

  • Karyogram An image of the chromosomes of a cell arranged in homologous pairs.
  • Non-disjunction Failure of chromosomes to separate during meiosis.
  • Trisomy Having three copies of one chromosome.
  • Centromere The constriction where sister chromatids join.

Common Mistakes

  • Saying a karyogram is taken from a cell in interphase; chromosomes are visible at metaphase.
  • Confusing trisomy 21 with having 21 chromosomes.
  • Believing karyograms can find single base mutations.
  • Ordering chromosomes by colour rather than size, centromere position and banding.

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