1.6.3Inquiry Process
Karyograms: Skills — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 17 of 160
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Making a Karyogram
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.
Section 2 of 5Reading a Karyogram
Pairing
- Pairing Homologous chromosomes are matched by length, centromere position and the pattern of dark bands.
Numbering
- Numbering Human autosomes are numbered 1 to 22 in descending size, with the sex chromosomes shown as pair 23.
Sex
- Sex Two X chromosomes indicate a female, while one X and one Y indicate a male.
Abnormal number
- Abnormal number Any pair with three chromosomes, or a missing chromosome, shows an error of chromosome number.
Down syndrome
- Down syndrome Three copies of chromosome 21, trisomy 21, leads to 47 chromosomes and characteristic developmental features.
Section 3 of 5Uses of Karyograms
Uses of Karyograms
Prenatal diagnosis
- Prenatal diagnosis Cells from the fetus, obtained by amniocentesis or chorionic villus sampling, are tested for abnormal chromosome number.
Sex determination
- Sex determination The karyogram gives certain information on the sex chromosomes of an individual.
Evolution
- Evolution Comparing the karyograms of related species shows fused or rearranged chromosomes.
Limits
- Limits Karyograms detect only large changes in number or structure, not single gene mutations.
| Karyotype | Description |
|---|---|
| 46,XX | Normal female |
| 46,XY | Normal male |
| 47,XY,+21 | Male with Down syndrome |
| 45,X | Female with Turner syndrome |
Section 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.
- Count the sex chromosomesone X and one Y means the patient is male.
- Normal diploid number is 46, so 47 shows one extra chromosome.
- Three copies of chromosome 21 is trisomy 21, which is Down syndrome.
- The condition arises from non-disjunction in meiosis in a parent.
Answer Male with Down syndrome (trisomy 21), written 47,XY,+21.
Section 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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