Chromosomes and karyotypes
Introduced| English | Meaning |
|---|---|
| karyotype | The number and appearance of a person's chromosomes. |
| autosomes | autosomes |
| meiosis/meɪˈəʊsɪs/ | A type of nuclear division that produces haploid nuclei with half the original chromosome number and creates genetic variation. |
| nondisjunction/ˌnɒndɪsˈdʒʌŋkʃn/ | Failure of chromosomes to separate at cell division, giving gametes with an extra or missing copy. |
| mosaicism | mosaicism |
| germline mosaicism | A mutation present in some of a parent's eggs or sperm but not their body cells. |
One chromosome too many
- About 1 baby in 700 is born with an extra copy of chromosome 21, the smallest human chromosome. That one extra copy changes the face, the heart and learning.
- The cause is a single mistake in how chromosomes are shared out when an egg or sperm is made, and the arithmetic of that mistake is exact.
The karyotype
- A human cell has 46 chromosomes in 23 pairs: 22 pairs of autosomes 常染色体 plus the sex chromosomes, XX or XY.
- A karyotype 核型 is the chromosome set photographed and counted at metaphase, when every chromosome is condensed and visible.
Meiosis: two divisions
- Meiosis 减数分裂 copies the DNA once and divides twice. Division I separates the two chromosomes of each pair; division II separates the two sister chromatids of each chromosome.
- The result is four haploid gametes with 23 chromosomes each.
Put the events of meiosis in order.
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One copy, two divisions; pairs part first, then sister chromatids.
When a pair fails to separate
- Nondisjunction 染色体不分离 at division I sends both chromosomes of a pair to one side, so all four gametes are abnormal: two have 24 chromosomes (n+1) and two have 22 (n−1).
- At division II only one sister pair sticks: one gamete n+1, one n−1 and two normal, a 1:1:2 ratio.
Nondisjunction at division I of meiosis gives which gametes?
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The whole pair goes to one side at division I, so both daughter cells are abnormal and so are all four gametes.
An egg with 24 chromosomes is fertilised by a normal sperm. How many chromosomes does the embryo have?
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24 + 23 = 47, a trisomy.
Which errors survive
- Most whole-chromosome errors end in early miscarriage. Trisomy 21 is the common survivor; about 95% of cases are a free extra 21, about 90% of those begin in the egg, mostly at division I, and the risk rises with maternal age.
- 45,X (Turner syndrome) is the only full monosomy that survives. 45,Y never does, because the X carries genes every cell needs.
A person with the karyotype 45,Y can survive because the Y carries the testis-determining gene.
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45,Y is never viable. The X carries genes that every cell needs; the Y cannot replace them.
Writing a karyotype
- The notation is a sentence: chromosome count, then sex chromosomes, then the change. 46,XX is a typical female; 47,XY,+21 is a male with trisomy 21.
- Extra sex chromosomes (47,XXY, 47,XYY, 47,XXX) are milder than extra autosomes: extra X chromosomes are switched off, and the Y carries few genes.
Match each karyotype to its meaning.
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Count, then sex chromosomes, then the change.
Mosaicism
- If the error happens in an early mitosis after fertilisation, the person has two cell lines, for example 46,XY and 47,XY,+21. This is mosaicism 嵌合现象, and the features are often milder.
- A parent can also carry an abnormal cell line only in the ovary or testis, germline mosaicism 生殖腺嵌合, so a condition can recur although both parents' blood karyotypes are normal.
A child with mild features of trisomy 21 has two cell lines, 46,XX and 47,XX,+21. When did the error happen?
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A meiotic error would put the extra 21 in every cell. Two cell lines in one person mean the error came after fertilisation.
Worked example: naming three karyotypes
- A boy with trisomy 21 is 47,XY,+21. A girl with a single X is 45,X. A tall man with an extra X is 47,XXY (Klinefelter syndrome).
- Read each one back aloud: count, sex chromosomes, change. If you cannot say it, you cannot write it.
Worked example: counting the sperm
- One primary spermatocyte makes four sperm. If one sister pair sticks at division II, one sperm has 24 chromosomes, one has 22 and two have 23.
- Fertilising a normal egg (23): 24 + 23 = 47, a trisomy; 22 + 23 = 45, a monosomy; 23 + 23 = 46, typical.
In one primary spermatocyte, one sister pair fails to separate at division II. How many of the four sperm are normal?
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The other secondary spermatocyte divides normally, giving two normal sperm; the faulty one gives one n+1 and one n−1.
Three common slips
- Mixing up the two divisions. Division I gives 2:2 (all abnormal); division II gives 1:1:2.
- "Down syndrome runs in families." It is usually a new error; the recurrence risk is about 1%, or the maternal-age risk if that is higher.
- "45,Y can survive." It never does: without an X, cells lack essential genes.
Recap
- 46 chromosomes = 22 autosome pairs + XX or XY, counted at metaphase.
- Nondisjunction at division I → 2 n+1 and 2 n−1; at division II → 1:1:2.
- Trisomy 21 (about 1 in 700) is mostly a maternal meiosis-I error; 45,X is the only viable monosomy.
- An error after fertilisation gives mosaicism and usually a milder picture.