Chromosomes and karyotypes
הוצג| English | משמעות |
|---|---|
| karyotype | The chromosome set photographed and counted at metaphase, when every chromosome is condensed and visible. |
| 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/ | nondisjunction |
| mosaicism | mosaicism |
| germline mosaicism | germline mosaicism |
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.
רמת קושי 3/5 · 200 מטבעות + בונוס קטן בהשלמה הראשונה היום
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?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
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?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
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.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
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.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
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?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
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?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
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.