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Tổ chức Baccalaureate Quốc tế · IB Diploma · Biology · SL Chủ đề 29 4:40 Giọng đọc tiếng Anh · phụ đề tiếng Anh + 中文 được ghi trực tiếp
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Gametes normally contain one set of chromosomes, while fertilization 受精 joins two sets.
配子通常含有一组染色体，而受精将两组染色体结合。
Maintaining chromosome number across generations requires both processes.
维持世代间的染色体数目需要这两个过程共同作用。
Meiosis reduces chromosome number when producing haploid cells from a diploid starting cell.
减数分裂在从二倍体起始细胞产生单倍体细胞时，会减少染色体数目。
Homologous chromosomes separate in meiosis one, while sister chromatids separate in meiosis two.
同源染色体在减数第一次分裂中分离，而姐妹染色单体在减数第二次分裂中分离。
Fertilization combines haploid gamete nuclei to form a diploid zygote.
受精作用将单倍体配子核结合，形成二倍体合子。
Sexual reproduction can create new combinations of existing alleles.
有性生殖可以产生现有等位基因的新组合。
Haploid means having one chromosome set in the stated life cycle.
单倍体指在所述生活史中具有一个染色体组。
Fertilization is the fusion of gamete nuclei to form a zygote.
受精作用是配子核融合形成合子的过程。
Distinguish homologous chromosomes from sister chromatids, and chromosome number from DNA quantity.
区分同源染色体与姐妹染色单体，以及染色体数目与DNA含量。
Replication before meiosis copies DNA without doubling the number of chromosome sets.
减数分裂前的复制会拷贝DNA，但不会使染色体组数加倍。
Crossing over and independent assortment contribute to genetic variation, while random fertilization further changes combinations.
交叉互换和自由组合导致遗传变异，而随机受精进一步改变基因组合。
These processes do not make every offspring genetically distinct under every conceivable condition.
这些过程并不能在所有设想条件下都使每个后代在遗传上互不相同。
In the lab: Use a labelled model organism with a stated chromosome number.
在实验室中，使用带有标签且染色体数目已明确说明的模式生物。
Track chromosome sets through replication, meiosis I, meiosis II and fertilization.
追踪染色体组在复制、减数分裂I、减数分裂II和受精过程中的变化。
Use anonymous model data rather than personal family or reproductive-health information.
使用匿名模型数据，而非个人家庭或生殖健康信息。
Compare organism life cycles carefully; flowering-plant and animal reproductive structures require their own additional teaching.
仔细比较不同生物的生命周期。 开花植物和动物的生殖结构需要单独进行额外教学。
Known: a model organism has diploid chromosome number 2n equals 12.
已知：某模式生物的二倍体染色体数为 2n = 12。
A gamete has n equals 6.
配子含有 n = 6 条染色体。
After fertilization, the zygote has 12 chromosomes.
受精后，合子含有 12 条染色体。
With six independently assorting chromosome pairs and ignoring crossing over, there are 2 to the 6th power, which equals 64 possible maternal or paternal chromosome combinations in a gamete.
在六对独立分配的染色体中忽略交叉互换的情况下，配子内可能存在的母源或父源染色体组合共有 2^6 = 64 种。
This is a model count, not a probability of an individual phenotype.
这是一个模型计数，并非个体表型的概率。
Your turn: A model organism has diploid chromosome number 20.
轮到你了。 某模式生物的二倍体染色体数目为20。
How many chromosomes are normally in a gamete?
其配子中通常含有多少条染色体？
Pause the video and work it out.
请暂停视频，自己算一算。
Check: a gamete is haploid, so it has half of 20, which is 10 chromosomes.
核对：配子是单倍体，20 的一半，即 10 条染色体。
A gamete is not made by halving the size of an ordinary cell.
配子并非通过减半普通细胞的体积而形成。
Independent assortment counts assume distinct homologous alternatives and do not include crossing-over outcomes.
独立分配计数假设存在不同的同源等位基因，且不包含交叉互换的结果。
Human and plant reproduction cannot be fully replaced by a monohybrid Punnett grid.
人类和植物的繁殖无法被单因子孟德尔棋盘格完全替代。
Not true: DNA replication alone changes a diploid cell into a tetraploid cell.
不正确。 仅靠DNA复制无法将二倍体细胞转变为四倍体细胞。
Replication doubles the DNA content but the chromosomes still count as one each (two sister chromatids); the cell remains diploid.
复制使DNA含量加倍，但染色体仍各计为一条（含两条姐妹染色单体），细胞保持二倍体状态。
Tetraploidy needs a failure of division, not replication.
四倍体的产生需要分裂失败，而非复制所致。
Meiosis reduces chromosome number when producing haploid cells from a diploid starting cell.
减数分裂在从二倍体起始细胞产生单倍体细胞时，会减少染色体数目。
Distinguish homologous chromosomes from sister chromatids, and chromosome number from DNA quantity.
请区分同源染色体与姐妹染色单体，以及染色体数目与DNA含量。
Watch for: A gamete is not made by halving the size of an ordinary cell.
注意：配子并非通过简单减半普通细胞的体积而形成。
A model organism has a diploid chromosome number of 46.
模式生物的二倍体染色体数目为46。
A gamete normally has half that number.
配子通常具有该数目的一半。
Replication alone does not turn a diploid cell into a tetraploid cell.
仅靠复制并不能使二倍体细胞转变为四倍体细胞。
Meiosis halves the chromosome number, so a gamete has 6.
减数分裂使染色体数目减半，所以配子有6条。
Fertilization joins two gametes and restores 12.
受精使两个配子结合，恢复为12条。
A model organism has 2n equal to 8.
某模型生物的 2n 等于 8。
Ignoring crossing over, how many chromosome combinations can a gamete receive?
不考虑交叉互换，一个配子可以得到多少种染色体组合？
Does this count predict a phenotype ratio?
这个数目能预测表型比例吗？
Four pairs give 2 to the power 4, which is 16 combinations under independent assortment.
四对染色体在自由组合下给出 2 的 4 次方、即 16 种组合。
Phenotype probabilities also need loci, dominance and linkage information; combinations are not phenotype categories.
表型概率还需要基因座、显隐性和连锁信息；染色体组合不等于表型类别。
Pause and check yourself. Explain each point on screen in your own words, then change one number in the last example and say which assumption must still hold.
暂停一下，自我检查：用自己的话解释屏幕上的每个要点，再改动最后一个例子中的一个数字，说明哪个假设仍然必须成立。