Genetics
A-Level Biology Topic 16 9:35 English narration · English + 中文 subtitles burned in
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You have twenty three pairs of chromosomes.
你有二十三对染色体。
When a gamete is made, each pair lines up independently — either way round — so the number of different gametes you can make is two to the power of twenty three.
制造配子时，每一对都独立排列——两种朝向都可能—— 所以你能造出的不同配子数是二的二十三次方。
That is over eight million, from the line-up alone.
仅仅靠排列方式，就超过八百万种。
Then crossing over swaps pieces between the pair, and then any one gamete can meet any other at fertilisation.
然后交叉互换又在这一对之间交换片段，接着在受精时任何一个配子都可能遇上任何另一个。
That is why no two siblings are ever the same, and it all comes out of one kind of cell division.
这就是为什么两个兄弟姐妹从来不会一样，而这一切都来自一种细胞分裂。
Here is that division.
这就是那种分裂。
Watch one cell with a full double set of chromosomes divide twice in a row.
看着一个带有完整双套染色体的细胞连续分裂两次。
The first division separates the pairs.
第一次分裂把成对的染色体分开。
The second division separates the chromatids.
第二次分裂把姐妹染色单体分开。
What comes out at the end is four cells, each with half the chromosome number of the one you started with.
最后出来的是四个细胞，每个的染色体数都是起始细胞的一半。
Now the language of genetics.
现在说术语。
Sort a cell's chromosomes into their pairs and you have a karyotype. A diploid cell, written two n, has two full sets — one set from each parent.
二倍体细胞，写作 2n，有两套完整的染色体——每个亲本各给一套。
A haploid cell, written n, has just one set, and a gamete must be haploid.
单倍体细胞，写作 n，只有一套，而配子必须是单倍体。
If gametes were diploid, the chromosome number would double every generation.
如果配子是二倍体，染色体数每一代都会翻倍。
Chromosomes come in homologous pairs: the two are the same length and carry the same genes at the same positions, though they may carry different alleles.
染色体是成对的同源染色体：两条长度相同，在相同位置上带有相同的基因， 但携带的等位基因可能不同。
Meiosis has two divisions, one straight after the other, giving eight named stages from prophase one through to telophase two.
减数分裂有两次分裂，一次紧接着一次， 从前期 I 一直到末期 II 共八个命名的阶段。
The first division separates the homologous pair.
第一次分裂把同源染色体对分开。
The second separates the chromatids.
第二次把染色单体分开。
So one diploid cell gives four haploid gametes.
所以一个二倍体细胞产生四个单倍体配子。
Meiosis does not just halve the number — it makes every gamete genetically different, and there are three separate sources.
减数分裂不只是把数目减半——它让每个配子都不同，而来源有三个，彼此独立。
Crossing over: while the homologous chromosomes are paired up, they swap matching pieces at a point called a chiasma, which mixes the alleles into new combinations. The nuclear envelope breaks down and a spindle forms, as in mitosis.
交叉互换：当同源染色体配对在一起时，它们在一个叫交叉点的位置交换相配的片段， 把等位基因混成新的组合。
Independent assortment: the pairs line up on the spindle in a completely random order, so each gamete gets a random mix of the mother's and father's chromosomes.
自由组合：各对染色体在纺锤体上完全随机地排列， 所以每个配子都得到母方和父方染色体的随机混合。
And then at fertilisation, any one gamete can fuse with any other.
再然后，在受精时，任何一个配子都可以与任何另一个融合。
Name at least the first two — a question asking how meiosis creates variation wants both, not one.
至少要说出前两个——问减数分裂如何产生变异的题目，两个都要，不能只写一个。
These words have exact meanings, and the marks depend on using them exactly.
这些词都有确切的含义，得分与否就取决于用得准不准。
A gene is a length of DNA that codes for a protein, and its position on the chromosome is its locus.
基因是一段编码蛋白质的 DNA，它在染色体上的位置叫基因座。
An allele is one version of a gene.
等位基因是一个基因的一个版本。
A dominant allele shows its effect with only one copy present; a recessive allele needs two copies to show; and codominant alleles both show their effect together.
显性等位基因只要有一份拷贝就表现出来； 隐性等位基因需要两份拷贝才表现；共显性等位基因则两者一起表现。
The genotype is the alleles the organism has; the phenotype is the features you can actually see.
基因型是这个生物所拥有的等位基因；表现型是你实际能看到的特征。
Homozygous means the two alleles are the same, heterozygous means they are different.
纯合意味着两个等位基因相同，杂合意味着它们不同。
And a test cross means crossing an organism with the recessive homozygote, to find out an unknown genotype.
测交是指把一个生物与隐性纯合子杂交，以查明未知的基因型。
Here is a monohybrid cross being worked through in a Punnett square.
这是一个杂交的完整过程。
Two heterozygous parents each make two kinds of gamete.
两个杂合亲本各自产生两种配子。
Combine every gamete from one with every gamete from the other, and you get four equally likely results.
把一方的每种配子与另一方的每种配子组合起来，就得到四个可能性相同的结果。
Count the phenotypes among those four and the famous ratio falls straight out.
数一数这四个当中的表现型，那个著名的比例就直接出来了。
Marks are lost here by rushing, so set it out in four steps every time: parental genotypes, then gametes in circles — the examiner looks for that — then the Punnett square, then the offspring ratio and phenotypes.
这里最容易因为赶时间而丢分，所以每次都要分四步写清楚。 写出亲本基因型。
Track two genes at once and it is a dihybrid cross, with sixteen boxes.
写出配子，并且把它们圈起来——阅卷人就找这个。
Take a heterozygous cross, big T little t crossed with big T little t.
画出方格本身。 然后读出比例，并说明表现型是什么。
The gametes are big T and little t from each parent.
拿一个杂合杂交来说，大写T小写t 与 大写T小写t 杂交。
Fill it in and you get big T big T, big T little t, big T little t, and little t little t.
两个亲本产生的配子都是大写T和小写t。 填进去就得到 大写T大写T、大写T小写t、大写T小写t，还有小写t小写t。
Three of those four have at least one dominant allele, so three are tall and one is short — the classic three to one.
这四个里有三个至少带一个显性等位基因，所以三个高、一个矮——经典的三比一。
Now, ratios often do not come out as three to one, and there are five usual reasons.
比例常常不是三比一，通常有五个原因。
Codominance: both alleles show, so you get a third phenotype rather than a blend.
共显性：两个等位基因都表现出来，所以你得到第三种表现型，而不是混合。
Multiple alleles: more than two versions exist in the population, as with human blood groups.
复等位基因：群体中存在多于两个版本，比如人的血型。
Sex linkage: the gene sits on the X chromosome, so the result differs between sons and daughters.
伴性遗传：基因位于 X 染色体上，所以儿子和女儿的结果不同。
Linkage means two genes are on the same autosome, so they tend to be inherited together instead of assorting independently.
连锁是指两个基因在同一条常染色体上，所以它们倾向于一起遗传，而不是自由组合。
And epistasis: one gene changes how another gene is shown.
上位性：一个基因改变另一个基因的表现方式。
If a ratio does not come out as you expected, one of these five is usually the reason.
如果比例不像你预期的那样，通常就是这五个当中的一个在起作用。
So how do you tell whether a difference from the expected ratio matters?
那么怎么判断与预期比例的差异是否重要呢？
You use the chi-squared test.
用卡方检验。
Here is this formula: for every group, take the observed number minus the expected number, square it, divide by the expected number, and add all of those up.
公式是这样的：对每一组，用观察值减去期望值，平方，再除以期望值，然后把所有这些加起来。
The observed numbers are what you actually counted.
观察值是你实际数出来的数目。
The expected numbers come from the ratio your genetic diagram predicts.
期望值来自你的遗传图解所预测的比例。
Then work out the degrees of freedom, which is the number of groups minus one, and use it to look up a critical value.
然后算出自由度，也就是组数减一，用它去查临界值。
Now compare it.
接着比较。
If your chi-squared is less than the critical value, the difference is not significant.
如果你的卡方值小于临界值，差异就不显著—— 结果符合这个比例，差异只是偶然。
If it is greater, something else is going on.
如果大于临界值，差异就是显著的，说明另有原因。
Let us do one.
我们来算一个。
A heterozygous cross gives a hundred and sixty offspring: a hundred and fourteen tall, and forty six short.
一次杂合杂交得到一百六十个后代：一百一十四个高，四十六个矮。
Does that fit three to one?
这符合三比一吗？
First the expected numbers.
先算期望值。
Three quarters of a hundred and sixty is a hundred and twenty tall, and one quarter is forty short.
一百六十的四分之三是一百二十个高，四分之一是四十个矮。
Put them in the formula.
把它们代入公式。
A hundred and fourteen minus a hundred and twenty, squared, over a hundred and twenty, is zero point three.
一百一十四减一百二十，平方，除以一百二十，等于零点三。
Forty six minus forty, squared, over forty, is zero point nine.
四十六减四十，平方，除以四十，等于零点九。
Add them and chi-squared is one point two.
加起来卡方值是一点二。
There are two groups, so degrees of freedom is one, and the critical value at p equals nought point nought five is three point eight four.
一共两组，所以自由度是一，而 p 等于零点零五时的临界值是三点八四。
One point two is less than three point eight four, so the difference is not significant.
一点二小于三点八四，所以差异不显著。
The results do fit three to one.
结果确实符合三比一。
Why does an allele change what you see?
为什么一个等位基因会改变你看到的东西？
Because a gene codes for a protein, and that protein does a job.
因为基因编码蛋白质，而蛋白质要做一份工作。
So a faulty gene makes a faulty protein, and the phenotype changes.
所以有缺陷的基因造出有缺陷的蛋白质，表现型就变了。
TYR codes for the enzyme tyrosinase, and a faulty allele gives albinism, because no pigment is made.
TYR 基因编码酪氨酸酶，有缺陷的等位基因导致白化病，因为造不出色素。
HBB codes for haemoglobin, and a faulty allele gives sickle cell anaemia.
HBB 编码血红蛋白，有缺陷的等位基因导致镰状细胞贫血。
F8 codes for factor eight, which helps clotting, and a faulty allele gives haemophilia.
F8 编码凝血因子八，它帮助血液凝固，有缺陷的等位基因导致血友病。
HTT codes for huntingtin, and a faulty allele gives Huntington's disease.
HTT 编码亨廷顿蛋白，有缺陷的等位基因导致亨廷顿病。
The same logic explains height in pea plants.
同样的道理也解释了豌豆的株高。
The dominant allele codes for a working enzyme that makes gibberellin, so the stem grows by elongation and the plant is tall.
显性等位基因编码一种能正常工作的酶，能造出赤霉素， 于是茎伸长，植株就高。
The recessive allele codes for a broken enzyme, so the plant is short.
隐性等位基因编码一种坏掉的酶，赤霉素造得很少，植株就矮。
A cell does not run all its genes all the time.
细胞并不是一直运行它所有的基因。
Structural genes code for useful proteins such as enzymes; regulatory genes control whether other genes are switched on.
结构基因编码有用的蛋白质，比如酶； 调节基因控制其他基因是否被开启。
An inducible enzyme is made only when needed; a repressible one is normally made but can be switched off.
可诱导酶只在需要时才制造； 可抑制酶平时一直制造，但可以被关掉。
In a prokaryote such as a bacterium, the lac operon shows how gene expression is controlled.
乳糖操纵子展示了这是怎么回事。
When there is no lactose, a repressor protein sits on the operator and blocks transcription, so the enzymes are not made.
没有乳糖时，一个阻遏蛋白坐在操纵基因上，挡住转录，所以消化乳糖的酶造不出来。
When lactose is present, it binds the repressor and pulls it off the operator.
有乳糖时，乳糖结合到阻遏物上，把它从操纵基因上拉下来。
Transcription can now happen and the enzymes are made — which is what inducible means.
于是转录可以进行，酶被制造出来——这正是"可诱导"的含义。
In eukaryotes it works differently: transcription factors bind to DNA and change the rate of transcription.
在真核生物里方式不同：转录因子是结合到 DNA 上并改变转录速率的蛋白质。
Gibberellin works this way, by causing the breakdown of DELLA repressor proteins.
赤霉素就是这样起作用的，它导致 DELLA 阻遏蛋白被降解。
Four things the examiner keeps asking for.
考官反复要的四点。
First, set the cross out in full: parental genotypes, gametes in circles, the square, then the ratio and the phenotypes.
第一，完整地写出杂交：亲本基因型、圈起来的配子、方格， 然后是比例和表现型。
Second, with chi-squared, always state the degrees of freedom and compare with the critical value — a bare number earns little.
第二，做卡方检验时，一定要写出自由度并与临界值比较—— 只给一个数字得不到多少分。
Third, when asked how meiosis creates variation, give both sources: crossing over and independent assortment.
第三，问到减数分裂如何产生变异时，两个来源都要给： 交叉互换和自由组合。
Fourth, for a sex-linked gene, write the alleles on the X, as a superscript, so the examiner can see you know where the gene sits.
第四，伴性基因要把等位基因写在 X 上，作为上标， 让阅卷人看出你知道这个基因在哪里。
So there it is.
就是这样了。
A division that halves and shuffles, a square that predicts, a test that checks the prediction, and a switch that decides which genes run at all.
一种既减半又洗牌的分裂，一个用来预测的方格，一个用来检验预测的检验， 还有一个决定哪些基因运行的开关。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。