Heredity
AP Biology Topic 5 7:53 English narration · English + 中文 subtitles burned in
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Two snails on the same tree.
同一棵树上的两只蜗牛。
Same species, same bark, same weather.
同一个物种，同一块树皮，同样的天气。
But look at the shells. One is pale with thin dark bands, the other dark with wide ones.
但看看它们的壳：一只颜色浅，带着细细的深色条纹；另一只颜色深，条纹很宽。
Nothing in their lives made that difference. They were born with it.
它们的经历并没有造成这个差别，它们生下来就是这样。
Here is where it comes from.
而它的来源就在这里。
These are one human cell's chromosomes, sorted into pairs. Forty-six in all, in twenty-three pairs.
这是一个人体细胞里的染色体，已经按对整理好： 一共四十六条，分成二十三对。
In each pair the partners carry the same genes, but one came from the mother and one from the father.
每一对里的两条携带相同的基因， 但一条来自母亲，一条来自父亲。
Each of us is a mixture of two people.
我们每个人都是两个人的一次混合。
Five steps today. Meiosis, the division that makes eggs and sperm. The three ways it shuffles the genes. Mendel's rules and the Punnett square. The patterns that break them. And how the environment changes what a gene shows.
今天分五步：减数分裂，也就是产生卵子和精子的那种分裂；它打乱基因的三种方式； 孟德尔的规律和庞纳特方格；那些不遵守规律的遗传方式； 以及环境如何改变一个基因表现出来的样子。
Meiosis starts with one diploid cell.
减数分裂从一个二倍体细胞开始。
Diploid means two full sets of chromosomes, one from each parent, so they come in homologous pairs.
二倍体的意思是有两整套染色体，每个亲本各给一套。
This cell divides twice, not once.
这个细胞要分裂两次，而不是一次。
The first division separates the homologous chromosomes — the alleles segregate — and the second separates the copies.
第一次分裂把成对的染色体分开， 第二次分裂把复制出来的拷贝分开。
Four haploid cells come out, each with half the chromosome number, and haploid means one set.
最后得到四个单倍体细胞，单倍体就是只有一套。
Those are the gametes.
它们就是配子。
Watch it happen.
我们来看这个过程。
One pair: one chromosome from the mother, one from the father.
一对染色体：一条来自母亲，一条来自父亲。
Each copies itself.
每一条都复制自己。
The pair lines up and swaps pieces.
这一对靠在一起排好，互相交换片段。
Then two divisions, and four gametes come out, no two alike.
然后是两次分裂， 出来四个配子，没有两个是一样的。
The exam loves this contrast.
考试特别喜欢考这个对比。
Mitosis is one division; meiosis is two.
有丝分裂只分裂一次，减数分裂分裂两次。
Mitosis makes two cells; meiosis makes four.
有丝分裂产生两个细胞，减数分裂产生四个。
Mitosis keeps the chromosome number, so a diploid cell stays diploid, while meiosis halves it.
有丝分裂保持染色体数目不变，二倍体细胞仍是二倍体，而减数分裂把数目减半。
And mitosis makes identical copies, while all four meiotic products differ.
而且有丝分裂产生完全相同的拷贝，减数分裂的四个产物却各不相同。
Same spindle, opposite purpose: one for growth, one for gametes.
同样的纺锤体，相反的目的：一个为了生长，一个为了产生配子。
Look closer at that swap.
我们把那次交换看得更仔细一些。
Early in the first division the two chromosomes of a pair lie against each other.
在第一次分裂的早期， 一对染色体中的两条紧紧贴在一起。
Where they touch they break and rejoin the other way.
在接触的地方，它们断开，再交叉接回去。
That contact point is a chiasma.
这个接触点叫做交叉点。
Each chromosome now mixes the mother's genes with the father's: a brand new combination.
现在每条染色体都混合了母亲和父亲的基因， 成为一种全新的组合。
The second shuffle is simpler.
第二种打乱方式更简单。
When the pairs line up in the middle of the cell, each pair picks its side at random, and the pairs do not affect each other.
当各对染色体在细胞中央排列时， 每一对随机选择自己朝向哪一边，而且各对之间互不影响。
Two pairs already give four different gametes.
仅仅两对，就已经给出四种不同的配子。
Turn one pair around and the gamete changes completely.
把其中一对翻转过来，配子就完全变了。
So meiosis is the engine of genetic diversity.
这带来多大的变异？
How much variation is that? Each of the twenty-three pairs can line up two ways, independently.
二十三对染色体中的每一对都可以有两种排列方式，而且彼此独立。
That gives more than eight million different gametes from one parent.
仅这一点，一个亲本就能产生八百多万种不同的配子。
Random fertilization then multiplies it again: any sperm can meet any egg, so the possible children number about seventy trillion.
任何一个精子都可能遇到任何一个卵子，所以可能的孩子大约有七十万亿种。
And crossing over changes the chromosomes themselves, so the true total is beyond counting.
而交叉互换还会改变染色体本身，所以真正的总数根本数不过来。
That is why no two children are ever the same.
这就是为什么没有任何两个孩子是一样的。
Now the words, used exactly.
现在讲术语，而且要用得准确。
A gene exists in different versions, and each version is an allele.
一个基因有不同的版本，每个版本叫做一个等位基因。
Your genotype is the pair of alleles you carry; your phenotype is what you show.
基因型是你携带的那一对等位基因，表型是你表现出来的性状。
Two of the same allele is homozygous, two different ones heterozygous.
两个相同的等位基因叫纯合，两个不同的叫杂合。
A dominant allele hides a recessive one, and segregation separates the two, so each gamete carries one.
显性等位基因会掩盖隐性等位基因； 而分离定律把这两个等位基因分开，所以每个配子只带其中一个。
Now watch the square do the bookkeeping.
现在看这个方格怎样记账。
Two heterozygotes, four boxes.
两个杂合子，四个格子。
One is homozygous dominant, two are heterozygous, one is homozygous recessive.
一个是显性纯合，两个是杂合，一个是隐性纯合。
Three of the four look dominant.
四个当中有三个看起来是显性的。
Here is a shortcut the exam rewards on a dihybrid cross.
这里有一个考试很吃香的捷径。
Cross two double heterozygotes. What fraction show both dominant traits?
让两个双杂合子杂交， 有多大比例同时表现两个显性性状？
Do not draw a sixteen-box square.
不要去画十六格的方格。
Take one gene alone: three quarters carry a dominant allele.
先只看一个基因：四分之三带有显性等位基因。
Now the second gene: again three quarters.
再看第二个基因：同样是四分之三。
The genes are independent, so multiply.
两个基因彼此独立，所以直接相乘。
Three quarters times three quarters is nine sixteenths.
四分之三乘四分之三等于十六分之九。
The fully recessive kind is one quarter times one quarter, one sixteenth.
完全隐性的那一种是四分之一乘四分之一，等于十六分之一。
Two ratios multiplied give the classic nine, three, three, one pattern.
两个比例相乘，正好给出经典的九、三、三、一。
Non-Mendelian genetics: many real traits break Mendel's simple rule.
很多真实的性状并不遵守孟德尔那条简单的规律。
In incomplete dominance neither allele wins, so red crossed with white gives pink.
在不完全显性中，两个等位基因谁也压不过谁，所以红花和白花杂交得到粉花。
In codominance both show fully, as in blood group AB. Multiple alleles: a gene can have more than two versions in a population, as in the ABO blood groups.
在共显性中，两个等位基因都完整地表现出来，比如AB血型。
In pleiotropy one gene affects several unrelated traits at once.
一个基因还可以有两个以上的等位基因。
And height or skin colour are polygenic: many genes each add a little, giving a smooth range.
而身高或肤色属于多基因遗传：许多基因各加一点，于是得到连续的范围。
Sex-linked inheritance is the pattern examiners test most.
伴性遗传是考官考得最多的一种。
The gene for red-green colour blindness is sex-linked — it sits on the X chromosome, and the Y carries no copy.
红绿色盲的基因位于X染色体上， 而Y染色体上没有它的拷贝。
A mother with one faulty allele sees normally.
一位带有一个缺陷等位基因的母亲，视觉是正常的。
Her daughters get a second X from the father, so a working allele covers the fault.
她的女儿会从父亲那里再得到一条X，一个正常的等位基因就把缺陷掩盖了。
Her sons get the Y, so their single X decides.
而她的儿子得到的是Y，所以由那唯一一条X决定。
Far more boys are colour blind.
色盲的男孩多得多。
One last idea, easy to forget — environmental effects on phenotype. Genes are not the whole story.
最后一个概念，很容易被忘掉：基因并不是故事的全部。
The same genotype gives different phenotypes in different conditions.
同样的基因型在不同条件下会给出不同的表型。
A hydrangea is blue in acid soil and pink in basic soil, with no change to its DNA.
绣球花在酸性土里开蓝花，在碱性土里开粉花，而它的DNA完全没有改变。
A Himalayan rabbit grows dark fur only where the skin is cool.
喜马拉雅兔只在皮肤温度低的地方长出深色的毛。
Now the calculation AP loves.
现在讲AP最爱考的这个计算。
A cross predicts three to one, but you count eighty offspring: fifty-five dominant, twenty-five recessive.
一次杂交预测三比一， 但你数了八十个后代：五十五个显性，二十五个隐性。
Is that difference real, or just chance?
这个差别是真实的，还是只是偶然？
First write the expected numbers. Three quarters of eighty is sixty; one quarter is twenty.
先写出期望值：八十的四分之三是六十，四分之一是二十。
Pause here and try the rest yourself. Ready?
先暂停，自己把剩下的算一算。
The chi-square formula takes each group, subtracts expected from observed, squares it, then divides by the expected value.
好了吗？ 卡方公式对每一组做同样的事：用观察值减去期望值，取平方，再除以期望值。
For the dominant group the difference is five, squared twenty-five, over sixty gives zero point four two.
对显性那一组，差是五，平方是二十五，除以六十得到零点四二。
For the recessive group the difference is five again, so twenty-five over twenty is one point two five.
对隐性那一组，差同样是五，二十五除以二十得到一点二五。
The total is one point six seven.
总和是一点六七。
Two groups means one degree of freedom, and the critical value at five percent is three point eight four.
两组意味着自由度为一，而百分之五水平上的临界值是三点八四。
Ours is smaller, so we fail to reject the null hypothesis.
我们的数值更小，所以不能拒绝零假设。
Four takeaways.
四点要带走。
Meiosis turns one diploid cell into four haploid gametes and halves the chromosome number.
第一，减数分裂把一个二倍体细胞变成四个单倍体配子，并把染色体数目减半。
Three shuffles create the variation: crossing over, independent assortment and random fertilisation.
第二，三种打乱方式创造出变异：交叉互换、自由组合和随机受精。
Mendel's rules with a Punnett square predict the ratios, and chi-square checks them.
第三，孟德尔的规律配上庞纳特方格可以预测比例，而卡方检验用来检查它们。
Beyond Mendel sit codominance, sex linkage and the environment.
第四，在孟德尔之外还有共显性、伴性遗传，以及环境的影响。
Three marks students throw away.
三个学生白白丢掉的分。
First, when a question asks where variation comes from, name all three sources: crossing over, independent assortment and random fertilisation.
第一，题目问变异从哪里来时， 要把三个来源都说出来：交叉互换、自由组合和随机受精。
One is not a full answer.
只说一个不算完整回答。
Second, keep genotype and phenotype apart: two genotypes can look exactly the same.
第二，把基因型和表型分清楚：两种基因型可以看起来一模一样。
Third, finish the chi-square. Compare with the critical value, then conclude in words.
第三，卡方要算完：把你的数值和临界值比较，再用文字写出结论。