Inheritance
IGCSE Biology Topic 17 8:11 English narration · English + 中文 subtitles burned in
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Two brown-eyed parents can have a blue-eyed child.
两个棕色眼睛的父母可能生出一个蓝眼睛的孩子。
That is not a mistake, and it is not a surprise once you can read genotypes.
这不是错误，只要你会读基因型，就一点也不奇怪。
By the end of this lesson you will be able to work out both parents' genes from that one child, and predict the chance for the next one.
学完这节课，你就能从那一个孩子推出父母双方的基因，并预测下一个孩子的概率。
Welcome to inheritance.
欢迎来到遗传。
Chromosomes and genes, cell division, Punnett squares, and sex linkage.
染色体和基因、细胞分裂、庞纳特方格，还有伴性遗传。
Let's begin.
让我们开始吧。
Start with three words that nest inside each other.
先看三个层层嵌套的词。
Chromosomes are made of D N A, which carries the genetic information in units called genes.
染色体由DNA构成，DNA以叫做基因的单位携带遗传信息。
A gene is a length of D N A that codes for one protein — that is the definition to learn.
基因是一段编码一种蛋白质的DNA——这就是要背的定义。
And an allele is an alternative form of a gene.
等位基因是一个基因的不同形式。
So a gene for eye colour may have a brown allele and a blue allele.
所以眼睛颜色的基因可能有棕色等位基因和蓝色等位基因。
Gene is the slot; allele is the version in it.
基因是那个位置；等位基因是位置上的版本。
Sex is inherited through the sex chromosomes.
性别通过性染色体遗传。
Females are X X and males are X Y.
女性是XX，男性是XY。
Now follow the gametes.
现在跟着配子看。
An egg always carries an X, because the mother only has X chromosomes to give.
卵子总是携带一个X，因为母亲只有X染色体可以给。
But a sperm carries either an X or a Y.
但精子携带X或者Y。
So the sperm decides the baby's sex — the father, not the mother.
所以是精子决定婴儿的性别——是父亲，不是母亲。
And because half the sperm carry X and half carry Y, the expected ratio of girls to boys is one to one.
因为一半的精子带X、一半带Y，所以女孩和男孩的预期比例是一比一。
For the supplement paper, here is the chain that explains everything.
补充部分有一条解释一切的链条。
The base sequence of a gene sets the order of amino acids in a protein.
基因的碱基序列决定蛋白质中氨基酸的顺序。
A different order folds the protein into a different shape — and the shape decides its job.
不同的顺序把蛋白质折叠成不同的形状——而形状决定它的功能。
So D N A controls the cell by controlling which proteins are made, including enzymes, membrane carriers and receptor proteins for neurotransmitters.
所以DNA通过控制制造哪些蛋白质来控制细胞，包括酶、膜上的载体和受体蛋白。
One more point: almost every body cell holds the same genes, but each cell only expresses the genes it needs — switches them on — so it makes only the proteins for its own job.
还有一点：几乎每个体细胞都含有相同的基因， 但每个细胞只表达它需要的基因——把它们打开—— 所以它只制造适合自己工作的蛋白质。
Same library, different books opened.
同一个图书馆，打开不同的书。
That raises a problem, and messenger R N A solves it.
这带来一个问题，信使RNA解决了它。
The gene stays in the nucleus, but proteins are made in the cytoplasm.
基因留在细胞核里，但蛋白质在细胞质中制造。
So making a protein takes three steps.
所以有三个步骤。
One: messenger R N A is made in the nucleus as a copy of the gene.
第一：信使RNA在细胞核中作为基因的拷贝被制造出来。
Two: that messenger R N A moves out into the cytoplasm and passes through a ribosome.
第二：这个信使RNA移出到细胞质，通过一个核糖体。
Three: the ribosome reads its bases and joins amino acids in the matching order to build the protein.
第三：核糖体读取它的碱基，按对应的顺序把氨基酸连起来，造出蛋白质。
The message travels; the gene never leaves.
信息在移动；基因从不离开。
Also for the supplement paper: two kinds of nuclear division.
同样是补充部分：两种细胞核分裂。
First the counts.
先看数目。
Body cells are diploid — two sets of chromosomes, twenty-three pairs in humans — and gametes are haploid, one set.
体细胞是二倍体——两套染色体，人类有二十三对——配子是单倍体，一套。
Mitosis makes two cells that are genetically identical to the parent cell.
有丝分裂产生两个和亲代细胞遗传上完全相同的细胞。
The chromosomes are copied exactly — replication — before division, so each daughter cell keeps the full chromosome number.
染色体在分裂前被精确复制，所以每个子细胞保持完整的染色体数目。
It is used for growth, repair, replacing old cells, and asexual reproduction — and stem cells, which are unspecialised, divide by mitosis before their daughters specialise.
它用于生长、修复、替换老细胞和无性生殖—— 干细胞是未特化的细胞，通过有丝分裂分裂，之后子细胞才特化。
Meiosis makes gametes.
减数分裂产生配子。
It is a reduction division: the chromosome number is halved, diploid to haploid, and the cells it makes are genetically different from one another.
它是一种减数的分裂：染色体数目减半，从二倍体到单倍体， 而且产生的细胞彼此遗传上不同。
Identical and full number, or different and half.
相同且全数，或者不同且减半。
Now the vocabulary, and it comes in three pairs.
现在看词汇，它们分成三对。
Genotype is the alleles an organism has — its genetic make-up.
基因型是一个生物拥有的等位基因——它的遗传组成。
Phenotype is the features you can actually observe.
表现型是你能实际观察到的特征。
A dominant allele shows in the phenotype even if only one copy is present, and we write it as a capital letter.
显性等位基因即使只有一个拷贝也会在表现型中显示，我们用大写字母写它。
A recessive allele shows only when no dominant allele is present, and we write it as a small letter.
隐性等位基因只有在没有显性等位基因时才显示，我们用小写字母写它。
Homozygous means two identical alleles — big B big B, or small b small b — and those breed true.
纯合子意思是两个相同的等位基因——大B大B，或者小b小b——它们是纯种的。
Heterozygous means two different alleles, big B small b, and those do not breed true.
杂合子意思是两个不同的等位基因，大B小b，它们不是纯种的。
To predict the offspring of a cross involving one gene, use a Punnett square.
要预测涉及一个基因的杂交的后代，就用庞纳特方格。
The method is always the same: write each parent's genotype, then write the gametes underneath — each gamete carries one allele — then combine them in a grid.
方法总是一样的： 写出每个亲本的基因型，然后在下面写出配子——每个配子带一个等位基因—— 再在方格中组合它们。
That is a monohybrid cross.
这就是单基因杂交。
Two results are worth memorising.
有两个结果值得背下来。
Two heterozygous parents give a ratio of three to one, dominant to recessive.
两个杂合子亲本给出三比一的比例，显性对隐性。
A heterozygous crossed with a homozygous recessive gives one to one.
一个杂合子和一个隐性纯合子杂交给出一比一。
Homozygous individuals breed true, so they are pure-breeding; heterozygous ones are not.
纯合子能稳定遗传，所以是纯种；杂合子则不是。
You can also read a pedigree diagram — a family tree — to follow a feature and work out genotypes.
你还可以读系谱图——也就是家系图——来追踪某个性状并推断基因型。
For the supplement paper, one more tool.
补充部分还有一个工具。
A test cross finds an unknown genotype: cross the unknown with a homozygous recessive.
测交用来找出未知的基因型：把未知的和隐性纯合子杂交。
If any offspring show the recessive feature, the unknown must have been heterozygous.
如果有任何后代表现出隐性特征，那个未知的一定是杂合子。
Now the question from the start.
现在回到开头的问题。
Brown eyes are dominant to blue.
棕色眼睛对蓝色是显性。
Two brown-eyed parents have a blue-eyed child.
两个棕眼父母生了一个蓝眼孩子。
Give the parents' genotypes, and the chance their next child has blue eyes.
给出父母的基因型，以及下一个孩子是蓝眼的概率。
Start from the child, because a recessive phenotype pins a genotype exactly.
从孩子开始推，因为隐性表现型能准确定出基因型。
Blue is recessive, so the child must be small b small b, and it got one small b from each parent.
蓝色是隐性的，所以孩子一定是小b小b，而且它从每个亲本各得到一个小b。
So each parent carries a small b.
所以每个亲本都带一个小b。
But both parents show brown eyes, so each must also carry a big B.
但两个亲本都表现为棕眼，所以每个还必须带一个大B。
Both parents are big B small b.
两个亲本都是大B小b。
The Punnett square then gives big B big B, big B small b, big B small b, and small b small b — so the chance of a blue-eyed child is one in four, or twenty-five percent.
庞纳特方格给出大B大B、大B小b、大B小b和小b小b—— 所以蓝眼孩子的概率是四分之一，也就是百分之二十五。
And one warning: that twenty-five percent applies to each child on its own.
还有一个警告： 那个百分之二十五对每个孩子单独适用。
Three brown-eyed children do not make the fourth blue.
三个棕眼孩子不会让第四个变成蓝眼。
For the supplement paper, codominance is where both alleles show in the phenotype of a heterozygous organism — neither one hides the other.
补充部分：共显性是指在杂合个体的表现型中两个等位基因都表现出来—— 谁也不掩盖谁。
The standard example is the ABO blood groups.
标准的例子是ABO血型。
There are three alleles: I A, I B and I O.
有三个等位基因：IA、IB和IO。
I A and I B are codominant with each other, while I O is recessive to both.
IA和IB彼此共显性，而IO对两者都是隐性的。
So two copies of I A, or one I A with I O, give group A.
所以两个IA，或者一个IA配一个IO，是A型。
Two I B, or I B with I O, give group B.
两个IB，或者IB配IO，是B型。
I O with I O gives group O.
IO配IO是O型。
And I A together with I B gives group A B — both alleles shown at once, which is exactly what codominance means.
而IA和IB在一起是AB型—— 两个等位基因同时表现出来，这正是共显性的意思。
Last, sex linkage.
最后是伴性遗传。
A sex-linked characteristic is controlled by a gene on a sex chromosome, usually the X.
伴性特征由性染色体上的基因控制，通常是X染色体。
Now the reasoning that earns the mark.
现在说能得分的推理。
Males have only one X chromosome, so there is no second copy that could carry a dominant allele to mask a recessive one.
男性只有一条X染色体， 所以没有第二个拷贝能带一个显性等位基因来掩盖隐性的。
A recessive allele on that single X therefore always shows in a male.
因此那唯一一条X上的隐性等位基因在男性身上总会表现出来。
Females have two X chromosomes, so they need two copies of the recessive allele.
女性有两条X染色体，所以她们需要两个隐性等位基因的拷贝。
That is why sex-linked features are more common in males.
这就是为什么伴性特征在男性中更常见。
Red-green colour blindness is the example to name: the recessive allele sits on the X chromosome, so it is much more common in boys than in girls.
红绿色盲是要说出名字的例子： 隐性等位基因位于X染色体上，所以它在男孩中比在女孩中常见得多。
Three marks to lock in.
锁住三个分。
First: genotype is what it has, phenotype is what you see; homozygous is two the same, heterozygous is two different.
第一：基因型是它拥有什么，表现型是你看到什么； 纯合子是两个相同，杂合子是两个不同。
Second: learn the two ratios — three to one, and one to one.
第二：记住两个比例——三比一，和一比一。
Third: in any cross question, work backwards from the recessive individual, because a recessive phenotype pins a genotype exactly.
第三：任何杂交题，都要从隐性个体倒着推，因为隐性表现型能准确定出基因型。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。