Inheritance and Genetic Diagrams | 遗传与遗传图解

📚 Inheritance and Genetic Diagrams | 遗传与遗传图解

Inheritance is the mechanism through which traits are transmitted from one generation to the next. Understanding genetic principles not only reveals the secrets of family resemblance and variation but also forms a cornerstone of biology, evolution and modern medicine. In the Edexcel IGCSE Science specification, you are expected to interpret genetic diagrams, predict outcomes of monohybrid crosses and explain the patterns behind inherited disorders.

遗传是将性状从一代传递到下一代的机制。理解遗传原理不仅能揭示家族相似性与变异的秘密，也是生物学、进化论和现代医学的重要基础。在Edexcel IGCSE科学课程中，你需要能够解读遗传图解、预测单基因杂交的结果并解释遗传病的传递模式。

1. What is Inheritance? | 什么是遗传？

Inheritance is the process by which genetic information, stored in DNA, is passed from parents to their offspring. This information is organised into units called genes, which determine specific characteristics, or traits, such as eye colour, height and blood type. Gametes (sperm and egg cells) carry half the genetic material of each parent, which combines during fertilisation to produce a unique individual.

遗传是指储存在DNA中的遗传信息从亲代传给子代的过程。这些信息组成称为基因的单位，基因决定了特定的性状，如眼色、身高和血型。配子（精子和卵细胞）携带每个亲本一半的遗传物质，在受精时结合形成一个独特的个体。

Offspring therefore inherit a mixture of alleles from both parents, leading to both similarities and differences within a family. This is why you may have your mother’s eye shape but your father’s hair colour.

子代因此从双亲继承混合的等位基因，使得同一家庭成员既有相似之处，也存在差异。这就是为什么你可能有母亲的眼型却继承了父亲的发色。

2. Key Genetic Terms | 关键遗传术语

Before drawing genetic diagrams, you must be clear about the following vocabulary:

在绘制遗传图解之前，你必须厘清以下术语：

Gene – a section of DNA that codes for a particular protein and determines a trait.
基因 – 编码特定蛋白质并决定某一性状的DNA片段。
Allele – a different version of the same gene. For example, there is an allele for brown eyes and another for blue eyes.
等位基因 – 同一基因的不同形式。例如，有褐色眼的等位基因和蓝色眼的等位基因。
Dominant allele – an allele that is always expressed if present, even when only one copy is inherited. Conventional symbol: capital letter, e.g. B.
显性等位基因 – 只要存在就会表达的等位基因，即使只继承了一个拷贝。常用大写字母表示，如B。
Recessive allele – an allele that is only expressed when two copies are present. Symbol: lower-case letter, e.g. b.
隐性等位基因 – 只有存在两个拷贝时才会表达的等位基因。用小写字母表示，如b。
Homozygous – having two identical alleles for a gene, e.g. BB or bb.
纯合 – 某个基因的两个等位基因相同，如BB或bb。
Heterozygous – having two different alleles, e.g. Bb.
杂合 – 两个等位基因不同，如Bb。
Genotype – the combination of alleles an organism possesses, e.g. Bb.
基因型 – 一个生物体拥有的等位基因组合，如Bb。
Phenotype – the observable characteristic resulting from the genotype, e.g. brown eyes.
表型 – 基因型所表现出的可观察性状，如褐色眼。

Mastering this terminology is essential for answering inheritance questions correctly.

掌握这些术语是正确回答遗传问题的关键。

3. Monohybrid Crosses | 单基因杂交

A monohybrid cross studies the inheritance of a single characteristic controlled by one gene with two alleles. For example, in pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). When a homozygous tall plant (TT) is crossed with a homozygous short plant (tt), all the F₁ offspring are heterozygous tall (Tt).

单基因杂交研究的是由一对等位基因控制的单一性状的遗传。例如，在豌豆中，高茎等位基因（T）对矮茎等位基因（t）为显性。当一株纯合高茎豌豆（TT）与一株纯合矮茎豌豆（tt）杂交时，所有F₁子代都是杂合高茎（Tt）。

If two F₁ individuals (Tt × Tt) are crossed, the expected genotype ratio in the offspring is 1 TT : 2 Tt : 1 tt, giving a phenotype ratio of 3 tall : 1 short.

若让两个F₁个体（Tt × Tt）杂交，子代预期的基因型比例为1 TT : 2 Tt : 1 tt，表型比例为3高茎：1矮茎。

4. Punnett Squares | 庞纳特方格

A Punnett square is a grid used to predict the possible genotypes of offspring from a genetic cross. You place the possible gametes of one parent along the top and those of the other parent down the side, then fill in the squares to show all possible fertilisation outcomes.

庞纳特方格是用来预测杂交后代可能基因型的网格。将一个亲本可能的配子列在顶行，将另一亲本的配子列在左侧列，然后在方格内填入所有可能的受精结果。

For a cross between two heterozygous brown-eyed parents (Bb × Bb):

以两个杂合褐色眼的亲本（Bb × Bb）杂交为例：

	B	b
B	BB	Bb
b	Bb	bb

The result: genotype ratio 1 BB : 2 Bb : 1 bb; phenotype ratio 3 brown eyes : 1 blue eye. The probability of a blue-eyed child is ¼ or 25%.

结果：基因型比例为1 BB : 2 Bb : 1 bb；表型比例为3褐色眼 : 1蓝色眼。蓝色眼孩子的概率是¼或25%。

5. Dominant and Recessive Alleles in Action | 显性与隐性等位基因的作用

A dominant allele masks the effect of a recessive allele in a heterozygous individual. Therefore, a person with genotype Bb will have brown eyes because the brown allele (B) is dominant over the blue allele (b). This explains why a recessive trait can skip a generation – two heterozygous parents, both showing the dominant trait, can produce a child who is homozygous recessive and displays the recessive trait.

显性等位基因在杂合个体中会遮盖隐性等位基因的作用。因此，基因型为Bb的人会表现出褐色眼，因为褐色等位基因（B）对蓝色（b）是显性的。这解释了为什么隐性性状会隔代出现——两个表现出显性性状的杂合亲本，可以生出纯合隐性从而表现隐性性状的孩子。

In a pedigree chart, this is often seen when two unaffected parents have an affected child; this indicates that both parents are carriers of a recessive allele.

在系谱图中，这常表现为两个未患病的父母生出一个患病的孩子，表明父母双方都是隐性等位基因的携带者。

6. Homozygous and Heterozygous Genotypes | 纯合与杂合基因型

An individual with two identical alleles for a trait is said to be homozygous. If the alleles are both dominant (e.g. BB) or both recessive (bb), the trait is expressed accordingly. A heterozygous individual (Bb) carries one dominant and one recessive allele; the dominant trait is expressed, but the recessive allele can still be passed on to offspring.

某个性状的等位基因相同的个体被称为纯合子。若两个都是显性（如BB）或都是隐性（bb），则相应地表现出该性状。杂合个体（Bb）携带一个显性和一个隐性等位基因；表现出显性性状，但仍可将隐性等位基因传给后代。

Carrier refers to a person who is heterozygous for a recessive disorder. They do not show symptoms but can transmit the disease-causing allele to their children.

携带者是指对某种隐性遗传病为杂合的人。他们自身不会表现出症状，但可能将致病等位基因传递给孩子。

7. Genetic Diagram Example: Eye Colour | 遗传图解示例：眼色

Let’s work through a complete genetic cross for eye colour. Stephen and Alice are both heterozygous for brown eyes (Bb). Draw a Punnett square and state the probability of them having a blue-eyed child.

我们完整地做一个眼色遗传杂交的示例。Stephen和Alice都是褐色眼的杂合子（Bb）。请画出庞纳特方格并说明他们生出一个蓝眼孩子的概率。

Parental genotypes: Bb × Bb. Gametes: B and b from each parent. Punnett square combination yields BB, Bb, Bb, bb. Probability of blue eyes (bb) = ¼ or 25%. It is crucial to write down the gametes carefully and link them to the genotypes in the offspring.

亲本基因型：Bb × Bb。配子：各自产生B和b。庞纳特方格组合得到BB, Bb, Bb, bb。蓝眼（bb）的概率为¼或25%。仔细写出配子并将其与子代基因型对应是很关键的。

This same method applies to any monohybrid cross, including those in animal and plant breeding.

同样的方法也适用于任何单基因杂交，包括动物和植物育种。

8. Co-dominance and Incomplete Dominance | 共显性与不完全显性

Not all alleles follow a simple dominant–recessive pattern. In co-dominance, both alleles contribute to the phenotype and are expressed equally in the heterozygote. A classic example is the ABO blood group system. The alleles I^A and I^B are co-dominant, while the allele I^O is recessive. A person with genotype I^AI^B has blood type AB, expressing both A and B antigens.

并非所有等位基因都遵循简单的显性-隐性模式。在共显性中，两个等位基因都对表型产生影响，且在杂合子中同时表达。经典的例子是ABO血型系统。等位基因Iᴬ和Iᴮ为共显性，而Iᴼ为隐性。基因型IᴬIᴮ的人血型为AB型，同时表达A和B抗原。

In incomplete dominance, the heterozygote shows an intermediate phenotype. For example, in snapdragon flowers, a cross between a red-flowered plant (R¹R¹) and a white-flowered plant (R²R²) yields pink-flowered offspring (R¹R²).

在不完全显性中，杂合子表现出中间表型。例如，在金鱼草中，红花植株（R¹R¹）与白花植株（R²R²）杂交，产生粉红色花（R¹R²）的后代。

When answering questions, always consider the type of dominance pattern described.

回答问题时，务必判断题目所述的显性类型。

9. Sex Determination | 性别决定

In humans, biological sex is determined by the 23rd pair of chromosomes, known as the sex chromosomes. Females typically have two X chromosomes (XX), while males have one X and one Y chromosome (XY).

在人类中，生物性别由第23对染色体，即性染色体决定。女性通常有两条X染色体（XX），男性有一条X和一条Y染色体（XY）。

During gamete formation, eggs always carry an X chromosome. Sperm carry either an X or a Y chromosome. Fertilisation therefore results in a 50% chance of a female (XX) and a 50% chance of a male (XY).

配子形成时，卵子总是携带一条X染色体。精子则携带X或Y染色体。因此受精后，有50%的概率是女性（XX），50%的概率是男性（XY）。

The genetic cross for sex determination can be summarised:

性别决定的遗传杂交可总结如下：

	X	Y
X	XX	XY
X	XX	XY

This explains why the sex ratio in most human populations is close to 1:1.

这解释了为什么大多数人类群体的性别比接近1:1。

10. Inherited Disorder: Cystic Fibrosis | 遗传病：囊性纤维化

Cystic fibrosis (CF) is an inherited disorder caused by a recessive allele. Let ‘F’ represent the normal allele and ‘f’ represent the faulty allele. A person with the genotype FF is healthy; Ff is a carrier (healthy, but can pass on the allele); ff suffers from cystic fibrosis.

囊性纤维化是由一个隐性等位基因引起的遗传病。用 F 代表正常等位基因，f 代表缺陷等位基因。基因型FF的人健康；Ff为携带者（健康，但可以传递等位基因）；ff则患病。

If two carriers (Ff × Ff) have children, the chance of an affected child is 1 in 4 (25%), the chance of a carrier child is 1 in 2 (50%), and the chance of a completely healthy non-carrier is 1 in 4 (25%).

如果两个携带者（Ff × Ff）生育孩子，患病孩子的概率为1/4（25%），携带者孩子的概率为1/2（50%），完全健康且非携带者的概率为1/4（25%）。

Being able to calculate these probabilities using a Punnett square is a key exam skill.

能够使用庞纳特方格计算出这些概率是重要的考试技能。

11. Pedigree Analysis | 系谱分析

A pedigree chart is a diagram showing the genetic relationships within a family over several generations. It uses standard symbols: squares for males, circles for females, shaded symbols for individuals expressing a trait, and half-shaded for carriers (where applicable).

系谱图是显示一个家族几代内遗传关系的图示。它使用标准符号：方框代表男性，圆圈代表女性，涂满的符号表示表现出该性状的个体，半涂表示携带者（在适用时）。

Pedigrees are particularly useful for tracing the inheritance of traits such as genetic disorders. By analysing who shows a trait and who does not, you can deduce whether the allele is dominant or recessive and predict the genotypes of family members.

系谱图对追踪遗传病等性状的遗传特别有用。通过分析谁表现出性状、谁未表现，你可以推断该等位基因是显性还是隐性，并预测家庭成员的基因型。

For example, if two unaffected parents have an affected child, the trait must be recessive, and both parents are heterozygous carriers.

例如，如果两个未患病的父母生出一个患病的孩子，该性状必然是隐性的，且父母双方都是杂合携带者。

12. Summary and Exam Tips | 总结与考试技巧

Inheritance questions often require you to construct and interpret Punnett squares, state ratios, and explain the patterns observed. Always define the alleles clearly, write down the parental genotypes and gametes before completing the cross, and then interpret the results in terms of genotype and phenotype probabilities.

遗传题目常要求你构建并解读庞纳特方格、陈述比例并解释观察到的模式。一定要清楚地定义等位基因，在完成杂交前先写出亲本基因型和配子，然后从基因型和表型概率两方面解释结果。

Use capital and lower-case letters consistently and explain your notation.
前后一致地使用大写和小写字母，并解释你的标记方式。
Remember that gametes contain only one allele from each gene pair.
记住配子只含有每个基因对中的一个等位基因。
When a question involves a pedigree, work step by step: identify the inheritance pattern, assign possible genotypes, and then test them.
当题目涉及系谱图时，要一步步来：先确定遗传模式，分配可能的基因型，再进行验证。
For co-dominance and incomplete dominance, ensure you use appropriate symbols (e.g. superscripts) and do not label alleles as simply dominant or recessive.
对于共显性和不完全显性，确保使用合适的符号（如上标），不要简单地将等位基因标为显性或隐性。

Regular practice with past paper questions will build confidence in tackling inheritance problems.

经常练习历年真题会让你在处理遗传问题时充满信心。

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Inheritance and Genetic Diagrams | 遗传与遗传图解