📚 Patterns of Inheritance | 遗传模式
Inheritance is the process by which genetic information is passed from parents to offspring. Understanding the patterns of inheritance helps explain why offspring resemble their parents but also show variation. This article covers key terms, monohybrid crosses, codominance, sex-linked traits and pedigree charts.
遗传是遗传信息从亲代传递给子代的过程。理解遗传模式有助于解释为什么后代既像父母又存在差异。本文涵盖关键术语、单基因杂交、共显性、性连锁性状和家系图。
1. Key Genetic Terms | 关键遗传学术语
A gene is a section of DNA that codes for a specific protein. Alleles are different forms of the same gene, and they occupy the same locus on homologous chromosomes. A dominant allele is expressed in the phenotype even when only one copy is present, while a recessive allele is only expressed when two copies are present.
基因是编码特定蛋白质的一段 DNA。等位基因是同一基因的不同形式，位于同源染色体的相同基因座上。显性等位基因即使只有一个拷贝也会在表现型中表达，而隐性等位基因只有在两个拷贝都存在时才表达。
Homozygous individuals have two identical alleles, such as AA or aa. Heterozygous individuals have two different alleles, such as Aa. The genotype is the genetic makeup, and the phenotype is the observable characteristic that results from the genotype and the environment.
纯合子个体具有两个相同的等位基因，例如 AA 或 aa。杂合子个体具有两个不同的等位基因，例如 Aa。基因型是遗传组成，表现型是由基因型和环境共同决定的可观察特征。
2. Monohybrid Inheritance and Punnett Squares | 单基因遗传与庞尼特方格
Monohybrid inheritance involves one gene with two alleles. In a cross between two heterozygous parents, such as Bb × Bb, each parent produces gametes carrying either B or b with equal probability. A Punnett square can be used to predict the genotypes of the offspring.
单基因遗传涉及一个基因的两个等位基因。在两个杂合亲本杂交中，如 Bb × Bb，每个亲本产生携带 B 或 b 的配子，概率相等。庞尼特方格可用于预测后代的基因型。
| B | b | |
| B | BB | Bb |
| b | Bb | bb |
The expected genotype ratio is 1 BB : 2 Bb : 1 bb. If B is completely dominant over b, the phenotype ratio is 3 dominant : 1 recessive.
预期的基因型比例为 1 BB : 2 Bb : 1 bb。如果 B 对 b 完全显性，则表现型比例为 3 显性 : 1 隐性。
3. Test Crosses | 测交
A test cross is used to determine whether an individual showing a dominant phenotype is homozygous dominant or heterozygous. The individual is crossed with a homozygous recessive individual. If any offspring show the recessive phenotype, the unknown parent must be heterozygous.
测交用于确定表现出显性表现型的个体是纯合显性还是杂合。将该个体与纯合隐性个体杂交。如果任何后代表现出隐性表现型，则未知亲本一定是杂合子。
For example, crossing a tall pea plant of unknown genotype with a dwarf plant tt will produce all tall offspring if the parent is TT, but a 1 tall : 1 dwarf ratio if the parent is Tt.
例如，将未知基因型的高茎豌豆与矮茎植株 tt 杂交，如果亲本是 TT，则后代全部为高茎；如果亲本是 Tt，则比例为 1 高茎 : 1 矮茎。
4. Codominance and Incomplete Dominance | 共显性与不完全显性
In codominance, both alleles are expressed equally in the heterozygous phenotype. For example, in cattle, a red-coated animal crossed with a white-coated animal may produce roan offspring with both red and white hairs.
在共显性中，两个等位基因在杂合表现型中均等表达。例如，在牛中，红色毛皮动物与白色毛皮动物杂交可能产生红白相间的花毛后代。
In incomplete dominance, the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. For example, crossing red snapdragon flowers with white snapdragon flowers produces pink flowers in the F1 generation.
在不完全显性中，杂合表现型是两个纯合表现型的中间混合。例如，红色金鱼草与白色金鱼草杂交，F1 代产生粉色花。
Both patterns differ from complete dominance, where one allele masks the other completely. It is important to use different notation, such as CR and CW for codominant alleles, to show that neither allele is recessive.
这两种模式都不同于完全显性，在完全显性中一个等位基因完全遮盖另一个。使用不同的符号很重要，例如共显性等位基因用 CR 和 CW 表示，以表明没有隐性等位基因。
5. Sex Determination | 性别决定
In humans, sex is determined by the sex chromosomes. Females have two X chromosomes XX, whereas males have one X and one Y chromosome XY. The X chromosome is much larger and carries many genes, while the Y chromosome is smaller and carries fewer genes, including the SRY gene that triggers male development.
人类的性别由性染色体决定。女性有两条 X 染色体 XX，男性有一条 X 和一条 Y 染色体 XY。X 染色体较大，携带许多基因；Y 染色体较小，携带较少基因，其中包括触发男性发育的 SRY 基因。
During gamete formation, a female can only pass on an X chromosome, but a male can pass on either an X or a Y. Therefore, the father’s sperm determines the sex of the child, giving a 1:1 ratio of male to female offspring.
在配子形成过程中，女性只能传递 X 染色体，而男性可以传递 X 或 Y 染色体。因此，父亲的精子决定孩子的性别，后代男女比例为 1:1。
6. Sex-linked Inheritance | 性连锁遗传
Sex-linked genes are located on the sex chromosomes, usually the X chromosome. Because males have only one X chromosome, they cannot be carriers for X-linked recessive conditions. A single recessive allele on the X chromosome will be expressed in males, whereas females need two copies to express the condition.
性连锁基因位于性染色体上，通常在 X 染色体上。由于男性只有一条 X 染色体，他们不可能成为 X 连锁隐性疾病的携带者。X 染色体上的一个隐性等位基因就会在男性中表达，而女性需要两个拷贝才会表达该疾病。
Examples include red-green colour blindness and haemophilia. A carrier female has the genotype XH Xh and passes the affected Xh allele to half of her sons, who will be affected, and to half of her daughters, who will be carriers.
例子包括红绿色盲和血友病。女性携带者的基因型为 XH Xh，她会将受影响的 Xh 等位基因传给一半的儿子，儿子会患病；传给一半的女儿，女儿会成为携带者。
X-linked recessive conditions are therefore more common in males than in females. Fathers cannot pass X-linked traits to their sons because sons inherit the Y chromosome from their father.
因此，X 连锁隐性遗传病在男性中比女性更常见。父亲不能将 X 连锁性状传给儿子，因为儿子从父亲那里遗传 Y 染色体。
7. Pedigree Charts | 家系图
A pedigree chart is a diagram that shows the inheritance of a trait through several generations. Squares represent males, circles represent females, and shaded symbols indicate individuals who express the trait. A horizontal line between a square and a circle represents mating, and vertical lines lead to offspring.
家系图是显示某一性状在几代人中遗传情况的图表。方块代表男性，圆圈代表女性，阴影符号表示表现出该性状的个体。方块和圆圈之间的水平线表示婚配，垂直线连接后代。
Pedigrees can reveal whether an allele is dominant or recessive and whether it is autosomal or sex-linked. A recessive trait can skip generations, while a dominant trait usually appears in every generation. X-linked recessive traits affect more males than females.
家系图可以揭示等位基因是显性还是隐性，以及是常染色体还是性连锁。隐性性状可能隔代出现，而显性性状通常每代都会出现。X 连锁隐性性状影响的男性多于女性。
8. Multiple Alleles and ABO Blood Groups | 复等位基因与 ABO 血型
Some genes have more than two alleles in the population. The ABO blood group system is controlled by three alleles: IA, IB and i. Alleles IA and IB are codominant, and both are dominant over i.
有些基因在群体中有两个以上等位基因。ABO 血型系统由三个等位基因控制：IA、IB 和 i。等位基因 IA 和 IB 共显性，且都对 i 显性。
The possible genotypes
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