📚 Dihybrid Crosses | 双因子杂交
A dihybrid cross examines the inheritance of two different genes at the same time. It is a natural extension of Mendel’s monohybrid work and is essential for understanding independent assortment, Punnett squares and the famous 9:3:3:1 F2 ratio.
双因子杂交同时研究两对等位基因的遗传。它是孟德尔单因子杂交研究的自然延伸，对于理解独立分配、庞纳特方格以及著名的 F₂ 9:3:3:1 比例至关重要。
1. What a Dihybrid Cross Is | 什么是双因子杂交
A dihybrid cross is a cross between two individuals that are heterozygous for two different genes. In standard notation, it can be written as AaBb × AaBb, where A and a are alleles of one gene, and B and b are alleles of a second gene.
双因子杂交是指两个个体在两对不同的基因上均为杂合子的杂交。用标准符号可写为 AaBb × AaBb，其中 A 和 a 是第一个基因的等位基因，B 和 b 是第二个基因的等位基因。
The two genes are usually located on different chromosomes, so their alleles are inherited independently. This is different from a monohybrid cross, which follows only one gene, such as Aa × Aa.
这两对基因通常位于不同的染色体上，因此它们的等位基因独立遗传。这与只跟踪一对基因的单因子杂交（如 Aa × Aa）不同。
2. Mendel’s Law of Independent Assortment | 孟德尔独立分配定律
Mendel’s second law states that alleles for different genes assort independently during gamete formation, provided the genes are on different chromosomes or are far apart on the same chromosome.
孟德尔第二定律指出，在配子形成过程中，不同基因的等位基因独立分配，前提是这些基因位于不同染色体上，或在同一染色体上相距很远。
This means that inheriting one allele does not influence which allele of the other gene is passed on. Independent assortment produces new combinations of alleles called recombinants.
这意味着继承一个等位基因不会影响另一个基因的哪个等位基因被传递。独立分配会产生新的等位基因组合，即重组型。
The independent behaviour of two genes is easier to see when we form gametes from a double heterozygote.
当我们从双杂合子形成配子时，两个基因的独立行为更容易观察。
3. Producing Gametes for Two Genes | 形成涉及两对等位基因的配子
For a dihybrid individual with genotype AaBb, each gamete receives one allele from each gene. The possible combinations are AB, Ab, aB and ab, and they are produced in equal proportions when the genes are unlinked.
对于基因型为 AaBb 的双杂合个体，每个配子从每个基因中获得一个等位基因。可能的组合是 AB、Ab、aB 和 ab，当基因不连锁时，它们以相等的比例产生。
AaBb gametes: AB, Ab, aB, ab (1 : 1 : 1 : 1)
This equal gamete ratio happens because allele A or a separates independently of allele B or b during meiosis. Each of the four combinations therefore has a probability of 1/2 × 1/2 = 1/4.
这一均等配子比例的出现，是因为等位基因 A 或 a 的分离独立于等位基因 B 或 b。四种组合中的每一种因此具有 1/2 × 1/2 = 1/4 的概率。
The four gamete types are the key to constructing a 4 × 4 Punnett square for a dihybrid cross.
四种配子类型是构建双因子杂交 4 × 4 庞纳特方格的关键。
4. Constructing a Dihybrid Punnett Square | 构建双因子庞纳特方格
Place the four possible gametes from one parent along the top and the four gametes from the other parent along the side. Each box is filled by combining the allele from the top and the side.
将一个亲本可能产生的四种配子放在顶部，将另一个亲本产生的四种配子放在左侧。每个方格通过组合顶部和左侧的等位基因来填写。
For AaBb × AaBb, the square contains 16 boxes. Each box represents an equally likely offspring genotype at fertilisation.
对于 AaBb × AaBb，方格表包含 16 个小格。每个小格代表受精时一个等概率的子代基因型。
Because both parents produce four gamete types with equal frequency, every one of the 16 boxes has the same probability of occurring.
由于两个亲本都以相同频率产生四种配子类型，16 个小格中的每一格出现的概率都相同。
When completing the square, write the alleles in a consistent order, for example A/a before B/b, to avoid missing combinations.
填写方格表时，请按固定顺序书写等位基因，例如先写 A/a 再写 B/b，以免遗漏组合。
5. The Classic F2 Phenotypic Ratio | 经典 F₂ 表型比
When both genes show complete dominance and are unlinked, self-crossing F1 dihybrids produces four phenotypes in the ratio 9 : 3 : 3 : 1.
当两对基因均表现完全显性且不连锁时，F₁ 双杂合子自交会产生四种表型，比例为 9 : 3 : 3 : 1。
F₂ phenotypic ratio = 9 : 3 : 3 : 1
Using the classic pea example, round seed shape (R) is dominant to wrinkled (r), and yellow seed colour (Y) is dominant to green (y). The F2 phenotypes are 9 round yellow, 3 round green, 3 wrinkled yellow and 1 wrinkled green.
以经典豌豆为例，圆粒种子（R）对皱粒（r）为显性，黄色种子（Y）对绿色（y）为显性。F₂ 表型为 9 圆黄 : 3 圆绿 : 3 皱黄 : 1 皱绿。
| Phenotype | Genotype(s) | Fraction |
|---|---|---|
| Round yellow | R_Y_ | 9/16 |
| Round green | R_yy | 3/16 |
| Wrinkled yellow | rrY_ | 3/16 |
|
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