📚 6.2 Patterns of Inheritance: Exam Practice | 6.2 遗传模式真题精练
Mastering patterns of inheritance requires not only understanding Mendelian principles but also applying them to unfamiliar contexts. This revision guide focuses on the most common exam-style questions for A-Level Biology, including monohybrid and dihybrid crosses, sex-linked traits, epistasis, and the chi-squared test. Each section is crafted to mirror the structure of past paper questions, helping you build confidence and accuracy.
要掌握遗传模式，不仅需要理解孟德尔原理，还需要将其应用于陌生的情境。本复习指南重点关注 A-Level 生物学最常见的考试题型，包括单性杂交、双性杂交、伴性遗传、上位效应以及卡方检验。每一部分都按照真题的结构设计，帮助你建立信心并提高解题准确性。
1. Monohybrid Crosses and Probability | 单性杂交与概率
A typical question will provide the genotypes of parents and ask for the phenotypic and genotypic ratios of the offspring. Always begin by defining alleles clearly: use a single letter, with uppercase for dominant and lowercase for recessive. For example, in pea plants, let T represent the tall allele and t the dwarf allele.
典型的考题会给出亲本的基因型，要求给出后代的表型和基因型比例。始终要先清晰地定义等位基因：用单个字母表示，显性用大写，隐性用小写。例如，在豌豆中，以 T 表示高茎等位基因，t 表示矮茎等位基因。
e to determine offspring: 1 TT : 2 Tt : 1 tt genotype ratio and 3 tall : 1 dwarf phenotype ratio. Always state the ratio in its simplest form and label phenotypes.
当杂交两个杂合个体（Tt × Tt）时，使用庞纳特方格确定后代：基因型比例为 1 TT : 2 Tt : 1 tt，表型比例为 3 高茎 : 1 矮茎。始终用最简形式表达比例并标明表型。
In a test cross, a dominant-phenotype individual of unknown genotype is crossed with a homozygous recessive. If any offspring show the recessive trait, the unknown parent must be heterozygous. This is frequently tested in exams.
在测交中，将表型为显性但基因型未知的个体与隐性纯合子杂交。如果后代出现隐性性状，则未知亲本必定是杂合子。这是考试中经常考查的内容。
2. Dihybrid Crosses and the Law of Independent Assortment | 双性杂交与自由组合定律
Dihybrid inheritance considers two genes located on different chromosomes. The standard cross is between two double heterozygotes, e.g., parents heterozygous for seed shape (Rr) and seed colour (Yy). According to Mendel's law of independent assortment, alleles for these traits segregate independently, producing gametes RY, Ry, rY, ry in equal proportions.
双性遗传考虑位于不同染色体上的两个基因。标准杂交是两个双杂合子之间的杂交，例如亲本在种子形状（Rr）和种子颜色（Yy）上均为杂合。根据孟德尔的自由组合定律，这些性状的等位基因独立分配，产生比例相等的配子 RY、Ry、rY、ry。
ted phenotypic ratio is 9 : 3 : 3 : 1. In an exam, you may be asked to explain deviations from this ratio due to linkage or epistasis. Always check whether the question specifies that genes are on the same chromosome (linked) or on different ones.
使用 4×4 庞纳特方格，预期的表型比例为 9 : 3 : 3 : 1。考试中可能要求解释由于连锁或上位效应导致的偏离比例。始终检查题目是否说明基因位于同一染色体（连锁）还是不同染色体。
Worked example: In rabbits, black fur (B) is dominant to white (b) and short fur (S) is dominant to long (s). A cross between two rabbits heterozygous for both traits yields offspring. Explain the expected phenotypic ratio and suggest why a breeder might obtain a different outcome.
例题：在兔子中，黑毛（B）对白毛（b）为显性，短毛（S）对长毛（s）为显性。将两只双杂合兔子杂交，得到后代。解释预期的表型比例，并说明为什么育种者可能得到不同的结果。
3. Codominance and
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